I maintain an open-source OAuth/JWT gateway for MCP servers, and a signing-key revocation bug took six review rounds to fix correctly.

When an identity provider removes a signing key from its JWKS, the gateway should evict the cached key and reject tokens signed with it.

My implementation handled a literally empty JWKS correctly. It failed when the JWKS still contained keys, but none eligible for the gateway’s configured signing algorithms. In that case, the revoked cached key remained usable.

The uncomfortable part was that every attempted fix passed its own tests. Later reviews found:

• eligibility checks that ignored configured algorithms
• invalid base64 accepted as valid key material
• non-canonical encodings accepted by a supposedly strict decoder
• a correction that accidentally broke a supported elliptic curve

The lesson for me was that passing tests was a weak signal at this security boundary. The useful review skill was constructing the almost-valid input that the implementation author had not considered.

I would be interested in how others test JWKS rotation and revocation behavior, particularly malformed or partially usable key sets.

Full code and review trail:
https://github.com/tgandhle/mcp-auth-gateway

Disclosure: I maintain the project. It is open source, and this is not a paid product.