diff --git a/config.yml.sample b/config.yml.sample
index 7033456..3be9ce5 100644
--- a/config.yml.sample
+++ b/config.yml.sample
@@ -113,4 +113,4 @@ backend:
       api_verify_tls: true
       api_timeout: 30s
       concurrency: 5
-      debug: false
\ No newline at end of file
+      debug: false
diff --git a/docs/documentation/usage/security.md b/docs/documentation/usage/security.md
index 08024b2..224a8b4 100644
--- a/docs/documentation/usage/security.md
+++ b/docs/documentation/usage/security.md
@@ -8,6 +8,23 @@ To enable encryption, set the [`encryption_passphrase`](../configuration/overvie
 > :warning: Important: Once encryption is enabled, it cannot be disabled, and the passphrase cannot be changed! 
 > Only new or updated records will be encrypted; existing data remains in plaintext until it’s next modified.
 
+## External Identity Provider Data Sanitization
+
+When users authenticate via LDAP, OIDC, or OAuth, WireGuard Portal sanitizes the field values received from the provider before storing them. This protects against several classes of attack that a compromised or misconfigured identity provider could introduce:
+
+- **Unsafe control characters** — Unicode control and format characters, null bytes, and invalid UTF-8 bytes are stripped from external profile fields before they reach the Vue.js UI or email templates.
+- **Email header injection** — carriage return and line feed characters in email fields are rejected entirely, and email fields must parse as plain email addresses.
+- **Log injection** — unsafe control and format characters are stripped from all external profile fields and from sanitization log context.
+- **Denial of service via oversized fields** — field lengths are capped (e.g., 256 runes for identifiers, 254 characters for email addresses).
+- **Reserved identifier collision** — reserved user identifiers such as `"all"`, `"new"`, `"id"`, and internal system user identifiers are rejected.
+- **Unsafe authorization groups** — OIDC/OAuth group claims are sanitized before group-based checks; groups changed by control/format stripping or truncation are dropped rather than repaired into allowed/admin matches.
+
+Sanitization is always enabled and cannot be disabled.
+
+When sanitization modifies or clears a field value, a `WARN` log entry is emitted with the provider name, provider type, and field name — but never the raw or sanitized value, to avoid leaking sensitive data into logs. This makes it straightforward to detect and investigate potentially malicious or misconfigured providers.
+
+---
+
 ## UI and API Access
 
 WireGuard Portal provides a web UI and a REST API for user interaction. It is important to secure these interfaces to prevent unauthorized access and data breaches.
@@ -21,4 +38,4 @@ A detailed explanation is available in the [Reverse Proxy](../getting-started/re
 ### Secure Authentication
 To prevent unauthorized access, WireGuard Portal supports integrating with secure authentication providers such as LDAP, OAuth2, or Passkeys, see [Authentication](./authentication.md) for more details.
 When possible, use centralized authentication and enforce multi-factor authentication (MFA) at the provider level for enhanced account security.
-For local accounts, administrators should enforce strong password requirements.
\ No newline at end of file
+For local accounts, administrators should enforce strong password requirements.
diff --git a/go.mod b/go.mod
index 76ece8d..c88340c 100644
--- a/go.mod
+++ b/go.mod
@@ -31,6 +31,7 @@ require (
 	gorm.io/driver/postgres v1.6.0
 	gorm.io/driver/sqlserver v1.6.3
 	gorm.io/gorm v1.31.1
+	pgregory.net/rapid v1.2.0
 )
 
 require (
diff --git a/go.sum b/go.sum
index ec35c76..808d6ee 100644
--- a/go.sum
+++ b/go.sum
@@ -432,5 +432,7 @@ modernc.org/strutil v1.2.1 h1:UneZBkQA+DX2Rp35KcM69cSsNES9ly8mQWD71HKlOA0=
 modernc.org/strutil v1.2.1/go.mod h1:EHkiggD70koQxjVdSBM3JKM7k6L0FbGE5eymy9i3B9A=
 modernc.org/token v1.1.0 h1:Xl7Ap9dKaEs5kLoOQeQmPWevfnk/DM5qcLcYlA8ys6Y=
 modernc.org/token v1.1.0/go.mod h1:UGzOrNV1mAFSEB63lOFHIpNRUVMvYTc6yu1SMY/XTDM=
+pgregory.net/rapid v1.2.0 h1:keKAYRcjm+e1F0oAuU5F5+YPAWcyxNNRK2wud503Gnk=
+pgregory.net/rapid v1.2.0/go.mod h1:PY5XlDGj0+V1FCq0o192FdRhpKHGTRIWBgqjDBTrq04=
 sigs.k8s.io/yaml v1.6.0 h1:G8fkbMSAFqgEFgh4b1wmtzDnioxFCUgTZhlbj5P9QYs=
 sigs.k8s.io/yaml v1.6.0/go.mod h1:796bPqUfzR/0jLAl6XjHl3Ck7MiyVv8dbTdyT3/pMf4=
diff --git a/internal/app/auth/auth_ldap.go b/internal/app/auth/auth_ldap.go
index ddcca83..95a16c9 100644
--- a/internal/app/auth/auth_ldap.go
+++ b/internal/app/auth/auth_ldap.go
@@ -149,5 +149,9 @@ func (l LdapAuthenticator) ParseUserInfo(raw map[string]any) (*domain.Authentica
 		AdminInfoAvailable: adminInfoAvailable,
 	}
 
+	if err := userInfo.Sanitize("ldap", l.cfg.ProviderName); err != nil {
+		return nil, err
+	}
+
 	return userInfo, nil
 }
diff --git a/internal/app/auth/auth_ldap_sanitize_test.go b/internal/app/auth/auth_ldap_sanitize_test.go
new file mode 100644
index 0000000..55daad1
--- /dev/null
+++ b/internal/app/auth/auth_ldap_sanitize_test.go
@@ -0,0 +1,98 @@
+package auth
+
+import (
+	"errors"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"github.com/h44z/wg-portal/internal/config"
+	"github.com/h44z/wg-portal/internal/domain"
+	"github.com/h44z/wg-portal/internal/testutil"
+)
+
+// makeLdapAuthenticator creates a minimal LdapAuthenticator for testing ParseUserInfo.
+func makeLdapAuthenticator() *LdapAuthenticator {
+	return &LdapAuthenticator{
+		cfg: &config.LdapProvider{
+			ProviderName: "test-ldap",
+			FieldMap: config.LdapFields{
+				BaseFields: config.BaseFields{
+					UserIdentifier: "uid",
+					Email:          "mail",
+					Firstname:      "givenName",
+					Lastname:       "sn",
+					Phone:          "telephoneNumber",
+					Department:     "department",
+				},
+				GroupMembership: "", // no group membership check
+			},
+		},
+	}
+}
+
+// makeRawLdapMap builds a minimal raw LDAP attribute map for ParseUserInfo.
+func makeRawLdapMap(uid, mail, givenName, sn, phone, department string) map[string]any {
+	return map[string]any{
+		"uid":             uid,
+		"mail":            mail,
+		"givenName":       givenName,
+		"sn":              sn,
+		"telephoneNumber": phone,
+		"department":      department,
+	}
+}
+
+// Test: firstname contains \x00 → output firstname has no null byte,
+// one WARN log entry with field: "firstname".
+func TestLdapParseUserInfo_NullByteInFirstname(t *testing.T) {
+	auth := makeLdapAuthenticator()
+	raw := makeRawLdapMap("alice", "alice@example.com", "Ali\x00ce", "Smith", "", "")
+
+	restore := testutil.CaptureWarnLogs(t)
+	info, err := auth.ParseUserInfo(raw)
+	records := restore()
+
+	require.NoError(t, err)
+	assert.NotContains(t, info.Firstname, "\x00", "firstname should have null byte removed")
+	assert.Equal(t, "Alice", info.Firstname)
+
+	warnCount := testutil.CountWarnEntries(records)
+	assert.Equal(t, 1, warnCount, "expected exactly one WARN log entry")
+
+	rec, found := testutil.FindWarnWithField(records, "firstname")
+	assert.True(t, found, "expected WARN log entry with field=firstname")
+	if found {
+		assert.Equal(t, "WARN", rec["level"])
+	}
+}
+
+// Test: all fields clean → no WARN log entries emitted.
+func TestLdapParseUserInfo_AllFieldsClean(t *testing.T) {
+	auth := makeLdapAuthenticator()
+	raw := makeRawLdapMap("alice", "alice@example.com", "Alice", "Smith", "+1 555-1234", "Engineering")
+
+	restore := testutil.CaptureWarnLogs(t)
+	info, err := auth.ParseUserInfo(raw)
+	records := restore()
+
+	require.NoError(t, err)
+	assert.Equal(t, domain.UserIdentifier("alice"), info.Identifier)
+
+	warnCount := testutil.CountWarnEntries(records)
+	assert.Equal(t, 0, warnCount, "expected no WARN log entries when all fields are clean")
+}
+
+// Test: identifier is "all" → returns ErrInvalidData.
+func TestLdapParseUserInfo_IdentifierAll(t *testing.T) {
+	auth := makeLdapAuthenticator()
+	raw := makeRawLdapMap("all", "all@example.com", "Alice", "Smith", "", "")
+
+	restore := testutil.CaptureWarnLogs(t)
+	_, err := auth.ParseUserInfo(raw)
+	_ = restore()
+
+	require.Error(t, err)
+	assert.True(t, errors.Is(err, domain.ErrInvalidData), "expected ErrInvalidData when identifier is 'all'")
+}
diff --git a/internal/app/auth/auth_oauth.go b/internal/app/auth/auth_oauth.go
index 37f6e39..55eb7e4 100644
--- a/internal/app/auth/auth_oauth.go
+++ b/internal/app/auth/auth_oauth.go
@@ -155,5 +155,5 @@ func (p PlainOauthAuthenticator) GetUserInfo(
 
