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

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

4.7 · CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:N/A:N

EPSS

• CVE-2026-73563: 0.17% estimated exploitation activity in the next 30 days; percentile 6.57 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

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…

Published by the source

24 Jul 2026, 16:20 UTC

Last modified by the source

13 Aug 2026, 17:55 UTC

Source last checked

26 Oct 2026, 13:01 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-38hq-7x33-php4

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-38hq-7x33-php4

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# Patch Next advisory brief: CVE-2026-73563

Advisory: @backstage/plugin-auth-backend: Unauthenticated OAuth account takeover via `redirect_uri` allowlist bypass
IDs: CVE-2026-73563, GHSA-38hq-7x33-php4
Record: https://patchnext.com/advisories/cve-2026-73563 (revision 2, source checked 26 Oct 2026, 13:01 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 4.7)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: @backstage/plugin-auth-backend 0.29.2. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm @backstage/plugin-auth-backend: affected <= 0.29.1; fixed in 0.29.2

## Suggested inspection (read-only)
- `npm ls @backstage/plugin-auth-backend --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on @backstage/plugin-auth-backend.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-38hq-7x33-php4 (OSV record GHSA-38hq-7x33-php4 lists affected ranges: npm @backstage/plugin-auth-backend >=0 <0.29.2 (fixed))
- Verified: https://github.com/advisories/GHSA-38hq-7x33-php4 (GitHub-reviewed advisory GHSA-38hq-7x33-php4 lists affected ranges: npm @backstage/plugin-auth-backend >=0 <=0.29.1)

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