@backstage/plugin-auth-backend: Unauthenticated OAuth account takeover via `redirect_uri` allowlist bypass

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 wildcard in the hostname, or patterns without an explicit protocol.

Patches

Patched in @backstage/plugin-auth-backend version 0.29.2. Patterns are now matched against each URL component separately so that wildcards no longer match across the host and path boundary, patterns without an explicit protocol are rejected as invalid configuration, and redirect URIs with embedded credentials are always rejected.

Note that as part of this fix, a wildcard port no longer implicitly matches every path: a pattern such as http://localhost:* now only matches the root path. Use http://localhost:*/* to allow any port and any path.

Workarounds

Disable the experimental features by removing auth.experimentalDynamicClientRegistration and auth.experimentalClientIdMetadataDocuments from your app-config, which is the default configuration. Alternatively, restrict the configured allowlist patterns to fully specified URLs with an explicit protocol and no wildcard in the hostname, which are not affected by this vulnerability.