A critical authentication bypass vulnerability exists in Open WebUI versions 0.8.0 through 0.11.0. When the ENABLE_OAUTH_TOKEN_EXCHANGE feature is enabled, the /oauth/{provider}/token/exchange endpoint accepts raw provider access tokens without verifying which OAuth client the token was originally issued to. This allows any attacker holding a valid access token from any client registered with the same OAuth provider to exchange it for a legitimate Open WebUI session. The flaw enables unauthorized access to user accounts, including tokens from applications the operator never authorized. The vulnerability stems from improper audience/client validation during the token exchange process. It has been patched in Open WebUI version 0.11.0 via two separate commits. Users are strongly advised to upgrade immediately or disable the OAuth token exchange feature as a mitigation.