Login Failed Due to Token-Based Validation Failure: Root Cause & Fix
When a SQL Server login fails despite valid credentials, the culprit can sometimes be token-based validation failure. A common root cause is a group-level DENY CONNECT permission.
Even if the individual user has been explicitly granted access, being part of a group with a DENY overrides any GRANT.
🔎 How to Detect It
Run this query to identify server-level principals (logins or groups) that have explicit DENY permissions, especially CONNECT:
SELECT
sp.[name],
sp.[type_desc],
perm.permission_name,
perm.state_desc
FROM
sys.server_principals sp
INNER JOIN
sys.server_permissions perm
ON sp.principal_id = perm.grantee_principal_id
WHERE
perm.state_desc = 'DENY';
This will highlight groups or logins that may be blocking valid users from connecting.
✅ Solution
- Identify the conflicting group(s) with the DENY CONNECT SQL permission.
- Decide how to resolve:
- Option A: Remove the user from the denied group.
- Option B: Revoke the deny and explicitly grant access:
REVOKE CONNECT SQL FROM [DOMAIN\GroupName];
-- Optionally:
GRANT CONNECT SQL TO [DOMAIN\GroupName];
⚠️ Important Notes
- DENY always takes precedence over GRANT.
- A user may be denied indirectly through group membership, even if they are explicitly granted access.
- This applies to both SQL Server logins and Windows/AD integrated logins.
By checking for group-level denies and carefully managing permissions, you can resolve these confusing login failures quickly and securely.
