LiteLLM JWT Authentication Bypass Vulnerability via OIDC Userinfo Cache Key Collision

A vulnerability in LiteLLM versions prior to 1.83.0 allows for authentication bypass when JWT authentication is enabled. The OIDC userinfo cache uses the first 20 characters of the JWT token as the cache key. Since JWT headers from the same signing algorithm produce identical initial 20 characters, an unauthenticated attacker can craft a token that matches a legitimate user's cached token. If the cache key collides, the attacker gains access to the user's identity and permissions. This issue affects deployments with JWT/OIDC authentication enabled, but most instances are not vulnerable as this configuration is not enabled by default.

Impact

Exploitation of this vulnerability allows an unauthenticated attacker to bypass JWT authentication, inheriting the identity and permissions of a legitimate user whose cached token is matched.

Remediation

Users can upgrade to LiteLLM version 1.83.0, where this vulnerability is fixed. Alternatively, OIDC userinfo caching can be disabled by setting the cache TTL to 0, or JWT authentication can be disabled entirely.