Portainer's Kubernetes middleware continues after token validation failure, bypassing endpoint authorization

Summary

Portainer proxies requests to Kubernetes clusters through a middleware layer (kubeClientMiddleware) that validates the requesting user's token before forwarding traffic to the cluster. When security.RetrieveTokenData returned an error, the middleware wrote an HTTP 403 response but was missing a return statement — execution continued into the handler with a nil tokenData value.

The Kubernetes endpoints sit behind Portainer's outer AuthenticatedAccess bouncer, so an attacker requires a valid Portainer session. However, a user whose secondary token validation fails in kubeClientMiddleware — for example a user without permission to access a given Kubernetes endpoint — would have their request forwarded to the cluster anyway, bypassing the authorization check. The same defect was present in both the CE and EE codebases.

Severity

High CWE-863 — Incorrect Authorization

Privilege required is Low — any valid Portainer session is sufficient to reach the middleware. Once the authorization outcome is bypassed, the attacker can read and modify Kubernetes resources on the target endpoint that their role should not permit — confidentiality and integrity impact are both High. No availability impact is introduced directly.

Affected Versions

The missing return statement has been present since Kubernetes proxy support was introduced.

Branch	First vulnerable	Fixed in
2.33.x (LTS)	2.33.0	2.33.8

Portainer 2.39.0 and later are not affected — the fix was present from the initial 2.39.0 release. All releases prior to 2.33.0 are end-of-life and will not receive a fix; users on EOL versions should upgrade to a supported release.

Workarounds

There is no configuration change that prevents the bypass directly. Administrators who cannot immediately upgrade can reduce exposure by:

• Restricting Kubernetes endpoint access. Remove Portainer access to Kubernetes endpoints for users who do not require it. A user without endpoint access cannot reach .