ZITADEL: Unauthorized Token Privilege Escalation in OAuth2 Token Exchange

### Summary A vulnerability in ZITADEL’s OAuth2 Token Exchange endpoint allows an authenticated user or client to exchange a low-privilege access token for a token with elevated permissions at a completely different application. This bypasses the intended authorization and separation policies configured within ZITADEL. ### Impact ZITADEL enables administrators to restrict token issuance based on client permissions, project roles, and specific scopes. Due to a missing verification step during the Token Exchange flow (`urn:ietf:params:oauth:grant-type:token-exchange`), ZITADEL fails to validate whether the incoming access token belongs to or is authorized for the client initiating the exchange. Additionally, ZITADEL does not enforce that the newly requested scopes must stay within the boundaries of the original token's scopes. An attacker with valid access to a low-privilege application can exploit this behavior by sending a token exchange request to a highly-privileged target application. This allows them to obtain administrative project roles, access sensitive profile data, or gain unauthorized access to other applications across the system. If public clients (which do not require client secrets) are used, this risk increases significantly as it requires no client authentication. ### Affected Versions Systems running one of the following versions are affected: * **4.x:** `4.0.0` through `4.15.2` (including RC versions) * **3.x:** `3.0.0` through `3.4.12` (including RC versions) ### Patches The vulnerability has been addressed in the latest releases. The patch resolves the issue by enforcing strict audience ownership verification and ensuring that requested scopes during a token exchange are restricted to a subset of the original token’s scopes. * **4.x**: Upgrade to $\ge$ [4.15.3](https://github.com/zitadel/zitadel/releases/tag/v4.15.3) * **3.x**: Upgrade to $\ge$ [4.15.3](https://github.com/zitadel/zitadel/releases/tag/v4.15.3) ### Workarounds The recommended solution is to upgrade to a patched version immediately. If an immediate upgrade is not possible, implement one of the following mitigations: - Disable via Feature Flag: Ensure the Token Exchange feature flag is explicitly turned off in your instance settings or environment variables (`ZITADEL_DEFAULTINSTANCE_FEATURES_TOKENEXCHANGE=false`). If the feature flag is disabled, the endpoint is unreachable and your system is not vulnerable. This is only possible in versions prior to v4.11.0, where the feature was not yet GA. - Remove the Grant Type: Ensure that the Token Exchange grant type is completely disabled or removed from all configured applications—especially high-privilege or public clients. ### Questions If you have any questions or comments about this advisory, please email us at [[email protected]](mailto:[email protected]). ### Credits Thanks to [thesecguy45](https://github.com/thesecguy45) and [Shubham Raj / Causal Security](https://github.com/cipher-creator) for reporting this vulnerability.

Quoted source text, attributed separately from HOL analysis.