 // ParseUserInfo parses the user information from the raw data.
 func (p PlainOauthAuthenticator) ParseUserInfo(raw map[string]any) (*domain.AuthenticatorUserInfo, error) {
-	return parseOauthUserInfo(p.userInfoMapping, p.userAdminMapping, raw)
+	return parseOauthUserInfo(p.userInfoMapping, p.userAdminMapping, raw, "oauth", p.name)
 }
diff --git a/internal/app/auth/auth_oidc.go b/internal/app/auth/auth_oidc.go
index ec53fdf..5bcdbc7 100644
--- a/internal/app/auth/auth_oidc.go
+++ b/internal/app/auth/auth_oidc.go
@@ -194,5 +194,5 @@ func (o OidcAuthenticator) GetUserInfo(ctx context.Context, token *oauth2.Token,
 
 // ParseUserInfo parses the user info.
 func (o OidcAuthenticator) ParseUserInfo(raw map[string]any) (*domain.AuthenticatorUserInfo, error) {
-	return parseOauthUserInfo(o.userInfoMapping, o.userAdminMapping, raw)
+	return parseOauthUserInfo(o.userInfoMapping, o.userAdminMapping, raw, "oidc", o.name)
 }
diff --git a/internal/app/auth/oauth_common.go b/internal/app/auth/oauth_common.go
index 855823c..586be15 100644
--- a/internal/app/auth/oauth_common.go
+++ b/internal/app/auth/oauth_common.go
@@ -13,6 +13,8 @@ func parseOauthUserInfo(
 	mapping config.OauthFields,
 	adminMapping *config.OauthAdminMapping,
 	raw map[string]any,
+	providerType string,
+	providerName string,
 ) (*domain.AuthenticatorUserInfo, error) {
 	var isAdmin bool
 	var adminInfoAvailable bool
@@ -27,18 +29,6 @@ func parseOauthUserInfo(
 		}
 	}
 
-	// next try to parse the user's groups
-	if !isAdmin && mapping.UserGroups != "" && adminMapping.AdminGroupRegex != "" {
-		adminInfoAvailable = true
-		re := adminMapping.GetAdminGroupRegex()
-		for _, group := range userGroups {
-			if re.MatchString(strings.TrimSpace(group)) {
-				isAdmin = true
-				break
-			}
-		}
-	}
-
 	userInfo := &domain.AuthenticatorUserInfo{
 		Identifier:         domain.UserIdentifier(internal.MapDefaultString(raw, mapping.UserIdentifier, "")),
 		Email:              internal.MapDefaultString(raw, mapping.Email, ""),
@@ -51,6 +41,24 @@ func parseOauthUserInfo(
 		AdminInfoAvailable: adminInfoAvailable,
 	}
 
+	if err := userInfo.Sanitize(providerType, providerName); err != nil {
+		return nil, err
+	}
+
+	// check admin group match after sanitization
+	if !isAdmin && mapping.UserGroups != "" && adminMapping.AdminGroupRegex != "" {
+		adminInfoAvailable = true
+		re := adminMapping.GetAdminGroupRegex()
+		for _, group := range userInfo.UserGroups {
+			if re.MatchString(group) {
+				isAdmin = true
+				break
+			}
+		}
+		userInfo.IsAdmin = isAdmin
+		userInfo.AdminInfoAvailable = adminInfoAvailable
+	}
+
 	return userInfo, nil
 }
 
diff --git a/internal/app/auth/oauth_common_sanitize_test.go b/internal/app/auth/oauth_common_sanitize_test.go
new file mode 100644
index 0000000..bb7a48b
--- /dev/null
+++ b/internal/app/auth/oauth_common_sanitize_test.go
@@ -0,0 +1,148 @@
+package auth
+
+import (
+	"errors"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"github.com/h44z/wg-portal/internal/config"
+	"github.com/h44z/wg-portal/internal/domain"
+	"github.com/h44z/wg-portal/internal/testutil"
+)
+
+// makeOauthFieldMapping returns a minimal OauthFields mapping for testing.
+func makeOauthFieldMapping() config.OauthFields {
+	return config.OauthFields{
+		BaseFields: config.BaseFields{
+			UserIdentifier: "sub",
+			Email:          "email",
+			Firstname:      "given_name",
+			Lastname:       "family_name",
+			Phone:          "phone",
+			Department:     "department",
+		},
+	}
+}
+
+// makeOauthRaw builds a minimal raw OAuth user info map.
+func makeOauthRaw(sub, email, givenName, familyName, phone, department string) map[string]any {
+	return map[string]any{
+		"sub":         sub,
+		"email":       email,
+		"given_name":  givenName,
+		"family_name": familyName,
+		"phone":       phone,
+		"department":  department,
+	}
+}
+
+// Test: email containing \r\n → output email is "",
+// one WARN log entry with field: "email" and cleared indication.
+func TestParseOauthUserInfo_CRLFInEmail(t *testing.T) {
+	mapping := makeOauthFieldMapping()
+	adminMapping := &config.OauthAdminMapping{}
+	raw := makeOauthRaw("user123", "user\r\n@example.com", "Alice", "Smith", "", "")
+
+	restore := testutil.CaptureWarnLogs(t)
+	info, err := parseOauthUserInfo(mapping, adminMapping, raw, "oauth", "test-provider")
+	records := restore()
+
+	require.NoError(t, err)
+	assert.Equal(t, "", info.Email, "email should be cleared when it contains CR/LF")
+
+	warnCount := testutil.CountWarnEntries(records)
+	assert.Equal(t, 1, warnCount, "expected exactly one WARN log entry")
+
+	rec, found := testutil.FindWarnWithField(records, "email")
+	assert.True(t, found, "expected WARN log entry with field=email")
+	if found {
+		msg, _ := rec["msg"].(string)
+		assert.Contains(t, msg, "cleared", "expected 'cleared' in log message when email is cleared")
+	}
+}
+
+// Test: two fields modified (email cleared, firstname truncated) →
+// two separate WARN log entries.
+func TestParseOauthUserInfo_TwoFieldsModified(t *testing.T) {
+	mapping := makeOauthFieldMapping()
+	adminMapping := &config.OauthAdminMapping{}
+
+	longFirstname := strings.Repeat("A", 200)
+	raw := makeOauthRaw("user123", "bad\r\nemail@example.com", longFirstname, "Smith", "", "")
+
+	restore := testutil.CaptureWarnLogs(t)
+	info, err := parseOauthUserInfo(mapping, adminMapping, raw, "oauth", "test-provider")
+	records := restore()
+
+	require.NoError(t, err)
+	assert.Equal(t, "", info.Email, "email should be cleared")
+	assert.Equal(t, 128, len([]rune(info.Firstname)), "firstname should be truncated to 128 runes")
+
+	warnCount := testutil.CountWarnEntries(records)
+	assert.Equal(t, 2, warnCount, "expected exactly two WARN log entries (one per modified field)")
+
+	_, emailFound := testutil.FindWarnWithField(records, "email")
+	assert.True(t, emailFound, "expected WARN log entry with field=email")
+
+	_, firstnameFound := testutil.FindWarnWithField(records, "firstname")
+	assert.True(t, firstnameFound, "expected WARN log entry with field=firstname")
+}
+
+// Test: identifier "all" → returns ErrInvalidData.
+func TestParseOauthUserInfo_IdentifierAll(t *testing.T) {
+	mapping := makeOauthFieldMapping()
+	adminMapping := &config.OauthAdminMapping{}
+	raw := makeOauthRaw("all", "all@example.com", "Alice", "Smith", "", "")
+
+	restore := testutil.CaptureWarnLogs(t)
+	_, err := parseOauthUserInfo(mapping, adminMapping, raw, "oauth", "test-provider")
+	_ = restore()
+
+	require.Error(t, err)
+	assert.True(t, errors.Is(err, domain.ErrInvalidData), "expected ErrInvalidData when identifier is 'all'")
+}
+
+func TestParseOauthUserInfo_DropsModifiedGroupBeforeAdminMatch(t *testing.T) {
+	mapping := makeOauthFieldMapping()
+	mapping.UserGroups = "groups"
+	adminMapping := &config.OauthAdminMapping{
+		AdminGroupRegex: "^wgportal-admins$",
+	}
+	raw := makeOauthRaw("user123", "user@example.com", "Alice", "Smith", "", "")
+	raw["groups"] = []any{"wgportal-\u200badmins"}
+
+	restore := testutil.CaptureWarnLogs(t)
+	info, err := parseOauthUserInfo(mapping, adminMapping, raw, "oidc", "test-provider")
+	records := restore()
+
+	require.NoError(t, err)
+	require.NotNil(t, info)
+	assert.False(t, info.IsAdmin, "sanitization must not repair a modified group into an admin match")
+	assert.Empty(t, info.UserGroups)
+
+	rec, found := testutil.FindWarnWithField(records, "user_group")
+	assert.True(t, found, "expected WARN log entry with field=user_group")
+	if found {
+		assert.Equal(t, "oidc", rec["provider_type"])
+	}
+}
+
+func TestParseOauthUserInfo_AllowsWhitespaceOnlyGroupTrim(t *testing.T) {
+	mapping := makeOauthFieldMapping()
+	mapping.UserGroups = "groups"
+	adminMapping := &config.OauthAdminMapping{
+		AdminGroupRegex: "^wgportal-admins$",
+	}
+	raw := makeOauthRaw("user123", "user@example.com", "Alice", "Smith", "", "")
+	raw["groups"] = []any{" wgportal-admins "}
+
+	info, err := parseOauthUserInfo(mapping, adminMapping, raw, "oidc", "test-provider")
+
+	require.NoError(t, err)
+	require.NotNil(t, info)
+	assert.True(t, info.IsAdmin)
+	assert.Equal(t, []string{"wgportal-admins"}, info.UserGroups)
+}
diff --git a/internal/app/auth/oauth_common_test.go b/internal/app/auth/oauth_common_test.go
index 8c08210..d3992e3 100644
--- a/internal/app/auth/oauth_common_test.go
+++ b/internal/app/auth/oauth_common_test.go
@@ -43,7 +43,7 @@ func Test_parseOauthUserInfo_no_admin(t *testing.T) {
 	})
 	adminMapping := &config.OauthAdminMapping{}
 
