[management] Handle single-string JWT group claim from IdPs (#6014)

shared/auth/jwt/extractor.go

@@ -146,7 +146,11 @@ func (c *ClaimsExtractor) ToGroups(token *jwt.Token, claimName string) []string
 	userJWTGroups := make([]string, 0)
 
 	if claim, ok := claims[claimName]; ok {
-		if claimGroups, ok := claim.([]interface{}); ok {
+		switch claimGroups := claim.(type) {
+		case string:
+			// Some IdPs emit a single group claim as a string instead of an array.
+			userJWTGroups = append(userJWTGroups, claimGroups)
+		case []any:
 			for _, g := range claimGroups {
 				if group, ok := g.(string); ok {
 					userJWTGroups = append(userJWTGroups, group)
@@ -154,9 +158,11 @@ func (c *ClaimsExtractor) ToGroups(token *jwt.Token, claimName string) []string
 					log.Debugf("JWT claim %q contains a non-string group (type: %T): %v", claimName, g, g)
 				}
 			}
+		default:
+			log.Debugf("JWT claim %q is not a string or string array (type: %T): %v", claimName, claim, claim)
 		}
 	} else {
-		log.Debugf("JWT claim %q is not a string array", claimName)
+		log.Debugf("JWT claim %q is missing", claimName)
 	}
 
 	return userJWTGroups

shared/auth/jwt/extractor_test.go

@@ -249,6 +249,15 @@ func TestClaimsExtractor_ToGroups(t *testing.T) {
 			groupClaimName: "groups",
 			expectedGroups: []string{},
 		},
+		{
+			name: "extracts single group string from claim",
+			claims: jwt.MapClaims{
+				"sub":    "user-123",
+				"groups": "admin",
+			},
+			groupClaimName: "groups",
+			expectedGroups: []string{"admin"},
+		},
 		{
 			name: "handles custom claim name",
 			claims: jwt.MapClaims{