diff --git a/components/backend/handlers/middleware.go b/components/backend/handlers/middleware.go
index 625f9f031..b6f311353 100644
--- a/components/backend/handlers/middleware.go
+++ b/components/backend/handlers/middleware.go
@@ -48,6 +48,27 @@ func isValidKubernetesName(name string) bool {
 	return kubernetesNameRegex.MatchString(name)
 }
 
+// isValidRBACSubject validates RBAC subject identifiers (user/group names in RoleBindings)
+// RBAC subjects can contain colons (e.g., system:authenticated, system:serviceaccount:ns:name)
+// Unlike Kubernetes resource names, RBAC subjects follow a different format (RFC 1123 subdomain)
+// Returns false if:
+//   - name is empty (prevents empty string injection)
+//   - name exceeds 253 characters (max length for DNS subdomain)
+//   - name contains control characters (prevents injection attacks)
+func isValidRBACSubject(name string) bool {
+	// Max length for RBAC subjects is 253 chars (same as DNS subdomain)
+	if len(name) == 0 || len(name) > 253 {
+		return false
+	}
+	// Reject control characters and newlines for security
+	for _, r := range name {
+		if r < 32 || r == 127 {
+			return false
+		}
+	}
+	return true
+}
+
 // ContentListItem represents a content list item for file browsing
 type ContentListItem struct {
 	Name       string `json:"name"`
diff --git a/components/backend/handlers/permissions.go b/components/backend/handlers/permissions.go
index 244d64923..4ed757d54 100644
--- a/components/backend/handlers/permissions.go
+++ b/components/backend/handlers/permissions.go
@@ -157,9 +157,9 @@ func AddProjectPermission(c *gin.Context) {
 		return
 	}
 
-	// Validate subject name is a valid Kubernetes resource name
-	if !isValidKubernetesName(req.SubjectName) {
-		c.JSON(http.StatusBadRequest, gin.H{"error": "Invalid userName format. Must be a valid Kubernetes resource name."})
+	// Validate subject name is a valid RBAC subject identifier
+	if !isValidRBACSubject(req.SubjectName) {
+		c.JSON(http.StatusBadRequest, gin.H{"error": "Invalid subject name format"})
 		return
 	}
 
diff --git a/components/backend/handlers/permissions_test.go b/components/backend/handlers/permissions_test.go
index f45bd400a..dc5dbcaf7 100644
--- a/components/backend/handlers/permissions_test.go
+++ b/components/backend/handlers/permissions_test.go
@@ -738,17 +738,11 @@ var _ = Describe("Permissions Handler", Ordered, Label(test_constants.LabelUnit,
 	Context("Input Validation", func() {
 		It("Should reject userNames with invalid characters", func() {
 			invalidUserNames := []string{
-				"user@domain.com",       // @ not allowed in k8s resource names
-				"user/name",             // / not allowed
-				"user\\name",            // \ not allowed
-				"user:name",             // : not allowed
-				"user name",             // space not allowed
-				"user.name",             // . not allowed in k8s resource names
-				"User-Name",             // uppercase not allowed
-				"user-name-",            // can't end with dash
-				"-user-name",            // can't start with dash
-				"user..name",            // consecutive dots not allowed
-				strings.Repeat("a", 64), // too long (>63 chars)
+				"user\nname",            // newline not allowed (control character)
+				"user\tname",            // tab not allowed (control character)
+				"user\x00name",          // null byte not allowed (control character)
+				"user\x1fname",          // control character not allowed
+				strings.Repeat("a", 254), // too long (>253 chars)
 			}
 
 			for _, userName := range invalidUserNames {
@@ -770,7 +764,7 @@ var _ = Describe("Permissions Handler", Ordered, Label(test_constants.LabelUnit,
 
 				httpUtils.AssertHTTPStatus(http.StatusBadRequest)
 				httpUtils.AssertJSONContains(map[string]interface{}{
-					"error": "Invalid userName format. Must be a valid Kubernetes resource name.",
+					"error": "Invalid subject name format",
 				})
 
 				logger.Log("Correctly rejected invalid userName: %s", userName)
@@ -782,8 +776,14 @@ var _ = Describe("Permissions Handler", Ordered, Label(test_constants.LabelUnit,
 				"user-name",
 				"user123",
 				"123user",
-				"a",                     // single character
-				strings.Repeat("a", 63), // exactly 63 chars (max allowed)
+				"a",                      // single character
+				"system:authenticated",   // RBAC group for all authenticated users
+				"system:unauthenticated", // RBAC group for anonymous access
+				"system:serviceaccount:test-project:default", // service account reference
+				"user@domain.com",        // email addresses are valid RBAC subjects
+				"user.name",              // dots are valid in RBAC subjects
+				"User-Name",              // uppercase is valid in RBAC subjects
+				strings.Repeat("a", 253), // exactly 253 chars (max allowed for RBAC subjects)
 			}
 
 			for _, userName := range validUserNames {