[BUG] JWKS JWT authenticator fails with EC keys (ClassCastException: ECKey cannot be cast to RSAKey)

[zaeemarshad]

What is the bug?
The jwt authenticator with jwks_uri fails when the JWKS endpoint returns EC (ECDSA) keys. The
JwtVerifier.getInitializedSignatureVerifier() method assumes all non-HMAC keys are RSA, causing a ClassCastException.

[2026-03-30T04:27:36,950][DEBUG][o.o.s.a.BackendRegistry  ] [osearch-store-client-49c5] 'java.lang.ClassCastException: class com.nimbusds.jose.jwk.ECKey cannot be cast to class com.nimbusds.jose.jwk.RSAKey (com.nimbusds.jose.jwk.ECKey and com.nimbusds.jose.jwk.RSAKey are in unnamed module of loader java.net.URLClassLoader @25ae1f80)' extracting credentials from jwt http authenticator
java.lang.ClassCastException: class com.nimbusds.jose.jwk.ECKey cannot be cast to class com.nimbusds.jose.jwk.RSAKey (com.nimbusds.jose.jwk.ECKey and com.nimbusds.jose.jwk.RSAKey are in unnamed module of loader java.net.URLClassLoader @25ae1f80)
	at com.nimbusds.jose.jwk.JWK.toRSAKey(JWK.java:637)
	at org.opensearch.security.auth.http.jwt.keybyoidc.JwtVerifier.getInitializedSignatureVerifier(JwtVerifier.java:110)
	at org.opensearch.security.auth.http.jwt.keybyoidc.JwtVerifier.getVerifiedJwtToken(JwtVerifier.java:65)
	at org.opensearch.security.auth.http.jwt.AbstractHTTPJwtAuthenticator.extractCredentials0(AbstractHTTPJwtAuthenticator.java:117)
	at org.opensearch.security.auth.http.jwt.AbstractHTTPJwtAuthenticator.lambda$extractCredentials$0(AbstractHTTPJwtAuthenticator.java:100)
	at org.opensearch.secure_sm.AccessController.doPrivileged(AccessController.java:76)
	at org.opensearch.security.auth.http.jwt.AbstractHTTPJwtAuthenticator.extractCredentials(AbstractHTTPJwtAuthenticator.java:100)
	at org.opensearch.security.auth.http.jwt.keybyjwks.HTTPJwtKeyByJWKSAuthenticator.extractCredentials(HTTPJwtKeyByJWKSAuthenticator.java:138)
...

How can one reproduce the bug?

1. Configure a JWT auth domain with jwks_uri pointing to a JWKS endpoint that serves EC P-256 keys:

jwt_auth_domain:
    http_enabled: true
    http_authenticator:
      type: jwt
      config:
        jwks_uri: "https://example.com/.well-known/jwks.json"
        jwt_header: "custom-jwt-header"
        subject_key: "username"
        roles_key: "roles"
    authentication_backend:
      type: noop

2. Send a request with a JWT signed with ES256 (EC P-256)

What is the expected behavior?

JWT is verified using the EC public key from the JWKS endpoint.

What is your host/environment?

• OS: Debian 12.13
• Version [3.5.0]
• Plugins

Do you have any additional context?

It seems that in JwtVerifier.java lines 107-112:

  if (key.getClass() == OctetSequenceKey.class) {
      result = new DefaultJWSVerifierFactory().createJWSVerifier(jwt.getHeader(), key.toOctetSequenceKey().toSecretKey());
  } else {
      result = new DefaultJWSVerifierFactory().createJWSVerifier(jwt.getHeader(), key.toRSAKey().toRSAPublicKey());
  }

hte else branch unconditionally calls key.toRSAKey() for all non-HMAC keys, including EC keys when it should be calling the key.toECKey().toECPublicKey()) on ECkey detection.

[cwperks]

[Triage] Thank you for the bug report @zaeemarshad and suggested fix. Labeling this issue as Help Wanted.

[wheresNasha]

Hi @cwperks and @zaeemarshad,

I’d like to pick this up. I understand the issue around EC keys being treated as RSA in JwtVerifier.

Just to confirm, is the expected fix to explicitly handle ECKey (in addition to existing HMAC/RSA handling)?
Also, are we planning to include a unit test covering ES256/JWKS with EC keys as part of the change?

Thanks!

[cwperks]

Also, are we planning to include a unit test covering ES256/JWKS with EC keys as part of the change?

Let's aim to always include tests. That could be of the form of unit tests, integration tests or both.

[wheresNasha]

Thanks @cwperks WIll plan to add UT.
Can you please assign the task to me?

[danhul]

Hello @cwperks I tried using Google's IAP to authenticate to opensearch dashboards but I am unable to use jwt because Google supports only the algorithm (ES256) and opensearch is treating is as RSA. It would be nice if this task is prioretized.
BR!