ZITADEL is an open source identity management platform. Prior to 4.15.3, ZITADEL's OAuth2 Token Exchange endpoint for urn:ietf:params:oauth:grant-type:token-exchange does not verify that the subject token belongs to the requesting client or that requested scopes remain within the original token's scopes, allowing a low-privilege token to be exchanged for elevated permissions at another application. This issue is fixed in version 4.15.3.

Affected: Go: github.com/zitadel/zitadel

### 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…