The vulnerability arises from a caching flaw in the toolbox-core package of the mcp-toolbox-sdk-python SDK. The module-level token cache does not differentiate cached tokens by audience, causing the same valid, unexpired Google ID token minted for one service (Service A) to be reused and sent to another service (Service B). This allows an attacker who operates or compromises Service B to capture the token and replay it to impersonate the victim application against Service A. The issue is classified as CWE-524 (Use of a Shared Cache with Insufficient Key Granularity).

An attacker who controls or monitors traffic to a secondary service can capture a token intended for a sensitive service and replay it to impersonate the victim application against that sensitive service. This can lead to unauthorized access and potential compromise of sensitive services relying on the SDK for authentication.

Patch status is not yet confirmed — check the vendor advisory for current remediation guidance. No official fix or patch links are provided in the available data. Until a fix is available, avoid using the SDK to authenticate to multiple audiences within the same process or implement custom token caching keyed by audience to prevent token reuse across audiences.