CVE-2026-73563 backstage CVE debrief

Who should care

Defenders responsible for Backstage deployments that have enabled the experimental features and configured custom wildcard-hostname or protocol-less patterns should assess exposure and prioritize verification and updates.

Why it matters

Defenders should care about CVE-2026-73563 because it affects Backstage deployments with enabled experimental features and custom wildcard-hostname or protocol-less patterns, potentially allowing attacker-controlled redirect URIs to pass the allowlist and receive OAuth authorization codes.

• Potential OAuth authorization code leakage to attacker-controlled redirect URIs
• Possible bypass of intended access controls due to wildcard hostname matching
• Risk of unintended protocol matching for redirect URIs without explicit protocol

Technical summary

The @backstage/plugin-auth-backend uses full-string matcher.isMatch glob matching for auth.experimentalDynamicClientRegistration.allowedRedirectUriPatterns and the auth.experimentalClientIdMetadataDocuments allowedClientIdPatterns and allowedRedirectUriPatterns options. This allows 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.

Defensive priority

Defenders should prioritize verifying and updating Backstage deployments that have enabled the experimental features and configured custom wildcard-hostname or protocol-less patterns.

Recommended defensive actions

• Verify Backstage deployments for enabled experimental features and custom wildcard-hostname or protocol-less patterns
• Update to prerelease version 0.29.2 or later
• Review and restrict allowedRedirectUriPatterns and allowedClientIdPatterns
• Confirm whether affected product deployments exist in managed environments and assign an owner for follow-up
• Review the supplied official advisory or CVE record to validate affected scope, severity, and vendor guidance
• Plan vendor-supported updates or mitigations through normal change control where exposure is confirmed
• Check relevant monitoring, detection, and logs for exposed assets that need extra review

Evidence notes

The CVE record and NVD entry provide details on the vulnerability in Backstage's experimental dynamic client registration and client ID metadata document features. The scope of affected deployments and the extent of potential impact require further verification. Defenders should verify whether custom wildcard-hostname or protocol-less patterns are configured and assess exposure. Verification tasks include reviewing deployment configurations, checking for enabled experimental features, and confirming the use of prerelease version 0.29.

Sources and references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.