GHSA-wqvh-63mv-9w92

@backstage/plugin-auth-backend: OAuth redirect URI allowlist bypass

### Impact The experimental OIDC provider in `@backstage/plugin-auth-backend` is vulnerable to a redirect URI allowlist bypass. Instances that have enabled experimental Dynamic Client Registration or Client ID Metadata Documents and configured `allowedRedirectUriPatterns` are affected. A specially crafted redirect URI can pass the allowlist validation while resolving to an attacker-controlled host. If a victim approves the resulting OAuth consent request, their authorization code is sent to the attacker, who can exchange it for a valid access token. This requires victim interaction and that one of the experimental features is explicitly enabled, which is not the default. ### Patches Upgrade to `@backstage/plugin-auth-backend` version 0.27.1 or later. ### Workarounds Disable experimental Dynamic Client Registration and Client ID Metadata Documents features if they are not required. ### References - [RFC 6749 Section 3.1.2 - Redirection Endpoint](https://datatracker.ietf.org/doc/html/rfc6749#section-3.1.2)