-	info, err := parseOauthUserInfo(fieldMapping, adminMapping, userInfo)
+	info, err := parseOauthUserInfo(fieldMapping, adminMapping, userInfo, "oauth", "test-provider")
 	assert.NoError(t, err)
 	assert.False(t, info.IsAdmin)
 	assert.Equal(t, info.Firstname, "Test User")
@@ -90,7 +90,7 @@ func Test_parseOauthUserInfo_admin_group(t *testing.T) {
 		AdminGroupRegex: "^wgportal-admins@mydomain.net$",
 	}
 
-	info, err := parseOauthUserInfo(fieldMapping, adminMapping, userInfo)
+	info, err := parseOauthUserInfo(fieldMapping, adminMapping, userInfo, "oauth", "test-provider")
 	assert.NoError(t, err)
 	assert.True(t, info.IsAdmin)
 	assert.Equal(t, info.Firstname, "Test User")
@@ -132,7 +132,7 @@ func Test_parseOauthUserInfo_admin_value(t *testing.T) {
 	})
 	adminMapping := &config.OauthAdminMapping{} // test with default regex
 
-	info, err := parseOauthUserInfo(fieldMapping, adminMapping, userInfo)
+	info, err := parseOauthUserInfo(fieldMapping, adminMapping, userInfo, "oauth", "test-provider")
 	assert.NoError(t, err)
 	assert.True(t, info.IsAdmin)
 	assert.Equal(t, info.Firstname, "Test User")
@@ -175,7 +175,7 @@ func Test_parseOauthUserInfo_admin_value_custom(t *testing.T) {
 		AdminValueRegex: "^1$",
 	}
 
-	info, err := parseOauthUserInfo(fieldMapping, adminMapping, userInfo)
+	info, err := parseOauthUserInfo(fieldMapping, adminMapping, userInfo, "oauth", "test-provider")
 	assert.NoError(t, err)
 	assert.True(t, info.IsAdmin)
 	assert.Equal(t, info.Firstname, "Test User")
diff --git a/internal/app/auth/sanitize_log_test.go b/internal/app/auth/sanitize_log_test.go
new file mode 100644
index 0000000..d64ba8f
--- /dev/null
+++ b/internal/app/auth/sanitize_log_test.go
@@ -0,0 +1,90 @@
+package auth
+
+import (
+	"bytes"
+	"encoding/json"
+	"log/slog"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+	"pgregory.net/rapid"
+
+	"github.com/h44z/wg-portal/internal/config"
+	"github.com/h44z/wg-portal/internal/domain"
+	"github.com/h44z/wg-portal/internal/testutil"
+)
+
+// captureWarnLogsInline redirects the default slog logger to a buffer, calls fn,
+// restores the original logger, and returns the captured log records.
+func captureWarnLogsInline(fn func()) []map[string]any {
+	original := slog.Default()
+	var buf bytes.Buffer
+	handler := slog.NewJSONHandler(&buf, &slog.HandlerOptions{Level: slog.LevelWarn})
+	slog.SetDefault(slog.New(handler))
+
+	fn()
+
+	slog.SetDefault(original)
+
+	var records []map[string]any
+	decoder := json.NewDecoder(&buf)
+	for decoder.More() {
+		var rec map[string]any
+		if err := decoder.Decode(&rec); err == nil {
+			records = append(records, rec)
+		}
+	}
+	return records
+}
+
+// Property 7: Sanitization change logging completeness
+func TestPropertySanitizationChangeLoggingCompleteness(t *testing.T) {
+	mapping := makeOauthFieldMapping()
+	adminMapping := &config.OauthAdminMapping{}
+
+	rapid.Check(t, func(t *rapid.T) {
+		sub := rapid.StringMatching(`[a-zA-Z0-9_@.-]{1,50}`).Draw(t, "sub")
+		email := rapid.String().Draw(t, "email")
+		firstname := rapid.String().Draw(t, "firstname")
+		lastname := rapid.String().Draw(t, "lastname")
+		phone := rapid.String().Draw(t, "phone")
+		department := rapid.String().Draw(t, "department")
+
+		if sub == "" {
+			sub = "testuser"
+		}
+
+		raw := makeOauthRaw(sub, email, firstname, lastname, phone, department)
+
+		// Count how many fields will actually change after sanitization
+		expectedChanges := 0
+		if domain.SanitizeIdentifier(sub, 256) != sub {
+			expectedChanges++
+		}
+		if domain.SanitizeEmail(email, 254) != email {
+			expectedChanges++
+		}
+		if domain.SanitizeString(firstname, 128) != firstname {
+			expectedChanges++
+		}
+		if domain.SanitizeString(lastname, 128) != lastname {
+			expectedChanges++
+		}
+		if domain.SanitizePhone(phone, 50) != phone {
+			expectedChanges++
+		}
+		if domain.SanitizeString(department, 128) != department {
+			expectedChanges++
+		}
+
+		var records []map[string]any
+		records = captureWarnLogsInline(func() {
+			_, _ = parseOauthUserInfo(mapping, adminMapping, raw, "oauth", "test-provider")
+		})
+
+		actualWarnCount := testutil.CountWarnEntries(records)
+		require.Equal(t, expectedChanges, actualWarnCount,
+			"number of WARN log entries (%d) must equal number of fields changed by sanitization (%d)",
+			actualWarnCount, expectedChanges)
+	})
+}
diff --git a/internal/app/users/ldap_helper.go b/internal/app/users/ldap_helper.go
index c202110..51430cb 100644
--- a/internal/app/users/ldap_helper.go
+++ b/internal/app/users/ldap_helper.go
@@ -28,7 +28,7 @@ func convertRawLdapUser(
 
 	uid := domain.UserIdentifier(internal.MapDefaultString(rawUser, fields.UserIdentifier, ""))
 
