CVE-2026-27478: Unity Catalog has a JWT Issuer Validation Bypass tht Allows Complete User Impersonation

Context: A critical authentication bypass vulnerability exists in the Unity Catalog token exchange endpoint (/api/1.0/unity-control/auth/tokens). The endpoint extracts the issuer (iss) claim from incoming JWTs and uses it to dynamically fetch the JWKS endpoint for signature validation without validating that the issuer is a trusted identity provider.

Way to exploit:

An attacker can exploit this by:

1. Hosting their own OIDC-compliant server with a valid JWKS endpoint
2. Signing a JWT with their own private key, setting the iss claim to their server
3. Setting the sub/email claim to any known user in the Unity Catalog system
4. Exchanging this crafted token for a valid internal access token

This results in complete impersonation of any user in the system, granting access to all catalogs, schemas, tables, and other resources that user has permissions to.

Additionally, the implementation does not validate the audience (aud) claim, allowing tokens intended for other services to be used.

Example

Example implementation doing token exchange with a self hosted .well-known/openid-configuration and jwks endpoint.

This can be run with python3 main.py and TARGET_USER, UC_SERVER and PORT adjusted to the testing setup.