Cryptobench

wycheproof-jwk-duplicate-kid

JWK: Duplicate kid

The keyset contains multiple keys with the same kid. The test expects that such keysets are rejected.

REJECT

Your service should reject this one.

Stated as the outcome to assert against, so a suite compares with this rather than hardcoding a result and getting it backwards.

The keyset contains multiple keys with the same kid. The test expects that such keysets are rejected. Demonstrated by 1 test vector in Project Wycheproof.

Provenance

Kind
vulnerabilityA published weakness exists for this. Failing it is a known exploit path.
Source
wycheproof
Clause
RFC 7517
CVE
none
Weakness
none
Produce this on the benchTake a token apart

Test keys only. Nothing here is a statement about any particular service: the expected outcome above is what an application ought to do, not evidence that yours does.

Reference

URL or embedded key?

Your app needs the public key to check a token. It can fetch it from a URL at runtime, or carry it as a file. Both are normal. The trade is always the same one: who controls rotation.

Fetch from the URLEmbed the key
RotationAutomatic on next fetchNeeds a redeploy
NetworkDepends on the URLNone
TrustWhatever the endpoint servesPinned to one key
SuitsA signer that rotates keysOffline, air-gapped, or pinned

Use the URL when the signer rotates its keys, which is what every identity provider does. That is why it became the norm, and it is usually a one-line config change.

Embed the key when you cannot make a network call while checking a token, or when you want your app pinned to one key so a compromised endpoint could not introduce another. Export the PEM from the Keysets tab.

One catch worth knowing: Spring Boot's public-key-location only loads RSA keys. An ES256 or EdDSA key needs a custom decoder, or the URL.