-	return &domain.User{
+	user := &domain.User{
 		BaseModel: domain.BaseModel{
 			CreatedBy: domain.CtxSystemLdapSyncer,
 			UpdatedBy: domain.CtxSystemLdapSyncer,
@@ -49,10 +49,16 @@ func convertRawLdapUser(
 		Lastname:   internal.MapDefaultString(rawUser, fields.Lastname, ""),
 		Phone:      internal.MapDefaultString(rawUser, fields.Phone, ""),
 		Department: internal.MapDefaultString(rawUser, fields.Department, ""),
-		Notes:      "",
-		Password:   "",
-		Disabled:   nil,
-	}, nil
+	}
+
+	if err := user.SanitizeExternalData("ldap", providerName); err != nil {
+		return nil, err
+	}
+
+	// Update authentication identifier after sanitization
+	user.Authentications[0].UserIdentifier = user.Identifier
+
+	return user, nil
 }
 
 func userChangedInLdap(dbUser, ldapUser *domain.User) bool {
diff --git a/internal/app/users/ldap_helper_sanitize_test.go b/internal/app/users/ldap_helper_sanitize_test.go
new file mode 100644
index 0000000..8333992
--- /dev/null
+++ b/internal/app/users/ldap_helper_sanitize_test.go
@@ -0,0 +1,136 @@
+package users
+
+import (
+	"errors"
+	"testing"
+
+	"github.com/go-ldap/ldap/v3"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"github.com/h44z/wg-portal/internal/config"
+	"github.com/h44z/wg-portal/internal/domain"
+	"github.com/h44z/wg-portal/internal/testutil"
+)
+
+// makeTestLdapFields returns a minimal LdapFields config for testing.
+func makeTestLdapFields() *config.LdapFields {
+	return &config.LdapFields{
+		BaseFields: config.BaseFields{
+			UserIdentifier: "uid",
+			Email:          "mail",
+			Firstname:      "givenName",
+			Lastname:       "sn",
+			Phone:          "telephoneNumber",
+			Department:     "department",
+		},
+		GroupMembership: "memberOf",
+	}
+}
+
+// makeTestAdminGroupDN returns a parsed DN for testing (a non-matching group).
+func makeTestAdminGroupDN(t *testing.T) *ldap.DN {
+	t.Helper()
+	dn, err := ldap.ParseDN("cn=admins,dc=example,dc=com")
+	require.NoError(t, err)
+	return dn
+}
+
+// makeRawLdapUser builds a raw LDAP user map for convertRawLdapUser.
+func makeRawLdapUser(uid, mail, givenName, sn, phone, department string) map[string]any {
+	return map[string]any{
+		"uid":             uid,
+		"mail":            mail,
+		"givenName":       givenName,
+		"sn":              sn,
+		"telephoneNumber": phone,
+		"department":      department,
+		"memberOf":        [][]byte{}, // no group memberships
+	}
+}
+
+// Test: identifier "all" → returns ErrInvalidData,
+// one WARN log entry with field: "identifier" and cleared indication.
+func TestConvertRawLdapUser_IdentifierAll(t *testing.T) {
+	fields := makeTestLdapFields()
+	adminGroupDN := makeTestAdminGroupDN(t)
+	raw := makeRawLdapUser("all", "all@example.com", "Alice", "Smith", "", "")
+
+	restore := testutil.CaptureWarnLogs(t)
+	user, err := convertRawLdapUser("test-ldap", raw, fields, adminGroupDN)
+	records := restore()
+
+	require.Error(t, err)
+	assert.True(t, errors.Is(err, domain.ErrInvalidData), "expected ErrInvalidData when identifier is 'all'")
+	assert.Nil(t, user)
+
+	warnCount := testutil.CountWarnEntries(records)
+	assert.Equal(t, 1, warnCount, "expected exactly one WARN log entry")
+
+	rec, found := testutil.FindWarnWithField(records, "identifier")
+	assert.True(t, found, "expected WARN log entry with field=identifier")
+	if found {
+		msg, _ := rec["msg"].(string)
+		assert.Contains(t, msg, "cleared", "expected 'cleared' in log message when identifier is cleared")
+	}
+}
+
+// Test: firstname contains \x00 → output firstname has null byte removed,
+// one WARN log entry with field: "firstname".
+func TestConvertRawLdapUser_NullByteInFirstname(t *testing.T) {
+	fields := makeTestLdapFields()
+	adminGroupDN := makeTestAdminGroupDN(t)
+	raw := makeRawLdapUser("alice", "alice@example.com", "Ali\x00ce", "Smith", "", "")
+
+	restore := testutil.CaptureWarnLogs(t)
+	user, err := convertRawLdapUser("test-ldap", raw, fields, adminGroupDN)
+	records := restore()
+
+	require.NoError(t, err)
+	require.NotNil(t, user)
+	assert.NotContains(t, user.Firstname, "\x00", "firstname should have null byte removed")
+	assert.Equal(t, "Alice", user.Firstname)
+
+	warnCount := testutil.CountWarnEntries(records)
+	assert.Equal(t, 1, warnCount, "expected exactly one WARN log entry")
+
+	rec, found := testutil.FindWarnWithField(records, "firstname")
+	assert.True(t, found, "expected WARN log entry with field=firstname")
+	if found {
+		assert.Equal(t, "WARN", rec["level"])
+	}
+}
+
+// Test: all fields clean → no WARN log entries emitted.
+func TestConvertRawLdapUser_AllFieldsClean(t *testing.T) {
+	fields := makeTestLdapFields()
+	adminGroupDN := makeTestAdminGroupDN(t)
+	raw := makeRawLdapUser("alice", "alice@example.com", "Alice", "Smith", "+1 555-1234", "Engineering")
+
+	restore := testutil.CaptureWarnLogs(t)
+	user, err := convertRawLdapUser("test-ldap", raw, fields, adminGroupDN)
+	records := restore()
+
+	require.NoError(t, err)
+	require.NotNil(t, user)
+	assert.Equal(t, domain.UserIdentifier("alice"), user.Identifier)
+
+	warnCount := testutil.CountWarnEntries(records)
+	assert.Equal(t, 0, warnCount, "expected no WARN log entries when all fields are clean")
+}
+
+func TestLdapUserIdentifier_NormalizesSyncComparisons(t *testing.T) {
+	raw := map[string]any{"uid": " alice\x00 "}
+
+	got := ldapUserIdentifier(raw, "uid")
+
+	assert.Equal(t, domain.UserIdentifier("alice"), got)
+}
+
+func TestLdapUserIdentifier_RejectsReservedIdentifier(t *testing.T) {
+	raw := map[string]any{"uid": " all "}
+
+	got := ldapUserIdentifier(raw, "uid")
+
+	assert.Empty(t, got)
+}
diff --git a/internal/app/users/ldap_sync.go b/internal/app/users/ldap_sync.go
index cc0fff9..56cc299 100644
--- a/internal/app/users/ldap_sync.go
+++ b/internal/app/users/ldap_sync.go
@@ -109,6 +109,11 @@ func (m Manager) updateLdapUsers(
 	for _, rawUser := range rawUsers {
 		user, err := convertRawLdapUser(provider.ProviderName, rawUser, fields, adminGroupDN)
 		if err != nil && !errors.Is(err, domain.ErrNotFound) {
+			if errors.Is(err, domain.ErrInvalidData) {
+				slog.Warn("skipping LDAP user with invalid data after sanitization",
+					"raw-dn", rawUser["dn"], "error", err)
+				continue
+			}
 			return fmt.Errorf("failed to convert LDAP data for %v: %w", rawUser["dn"], err)
 		}
 
@@ -212,7 +217,7 @@ func (m Manager) disableMissingLdapUsers(
 
 		existsInLDAP := false
 		for _, rawUser := range rawUsers {
-			userId := domain.UserIdentifier(internal.MapDefaultString(rawUser, fields.UserIdentifier, ""))
+			userId := ldapUserIdentifier(rawUser, fields.UserIdentifier)
 			if user.Identifier == userId {
 				existsInLDAP = true
 				break
@@ -258,19 +263,19 @@ func (m Manager) updateInterfaceLdapFilters(
 
