Backstage is an open framework for building developer portals. Prior to 0.29.2, the experimental dynamic client registration and client ID metadata document features in the @backstage/plugin-auth-backend use full-string matcher.isMatch glob matching for auth.experimentalDynamicClientRegistration.allowedRedirectUriPatterns and the auth.experimentalClientIdMetadataDocuments allowedClientIdPatterns and allowedRedirectUriPatterns options. A hostname wildcard can match across URL component boundaries, allowing an attacker-controlled redirect URI with a trusted hostname suffix in its path to pass the allowlist and receive an OAuth authorization code after a victim completes the flow. Patterns without an explicit protocol can match unintended protocols, and redirect URIs containing embedded credentials are accepted after user information is stripped for matching. The features are experimental and disabled by default; only deployments that enable them and configure custom wildcard-hostname or protocol-less patterns are affected. This issue is first fixed in prerelease version 0.29.2.

Affected: npm: @backstage/plugin-auth-backend

### Impact The allowlist matching used by the experimental dynamic client registration and client ID metadata document (CIMD) features in `@backstage/plugin-auth-backend` matched glob patterns against the full URL string. A * wildcard could therefore match across URL component boundaries: a pattern such as `https://*.example.com/callback`, intended to allow subdomains of a trusted host, would also match an attacker-controlled URL such as `https://attacker.example/x.example.com/callback`. This applies to `auth.experimentalDynamicClientRegistration.allowedRedirectUriPatterns` as well as the `allowedClientIdPatterns` and `allowedRedirectUriPatterns` options of `auth.experimentalClientIdMetadataDocuments`.

An attacker could use this to register an OAuth client whose redirect URI points to a host they control while still passing the allowlist, causing authorization codes to be delivered to the attacker when a victim completes an authorization flow. In addition, allowlist patterns without an explicit protocol could match URLs with any protocol, and redirect URIs containing embedded credentials (user:pass@host) were accepted after the credentials were stripped for matching.

The practical impact is limited. Both features are experimental and disabled by default, and the default allowlist patterns only reference fixed or loopback hosts and are not affected. Deployments are only impacted if they enable one of these features and configure custom allowlist patterns that contain a wild…

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