@backstage/plugin-auth-backend: SSRF in experimental CIMD metadata fetch

Impact

A Server-Side Request Forgery (SSRF) vulnerability exists in @backstage/plugin-auth-backend when auth.experimentalClientIdMetadataDocuments.enabled is set to true. The CIMD
metadata fetch validates the initial client_id hostname against private IP ranges but does not apply the same validation after HTTP redirects.

The practical impact is limited. The attacker cannot read the response body from the internal request, cannot control request headers or method, and the feature must be explicitly
enabled via an experimental flag that is off by default. Deployments that restrict allowedClientIdPatterns to specific trusted domains are not affected.

Patches

Patched in @backstage/plugin-auth-backend version 0.27.1. The fix disables HTTP redirect following when fetching CIMD metadata documents.

Workarounds

Disable the experimental CIMD feature by removing or setting auth.experimentalClientIdMetadataDocuments.enabled to false in your app-config. This is the default configuration.
Alternatively, restrict allowedClientIdPatterns to specific trusted domains rather than using the default wildcard pattern.

References

• IETF Client ID Metadata Document draft
• MCP Authorization Specification - Client ID Metadata Documents