 		// Combined filter: user must match the provider's base SyncFilter AND the interface's LdapGroupFilter
 		combinedFilter := fmt.Sprintf("(&(%s)(%s))", provider.SyncFilter, groupFilter)
-		
+
 		rawUsers, err := internal.LdapFindAllUsers(conn, provider.BaseDN, combinedFilter, &provider.FieldMap)
 		if err != nil {
-			slog.Error("failed to find users for interface filter", 
-				"interface", ifaceId, 
-				"provider", provider.ProviderName, 
+			slog.Error("failed to find users for interface filter",
+				"interface", ifaceId,
+				"provider", provider.ProviderName,
 				"error", err)
 			continue
 		}
 
 		matchedUserIds := make([]domain.UserIdentifier, 0, len(rawUsers))
 		for _, rawUser := range rawUsers {
-			userId := domain.UserIdentifier(internal.MapDefaultString(rawUser, provider.FieldMap.UserIdentifier, ""))
+			userId := ldapUserIdentifier(rawUser, provider.FieldMap.UserIdentifier)
 			if userId != "" {
 				matchedUserIds = append(matchedUserIds, userId)
 			}
@@ -285,17 +290,26 @@ func (m Manager) updateInterfaceLdapFilters(
 			return i, nil
 		})
 		if err != nil {
-			slog.Error("failed to save interface ldap allowed users", 
-				"interface", ifaceId, 
-				"provider", provider.ProviderName, 
+			slog.Error("failed to save interface ldap allowed users",
+				"interface", ifaceId,
+				"provider", provider.ProviderName,
 				"error", err)
 		} else {
-			slog.Debug("updated interface ldap allowed users", 
-				"interface", ifaceId, 
-				"provider", provider.ProviderName, 
+			slog.Debug("updated interface ldap allowed users",
+				"interface", ifaceId,
+				"provider", provider.ProviderName,
 				"matched_count", len(matchedUserIds))
 		}
 	}
 
 	return nil
 }
+
+func ldapUserIdentifier(rawUser map[string]any, field string) domain.UserIdentifier {
+	identifier := internal.MapDefaultString(rawUser, field, "")
+	identifier = domain.SanitizeIdentifier(identifier, 256)
+	if identifier == "" {
+		return ""
+	}
+	return domain.UserIdentifier(identifier)
+}
diff --git a/internal/app/users/user_manager.go b/internal/app/users/user_manager.go
index bb2d3a6..06dbc11 100644
--- a/internal/app/users/user_manager.go
+++ b/internal/app/users/user_manager.go
@@ -365,19 +365,7 @@ func (m Manager) validateCreation(ctx context.Context, new *domain.User) error {
 		return fmt.Errorf("invalid user identifier: %w", domain.ErrInvalidData)
 	}
 
-	if new.Identifier == "all" { // the 'all' user identifier collides with the rest api routes
-		return fmt.Errorf("reserved user identifier: %w", domain.ErrInvalidData)
-	}
-
-	if new.Identifier == "new" { // the 'new' user identifier collides with the rest api routes
-		return fmt.Errorf("reserved user identifier: %w", domain.ErrInvalidData)
-	}
-
-	if new.Identifier == "id" { // the 'id' user identifier collides with the rest api routes
-		return fmt.Errorf("reserved user identifier: %w", domain.ErrInvalidData)
-	}
-
-	if new.Identifier == domain.CtxSystemAdminId || new.Identifier == domain.CtxUnknownUserId {
+	if domain.IsReservedUserIdentifier(new.Identifier) {
 		return fmt.Errorf("reserved user identifier: %w", domain.ErrInvalidData)
 	}
 
diff --git a/internal/domain/auth.go b/internal/domain/auth.go
index 02853b7..a12e328 100644
--- a/internal/domain/auth.go
+++ b/internal/domain/auth.go
@@ -1,5 +1,10 @@
 package domain
 
