Impact

An authenticated Backstage user could craft a request URL that causes the proxy-backend to forward the request to a path outside the configured base path on the target server. This is limited to target servers already configured as proxy endpoints and requires Backstage authentication by default.

Patches

Patched in @backstage/plugin-proxy-backend version 0.6.17

Workarounds

- Deploy a reverse proxy or WAF in front of Backstage that normalizes request paths before they reach the backend.

Impact

An operator can configure overlapping proxy paths with different credential requirements. When a parent path permits unauthenticated access and a nested path requires credentials, the parent exemption can also cover requests handled by the nested proxy. An unauthenticated caller may therefore reach the nested upstream through Backstage, including with static upstream credentials configured for that proxy.

Exploitation requires this specific overlapping route configuration. The resulting confidentiality and integrity impact depends on the capabilities exposed by the nested upstream.

Patches

Upgrade @backstage/plugin-proxy-backend to version 0.6.18 or later. The fixed package is available in Backstage v1.55.0.

Workarounds

- Avoid nesting a credential-protected proxy path below a path configured with dangerously-allow-unauthenticated. - Apply the same credential requirement to overlapping proxy paths. - Restrict direct network access to the Backstage backend until the patched package is deployed.