diff --git a/dashcaddy-api/__tests__/backup-manager.test.js b/dashcaddy-api/__tests__/backup-manager.test.js
index 425ee5a..6f122b2 100644
--- a/dashcaddy-api/__tests__/backup-manager.test.js
+++ b/dashcaddy-api/__tests__/backup-manager.test.js
@@ -184,9 +184,16 @@ describe('BackupManager — backup/restore lifecycle', () => {
     it('rejects tampered data (auth tag mismatch)', async () => {
       const data = Buffer.from('test');
       const encrypted = await backupManager.encryptBackup(data, testKey);
-      // Corrupt the first character of the IV
-      const str = encrypted.toString();
-      const tampered = Buffer.from('X' + str.substring(1));
+      // Corrupt the authTag so the GCM integrity check is guaranteed to fail.
+      // The format is iv:authTag:ciphertext (all base64). We flip all bits of
+      // the first authTag byte — XOR with 0xFF always changes the value, so
+      // this can never be a no-op (unlike replacing a base64 char with a fixed
+      // char, which collides ~1/64 of the time when that char already matches).
+      const parts = encrypted.toString().split(':');
+      const authTagBuf = Buffer.from(parts[1], 'base64');
+      authTagBuf[0] ^= 0xFF;
+      parts[1] = authTagBuf.toString('base64');
+      const tampered = Buffer.from(parts.join(':'));
       await expect(backupManager.decryptBackup(tampered, testKey))
         .rejects.toThrow();
     });