+import (
+	"fmt"
+	"strings"
+)
+
 type LoginProvider string
 
 type LoginProviderInfo struct {
@@ -20,3 +25,52 @@ type AuthenticatorUserInfo struct {
 	IsAdmin            bool
 	AdminInfoAvailable bool // true if the IsAdmin flag is valid
 }
+
+// Sanitize sanitizes all external identity provider fields in place.
+// Returns ErrInvalidData if the identifier becomes empty after sanitization.
+func (u *AuthenticatorUserInfo) Sanitize(providerType, providerName string) error {
+	identifier := string(u.Identifier)
+	LogSanitizeChange(providerType, providerName, "identifier", identifier,
+		func() string { return SanitizeIdentifier(identifier, 256) }, &identifier)
+	u.Identifier = UserIdentifier(identifier)
+
+	email := u.Email
+	LogSanitizeChange(providerType, providerName, "email", email,
+		func() string { return SanitizeEmail(email, 254) }, &u.Email)
+	LogSanitizeChange(providerType, providerName, "firstname", u.Firstname,
+		func() string { return SanitizeString(u.Firstname, 128) }, &u.Firstname)
+	LogSanitizeChange(providerType, providerName, "lastname", u.Lastname,
+		func() string { return SanitizeString(u.Lastname, 128) }, &u.Lastname)
+	LogSanitizeChange(providerType, providerName, "phone", u.Phone,
+		func() string { return SanitizePhone(u.Phone, 50) }, &u.Phone)
+	LogSanitizeChange(providerType, providerName, "department", u.Department,
+		func() string { return SanitizeString(u.Department, 128) }, &u.Department)
+
+	u.UserGroups = sanitizeGroups(providerType, providerName, u.UserGroups)
+
+	if u.Identifier == "" {
+		return fmt.Errorf("empty user identifier: %w", ErrInvalidData)
+	}
+
+	return nil
+}
+
+// sanitizeGroups sanitizes group names, dropping any that were modified by sanitization.
+func sanitizeGroups(providerType, providerName string, rawGroups []string) []string {
+	if len(rawGroups) == 0 {
+		return rawGroups
+	}
+
+	groups := make([]string, 0, len(rawGroups))
+	for _, rawGroup := range rawGroups {
+		sanitized := rawGroup
+		LogSanitizeChange(providerType, providerName, "user_group", rawGroup,
+			func() string { return SanitizeString(rawGroup, 256) }, &sanitized)
+		if sanitized == "" || sanitized != strings.TrimSpace(rawGroup) {
+			continue
+		}
+		groups = append(groups, sanitized)
+	}
+
+	return groups
+}
diff --git a/internal/domain/auth_sanitize_test.go b/internal/domain/auth_sanitize_test.go
new file mode 100644
index 0000000..5098f35
--- /dev/null
+++ b/internal/domain/auth_sanitize_test.go
@@ -0,0 +1,103 @@
+package domain
+
+import (
+	"errors"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"github.com/h44z/wg-portal/internal/testutil"
+)
+
+func TestAuthenticatorUserInfo_Sanitize_NullByteInFirstname(t *testing.T) {
+	info := &AuthenticatorUserInfo{
+		Identifier: "alice",
+		Email:      "alice@example.com",
+		Firstname:  "Ali\x00ce",
+		Lastname:   "Smith",
+	}
+
+	restore := testutil.CaptureWarnLogs(t)
+	err := info.Sanitize("ldap", "test-provider")
+	records := restore()
+
+	require.NoError(t, err)
+	assert.Equal(t, "Alice", info.Firstname)
+
+	warnCount := testutil.CountWarnEntries(records)
+	assert.Equal(t, 1, warnCount)
+
+	_, found := testutil.FindWarnWithField(records, "firstname")
+	assert.True(t, found)
+}
+
+func TestAuthenticatorUserInfo_Sanitize_AllFieldsClean(t *testing.T) {
+	info := &AuthenticatorUserInfo{
+		Identifier: "alice",
+		Email:      "alice@example.com",
+		Firstname:  "Alice",
+		Lastname:   "Smith",
+		Phone:      "+1 555-1234",
+		Department: "Engineering",
+	}
+
+	restore := testutil.CaptureWarnLogs(t)
+	err := info.Sanitize("ldap", "test-provider")
+	records := restore()
+
+	require.NoError(t, err)
+	assert.Equal(t, UserIdentifier("alice"), info.Identifier)
+	assert.Equal(t, 0, testutil.CountWarnEntries(records))
+}
+
+func TestAuthenticatorUserInfo_Sanitize_IdentifierAll(t *testing.T) {
+	info := &AuthenticatorUserInfo{
+		Identifier: "all",
+		Email:      "all@example.com",
+		Firstname:  "Alice",
+		Lastname:   "Smith",
+	}
+
+	err := info.Sanitize("ldap", "test-provider")
+
+	require.Error(t, err)
+	assert.True(t, errors.Is(err, ErrInvalidData))
+}
+
+func TestAuthenticatorUserInfo_Sanitize_CRLFInEmail(t *testing.T) {
+	info := &AuthenticatorUserInfo{
+		Identifier: "user123",
+		Email:      "user\r\n@example.com",
+		Firstname:  "Alice",
+		Lastname:   "Smith",
+	}
+
+	restore := testutil.CaptureWarnLogs(t)
+	err := info.Sanitize("oauth", "test-provider")
+	records := restore()
+
+	require.NoError(t, err)
+	assert.Equal(t, "", info.Email)
+
+	_, found := testutil.FindWarnWithField(records, "email")
+	assert.True(t, found)
+}
+
+func TestAuthenticatorUserInfo_Sanitize_GroupsWithZeroWidthChars(t *testing.T) {
+	info := &AuthenticatorUserInfo{
+		Identifier: "user123",
+		Email:      "user@example.com",
+		UserGroups: []string{"wgportal-\u200badmins"},
+	}
+
+	restore := testutil.CaptureWarnLogs(t)
+	err := info.Sanitize("oidc", "test-provider")
+	records := restore()
+
+	require.NoError(t, err)
+	assert.Empty(t, info.UserGroups)
+
+	_, found := testutil.FindWarnWithField(records, "user_group")
+	assert.True(t, found)
+}
diff --git a/internal/domain/sanitize.go b/internal/domain/sanitize.go
new file mode 100644
index 0000000..9999a60
--- /dev/null
+++ b/internal/domain/sanitize.go
@@ -0,0 +1,151 @@
+package domain
+
+import (
+	"log/slog"
+	"net/mail"
+	"strings"
+	"unicode"
+	"unicode/utf8"
+
+	"golang.org/x/text/unicode/norm"
+)
+
+// LogSanitizeChange applies sanitizeFn to raw, logs when the value changes, and writes
+// the sanitized value to dest. Raw and sanitized values are intentionally omitted.
+func LogSanitizeChange(
+	providerType string,
+	providerName string,
+	field string,
+	raw string,
+	sanitizeFn func() string,
+	dest *string,
+) {
+	sanitized := sanitizeFn()
+	if sanitized != raw {
+		message := "sanitization modified field value from external provider"
+		if sanitized == "" {
+			message = "sanitization cleared field value from external provider"
+		}
+		slog.Warn(message,
+			"provider_type", SanitizeString(providerType, 64),
+			"provider", SanitizeString(providerName, 128),
+			"field", SanitizeString(field, 64),
+		)
+	}
+	*dest = sanitized
+}
+
+var reservedUserIdentifiers = map[string]struct{}{
+	"all":               {},
+	"new":               {},
+	"id":                {},
+	CtxSystemAdminId:    {},
+	CtxUnknownUserId:    {},
+	CtxSystemLdapSyncer: {},
+	CtxSystemWgImporter: {},
+	CtxSystemV1Migrator: {},
+	CtxSystemDBMigrator: {},
+}
+
+// SanitizeString normalizes to NFC, trims leading and trailing whitespace, strips Unicode
+// control and format characters, drops invalid UTF-8 bytes, and truncates the result to
+// maxLen runes. If maxLen <= 0, returns "".
+func SanitizeString(s string, maxLen int) string {
+	if maxLen <= 0 {
+		return ""
+	}
+
+	s = norm.NFC.String(strings.TrimSpace(s))
+
+	var b strings.Builder
+	b.Grow(len(s))
+	for len(s) > 0 {
+		r, size := utf8.DecodeRuneInString(s)
+		s = s[size:]
+		if r == utf8.RuneError && size == 1 {
+			continue
+		}
+		if !unicode.IsControl(r) && !unicode.Is(unicode.Cf, r) {
+			b.WriteRune(r)
+		}
+	}
+	s = b.String()
+
+	if utf8.RuneCountInString(s) > maxLen {
+		runes := []rune(s)
+		s = string(runes[:maxLen])
+	}
+
+	return strings.TrimSpace(s)
+}
+
+// SanitizeEmail applies SanitizeString first, then returns "" if the original s
+// contains CR/LF or if the sanitized result is not a plain email address.
+func SanitizeEmail(s string, maxLen int) string {
+	if strings.ContainsRune(s, '\r') || strings.ContainsRune(s, '\n') {
+		return ""
+	}
+
+	sanitized := SanitizeString(s, maxLen)
+
+	if sanitized == "" || strings.Count(sanitized, "@") != 1 {
+		return ""
+	}
+	addr, err := mail.ParseAddress(sanitized)
+	if err != nil || addr.Name != "" || addr.Address != sanitized {
+		return ""
+	}
+
+	return sanitized
+}
+
+// SanitizePhone applies SanitizeString first, then removes all characters not in the
+// set [0-9+\-() .]. Returns "" if the result after filtering is empty.
+func SanitizePhone(s string, maxLen int) string {
+	sanitized := SanitizeString(s, maxLen)
+
+	// Remove all characters not in [0-9+\-() .]
+	var b strings.Builder
+	b.Grow(len(sanitized))
+	for _, r := range sanitized {
+		if isAllowedPhoneRune(r) {
+			b.WriteRune(r)
+		}
+	}
+	result := strings.TrimSpace(b.String())
+
+	if result == "" {
+		return ""
+	}
+
+	return result
+}
+
+// isAllowedPhoneRune reports whether r is in the allowed phone character set [0-9+\-() .].
+func isAllowedPhoneRune(r rune) bool {
+	switch {
+	case r >= '0' && r <= '9':
+		return true
+	case r == '+', r == '-', r == '(', r == ')', r == ' ', r == '.':
+		return true
+	default:
+		return false
+	}
+}
+
+// SanitizeIdentifier applies SanitizeString first, then returns "" if the result equals
+// a reserved user identifier (case-sensitive, exact match).
+func SanitizeIdentifier(s string, maxLen int) string {
+	sanitized := SanitizeString(s, maxLen)
+
+	if IsReservedUserIdentifier(UserIdentifier(sanitized)) {
+		return ""
+	}
+
+	return sanitized
+}
+
+func IsReservedUserIdentifier(identifier UserIdentifier) bool {
+	_, reserved := reservedUserIdentifiers[string(identifier)]
+	return reserved
+}
diff --git a/internal/domain/sanitize_test.go b/internal/domain/sanitize_test.go
new file mode 100644
index 0000000..d3e5b6a
--- /dev/null
+++ b/internal/domain/sanitize_test.go
@@ -0,0 +1,503 @@
+package domain
+
+import (
+	"net/mail"
+	"strings"
+	"testing"
+	"unicode"
+	"unicode/utf8"
+
+	"pgregory.net/rapid"
+)
+
+func TestSanitizeString(t *testing.T) {
+	tests := []struct {
+		name   string
+		input  string
+		maxLen int
+		want   string
+	}{
+		{
+			name:   "null byte removed",
+			input:  "\x00",
+			maxLen: 64,
+			want:   "",
+		},
+		{
+			name:   "CR removed",
+			input:  "\r",
+			maxLen: 64,
+			want:   "",
+		},
+		{
+			name:   "LF removed",
+			input:  "\n",
+			maxLen: 64,
+			want:   "",
+		},
+		{
+			name:   "tab removed",
+			input:  "\t",
+			maxLen: 64,
+			want:   "",
+		},
+		{
+			name:   "leading and trailing whitespace trimmed",
+			input:  "  hello  ",
+			maxLen: 64,
+			want:   "hello",
+		},
+		{
+			name:   "multi-byte UTF-8 truncation at rune boundary",
+			input:  "héllo",
+			maxLen: 3,
+			want:   "hél", // 3 runes, not 3 bytes
+		},
+		{
+			name:   "empty input",
+			input:  "",
+			maxLen: 64,
+			want:   "",
+		},
+		{
+			name:   "maxLen zero returns empty",
+			input:  "hello",
+			maxLen: 0,
+			want:   "",
+		},
+		{
+			name:   "string longer than maxLen truncated",
+			input:  "abcdefgh",
+			maxLen: 4,
+			want:   "abcd",
+		},
+		{
+			name:   "mixed control chars and normal chars",
+			input:  "hel\x00lo\r\nworld",
+			maxLen: 64,
+			want:   "helloworld",
+		},
+		{
+			name:   "only whitespace returns empty",
+			input:  "   ",
+			maxLen: 64,
+			want:   "",
+		},
+		{
+			name:   "string exactly at maxLen not truncated",
+			input:  "abc",
+			maxLen: 3,
+			want:   "abc",
+		},
+		{
+			name:   "negative maxLen returns empty",
+			input:  "hello",
+			maxLen: -1,
+			want:   "",
+		},
+		{
+			name:   "DEL control removed",
+			input:  "hel\x7flo",
+			maxLen: 64,
+			want:   "hello",
+		},
+		{
+			name:   "zero-width format character removed",
+			input:  "ali\u200bce",
+			maxLen: 64,
+			want:   "alice",
+		},
+		{
+			name:   "invalid UTF-8 byte removed",
+			input:  "a\xffb",
+			maxLen: 64,
+			want:   "ab",
+		},
+		{
+			name:   "unicode normalized to NFC",
+			input:  "e\u0301",
+			maxLen: 64,
+			want:   "é",
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			got := SanitizeString(tc.input, tc.maxLen)
+			if got != tc.want {
+				t.Errorf("SanitizeString(%q, %d) = %q; want %q", tc.input, tc.maxLen, got, tc.want)
+			}
+		})
+	}
+}
+
+func TestSanitizeEmail(t *testing.T) {
+	tests := []struct {
+		name   string
+		input  string
+		maxLen int
+		want   string
+	}{
+		{
+			name:   "valid email passes through unchanged",
+			input:  "user@example.com",
+			maxLen: 254,
+			want:   "user@example.com",
+		},
+		{
+			name:   "CR in email returns empty",
+			input:  "user\r@example.com",
+			maxLen: 254,
+			want:   "",
+		},
+		{
+			name:   "LF in email returns empty",
+			input:  "user\n@example.com",
+			maxLen: 254,
+			want:   "",
+		},
+		{
+			name:   "missing @ returns empty",
+			input:  "userexample.com",
+			maxLen: 254,
+			want:   "",
+		},
+		{
+			name:   "whitespace-only returns empty",
+			input:  "   ",
+			maxLen: 254,
+			want:   "",
+		},
+		{
+			name:   "email with leading/trailing whitespace trimmed and returned",
+			input:  "  user@example.com  ",
+			maxLen: 254,
+			want:   "user@example.com",
+		},
+		{
+			name:   "empty input returns empty",
+			input:  "",
+			maxLen: 254,
+			want:   "",
+		},
+		{
+			name:   "display-name address rejected",
+			input:  "User <user@example.com>",
+			maxLen: 254,
+			want:   "",
+		},
+		{
+			name:   "multiple at signs rejected",
+			input:  "user@@example.com",
+			maxLen: 254,
+			want:   "",
+		},
+		{
+			name:   "invalid address rejected",
+			input:  "user@",
+			maxLen: 254,
+			want:   "",
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			got := SanitizeEmail(tc.input, tc.maxLen)
+			if got != tc.want {
+				t.Errorf("SanitizeEmail(%q, %d) = %q; want %q", tc.input, tc.maxLen, got, tc.want)
+			}
+		})
+	}
+}
+
+func TestSanitizePhone(t *testing.T) {
+	tests := []struct {
+		name   string
+		input  string
+		maxLen int
+		want   string
+	}{
+		{
+			name:   "valid phone passes through unchanged",
+			input:  "+1 (555) 123-4567",
+			maxLen: 50,
+			want:   "+1 (555) 123-4567",
+		},
+		{
+			name:   "non-allowed chars stripped",
+			input:  "abc+1def",
+			maxLen: 50,
+			want:   "+1",
+		},
+		{
+			name:   "all-stripped input returns empty",
+			input:  "abc",
+			maxLen: 50,
+			want:   "",
+		},
+		{
+			name:   "mixed allowed and non-allowed chars",
+			input:  "+49 (0) 123-456.789",
+			maxLen: 50,
+			want:   "+49 (0) 123-456.789",
+		},
+		{
+			name:   "empty input returns empty",
+			input:  "",
+			maxLen: 50,
+			want:   "",
+		},
+		{
+			name:   "only digits passes through",
+			input:  "1234567890",
+			maxLen: 50,
+			want:   "1234567890",
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			got := SanitizePhone(tc.input, tc.maxLen)
+			if got != tc.want {
+				t.Errorf("SanitizePhone(%q, %d) = %q; want %q", tc.input, tc.maxLen, got, tc.want)
+			}
+		})
+	}
+}
+
+func TestSanitizeIdentifier(t *testing.T) {
+	tests := []struct {
+		name   string
+		input  string
+		maxLen int
+		want   string
+	}{
+		{
+			name:   "reserved value all returns empty",
+			input:  "all",
+			maxLen: 256,
+			want:   "",
+		},
+		{
+			name:   "all with surrounding whitespace returns empty",
+			input:  " all ",
+			maxLen: 256,
+			want:   "",
+		},
+		{
+			name:   "reserved value new returns empty",
+			input:  "new",
+			maxLen: 256,
+			want:   "",
+		},
+		{
+			name:   "reserved value id returns empty",
+			input:  "id",
+			maxLen: 256,
+			want:   "",
+		},
+		{
+			name:   "system admin identifier returns empty",
+			input:  string(CtxSystemAdminId),
+			maxLen: 256,
+			want:   "",
+		},
+		{
+			name:   "unknown user identifier returns empty",
+			input:  string(CtxUnknownUserId),
+			maxLen: 256,
+			want:   "",
+		},
+		{
+			name:   "LDAP syncer identifier returns empty",
+			input:  string(CtxSystemLdapSyncer),
+			maxLen: 256,
+			want:   "",
+		},
+		{
+			name:   "ALL uppercase passes through (case-sensitive)",
+			input:  "ALL",
+			maxLen: 256,
+			want:   "ALL",
+		},
+		{
+			name:   "valid email identifier passes through",
+			input:  "alice@example.com",
+			maxLen: 256,
+			want:   "alice@example.com",
+		},
+		{
+			name:   "normal identifier passes through",
+			input:  "alice",
+			maxLen: 256,
+			want:   "alice",
+		},
+		{
+			name:   "empty input returns empty",
+			input:  "",
+			maxLen: 256,
+			want:   "",
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			got := SanitizeIdentifier(tc.input, tc.maxLen)
+			if got != tc.want {
+				t.Errorf("SanitizeIdentifier(%q, %d) = %q; want %q", tc.input, tc.maxLen, got, tc.want)
+			}
+		})
+	}
+}
+
+func TestSanitizeXSSPayload(t *testing.T) {
+	// XSS payload: null byte removed, angle brackets preserved
+	input := "<script>\x00alert(1)</script>"
+	want := "<script>alert(1)</script>"
+	got := SanitizeString(input, 256)
+	if got != want {
+		t.Errorf("SanitizeString(%q, 256) = %q; want %q", input, got, want)
+	}
+}
+
+// ---------------------------------------------------------------------------
+// Property 1: SanitizeString output invariants
+// ---------------------------------------------------------------------------
+
+// Feature: external-identity-sanitization, Property 1: SanitizeString output is free of control characters and bounded in length
+func TestPropertySanitizeStringOutputInvariants(t *testing.T) {
+	rapid.Check(t, func(t *rapid.T) {
+		s := rapid.String().Draw(t, "s")
+		maxLen := rapid.IntRange(0, 512).Draw(t, "maxLen")
+		result := SanitizeString(s, maxLen)
+
+		// No control or format runes in result
+		for _, r := range result {
+			if unicode.IsControl(r) || unicode.Is(unicode.Cf, r) {
+				t.Fatalf("result %q contains unsafe character %U", result, r)
+			}
+		}
+
+		if !utf8.ValidString(result) {
+			t.Fatalf("result %q is not valid UTF-8", result)
+		}
+
+		// No leading or trailing whitespace
+		if result != strings.TrimSpace(result) {
+			t.Fatalf("result %q has leading or trailing whitespace", result)
+		}
+
+		// Rune count <= maxLen
+		runeCount := utf8.RuneCountInString(result)
+		if runeCount > maxLen {
+			t.Fatalf("result %q has %d runes, exceeds maxLen %d", result, runeCount, maxLen)
+		}
+	})
+}
+
+// ---------------------------------------------------------------------------
+// Property 2: SanitizeString is idempotent
+// ---------------------------------------------------------------------------
+
+// Feature: external-identity-sanitization, Property 2: SanitizeString is idempotent
+func TestPropertySanitizeStringIdempotent(t *testing.T) {
+	rapid.Check(t, func(t *rapid.T) {
+		s := rapid.String().Draw(t, "s")
+		maxLen := rapid.IntRange(1, 512).Draw(t, "maxLen")
+
+		once := SanitizeString(s, maxLen)
+		twice := SanitizeString(once, maxLen)
+
+		if once != twice {
+			t.Fatalf("SanitizeString is not idempotent: once=%q, twice=%q (input=%q, maxLen=%d)",
+				once, twice, s, maxLen)
+		}
+	})
+}
+
+// ---------------------------------------------------------------------------
+// Property 3: SanitizeEmail rejection rules
+// ---------------------------------------------------------------------------
+
+// Feature: external-identity-sanitization, Property 3: SanitizeEmail rejects strings without "@" or containing CR/LF
+func TestPropertySanitizeEmailRejectionRules(t *testing.T) {
+	rapid.Check(t, func(t *rapid.T) {
+		s := rapid.String().Draw(t, "s")
+		maxLen := rapid.IntRange(1, 512).Draw(t, "maxLen")
+		result := SanitizeEmail(s, maxLen)
+
+		sanitized := SanitizeString(s, maxLen)
+		addr, parseErr := mail.ParseAddress(sanitized)
+		reject := strings.ContainsAny(s, "\r\n") ||
+			sanitized == "" ||
+			strings.Count(sanitized, "@") != 1 ||
+			parseErr != nil ||
+			addr.Name != "" ||
+			addr.Address != sanitized
+		if reject {
+			if result != "" {
+				t.Fatalf("SanitizeEmail(%q, %d) = %q; expected empty string (contains CR/LF or no @)",
+					s, maxLen, result)
+			}
+		}
+	})
+}
+
+// ---------------------------------------------------------------------------
+// Property 4: SanitizePhone allowed character set
+// ---------------------------------------------------------------------------
+
+// isAllowedPhoneCharTest mirrors the internal isAllowedPhoneRune logic for test assertions.
+func isAllowedPhoneCharTest(r rune) bool {
+	switch {
+	case r >= '0' && r <= '9':
+		return true
+	case r == '+', r == '-', r == '(', r == ')', r == ' ', r == '.':
+		return true
+	default:
+		return false
+	}
+}
+
+// Feature: external-identity-sanitization, Property 4: SanitizePhone output contains only allowed characters
+func TestPropertySanitizePhoneAllowedChars(t *testing.T) {
+	rapid.Check(t, func(t *rapid.T) {
+		s := rapid.String().Draw(t, "s")
+		maxLen := rapid.IntRange(1, 512).Draw(t, "maxLen")
+		result := SanitizePhone(s, maxLen)
+
+		for _, r := range result {
+			if !isAllowedPhoneCharTest(r) {
+				t.Fatalf("SanitizePhone(%q, %d) = %q; contains disallowed rune %U (%c)",
+					s, maxLen, result, r, r)
+			}
+		}
+	})
+}
+
+// ---------------------------------------------------------------------------
+// Property 5: SanitizeIdentifier rejects reserved identifiers
+// ---------------------------------------------------------------------------
+
+// Feature: external-identity-sanitization, Property 5: SanitizeIdentifier rejects reserved values
+func TestPropertySanitizeIdentifierRejectsReservedValues(t *testing.T) {
+	rapid.Check(t, func(t *rapid.T) {
+		s := rapid.String().Draw(t, "s")
+		maxLen := rapid.IntRange(1, 512).Draw(t, "maxLen")
+		result := SanitizeIdentifier(s, maxLen)
+		sanitized := SanitizeString(s, maxLen)
+
+		_, reserved := reservedUserIdentifiers[sanitized]
+		if reserved {
+			if result != "" {
+				t.Fatalf("SanitizeIdentifier(%q, %d) = %q; expected empty string when sanitized is reserved",
+					s, maxLen, result)
+			}
+		} else {
+			if result != sanitized {
+				t.Fatalf("SanitizeIdentifier(%q, %d) = %q; expected %q (== SanitizeString result)",
+					s, maxLen, result, sanitized)
+			}
+		}
+	})
+}
diff --git a/internal/domain/user.go b/internal/domain/user.go
index 39fc982..4eed5f2 100644
--- a/internal/domain/user.go
+++ b/internal/domain/user.go
@@ -282,6 +282,32 @@ func (u *User) CreateDefaultPeers() bool {
 	return true
 }
 
