Portainer Community Edition is a lightweight service delivery platform for containerized applications that can be used to manage Docker, Swarm, Kubernetes and ACI environments. From 2.33.0 to before 2.33., 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. This vulnerability is fixed in 2.33.8.

Affected: Go: github.com/portainer/portainer

## 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) |…