n8n: Per-Resource OAuth Consent Bypass via Unbound Refresh Token Resource Substitution

Impact

The OAuth token endpoint bound an authorization code's first access token to the consented resource, but not its refresh token. Refreshing only checked that the requested resource was registered, not that it matched the original grant. An OAuth client approved for one workflow could refresh with a different workflow's URL and get a valid, unapproved token for it. The patch binds refresh tokens to the granted resource and rejects mismatches.

Exploitation requires the attacker to register an OAuth client, convince an authenticated user to approve that client for one known protected resource, and know the URL of a second protected resource that the consenting user is permitted to execute.

Patches

The issue has been fixed in n8n versions 2.38.1 and 2.37.7. Users should upgrade to this version or later to remediate the vulnerability.

Workarounds

If upgrading is not immediately possible, administrators should consider the following temporary mitigations:

• Restrict n8n instance access to fully trusted users only.
• Deactivate MCP Trigger, form, and webhook workflows that are protected by the n8n OAuth server if they are not required.
• Audit connected OAuth clients and revoke any that are not recognized or no longer needed.
• Require re-authorization for all existing OAuth clients after upgrading, as previously issued refresh tokens did not store the original resource binding.

These workarounds do not fully remediate the risk and should only be used as short-term mitigation measures.