+// SanitizeExternalData sanitizes user profile fields received from an external identity provider.
+// Returns ErrInvalidData if the identifier becomes empty after sanitization.
+func (u *User) SanitizeExternalData(providerType, providerName string) error {
+	identifier := string(u.Identifier)
+	LogSanitizeChange(providerType, providerName, "identifier", identifier,
+		func() string { return SanitizeIdentifier(identifier, 256) }, &identifier)
+	u.Identifier = UserIdentifier(identifier)
+
+	LogSanitizeChange(providerType, providerName, "email", u.Email,
+		func() string { return SanitizeEmail(u.Email, 254) }, &u.Email)
+	LogSanitizeChange(providerType, providerName, "firstname", u.Firstname,
+		func() string { return SanitizeString(u.Firstname, 128) }, &u.Firstname)
+	LogSanitizeChange(providerType, providerName, "lastname", u.Lastname,
+		func() string { return SanitizeString(u.Lastname, 128) }, &u.Lastname)
+	LogSanitizeChange(providerType, providerName, "phone", u.Phone,
+		func() string { return SanitizePhone(u.Phone, 50) }, &u.Phone)
+	LogSanitizeChange(providerType, providerName, "department", u.Department,
+		func() string { return SanitizeString(u.Department, 128) }, &u.Department)
+
+	if u.Identifier == "" {
+		return fmt.Errorf("empty user identifier: %w", ErrInvalidData)
+	}
+
+	return nil
+}
+
 // region webauthn
 
 func (u *User) WebAuthnID() []byte {
diff --git a/internal/domain/user_sanitize_test.go b/internal/domain/user_sanitize_test.go
new file mode 100644
index 0000000..0d5f8a5
--- /dev/null
+++ b/internal/domain/user_sanitize_test.go
@@ -0,0 +1,64 @@
+package domain
+
+import (
+	"errors"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"github.com/h44z/wg-portal/internal/testutil"
+)
+
+func TestUser_SanitizeExternalData_NullByteInFirstname(t *testing.T) {
+	u := &User{
+		Identifier: "alice",
+		Email:      "alice@example.com",
+		Firstname:  "Ali\x00ce",
+		Lastname:   "Smith",
+	}
+
+	restore := testutil.CaptureWarnLogs(t)
+	err := u.SanitizeExternalData("ldap", "test-provider")
+	records := restore()
+
+	require.NoError(t, err)
+	assert.Equal(t, "Alice", u.Firstname)
+	assert.Equal(t, 1, testutil.CountWarnEntries(records))
+
+	_, found := testutil.FindWarnWithField(records, "firstname")
+	assert.True(t, found)
+}
+
+func TestUser_SanitizeExternalData_IdentifierAll(t *testing.T) {
+	u := &User{
+		Identifier: "all",
+		Email:      "all@example.com",
+		Firstname:  "Alice",
+		Lastname:   "Smith",
+	}
+
+	err := u.SanitizeExternalData("ldap", "test-provider")
+
+	require.Error(t, err)
+	assert.True(t, errors.Is(err, ErrInvalidData))
+}
+
+func TestUser_SanitizeExternalData_AllFieldsClean(t *testing.T) {
+	u := &User{
+		Identifier: "alice",
+		Email:      "alice@example.com",
+		Firstname:  "Alice",
+		Lastname:   "Smith",
+		Phone:      "+1 555-1234",
+		Department: "Engineering",
+	}
+
+	restore := testutil.CaptureWarnLogs(t)
+	err := u.SanitizeExternalData("ldap", "test-provider")
+	records := restore()
+
+	require.NoError(t, err)
+	assert.Equal(t, UserIdentifier("alice"), u.Identifier)
+	assert.Equal(t, 0, testutil.CountWarnEntries(records))
+}
diff --git a/internal/sanitize/log.go b/internal/sanitize/log.go
new file mode 100644
index 0000000..51e4d9b
--- /dev/null
+++ b/internal/sanitize/log.go
@@ -0,0 +1,32 @@
+package sanitize
+
+import (
+	"log/slog"
+
+	"github.com/h44z/wg-portal/internal/domain"
+)
+
+// LogChange applies sanitizeFn to raw, logs when the value changes, and writes
+// the sanitized value to dest. Raw and sanitized values are intentionally omitted.
+func LogChange(
+	providerType string,
+	providerName string,
+	field string,
+	raw string,
+	sanitizeFn func() string,
+	dest *string,
+) {
+	sanitized := sanitizeFn()
+	if sanitized != raw {
+		message := "sanitization modified field value from external provider"
+		if sanitized == "" {
+			message = "sanitization cleared field value from external provider"
+		}
+		slog.Warn(message,
+			"provider_type", domain.SanitizeString(providerType, 64),
+			"provider", domain.SanitizeString(providerName, 128),
+			"field", domain.SanitizeString(field, 64),
+		)
+	}
+	*dest = sanitized
+}
diff --git a/internal/testutil/testutil.go b/internal/testutil/testutil.go
new file mode 100644
index 0000000..c34c37a
--- /dev/null
+++ b/internal/testutil/testutil.go
@@ -0,0 +1,50 @@
+package testutil
+
+import (
+	"bytes"
+	"encoding/json"
+	"log/slog"
+	"testing"
+)
+
+func CaptureWarnLogs(t *testing.T) (restore func() []map[string]any) {
+	t.Helper()
+	original := slog.Default()
+	var buf bytes.Buffer
+	handler := slog.NewJSONHandler(&buf, &slog.HandlerOptions{Level: slog.LevelWarn})
+	slog.SetDefault(slog.New(handler))
+
+	return func() []map[string]any {
+		slog.SetDefault(original)
+		var records []map[string]any
+		decoder := json.NewDecoder(&buf)
+		for decoder.More() {
+			var rec map[string]any
+			if err := decoder.Decode(&rec); err == nil {
+				records = append(records, rec)
+			}
+		}
+		return records
+	}
+}
+
+func CountWarnEntries(records []map[string]any) int {
+	count := 0
+	for _, r := range records {
+		if lvl, ok := r["level"].(string); ok && lvl == "WARN" {
+			count++
+		}
+	}
+	return count
+}
+
+func FindWarnWithField(records []map[string]any, fieldName string) (map[string]any, bool) {
+	for _, r := range records {
+		if lvl, ok := r["level"].(string); ok && lvl == "WARN" {
+			if f, ok := r["field"].(string); ok && f == fieldName {
+				return r, true
+			}
+		}
+	}
+	return nil, false
+}