# CVE-2026-73563: @backstage/plugin-auth-backend: Unauthenticated OAuth account takeover via `redirect_uri` allowlist bypass

> Attackers can bypass OAuth redirect URI allowlists in experimental Backstage auth features to steal authorization codes and take over accounts.

- **Severity:** Medium 4.7 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:N/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** A user has to click or open something
- **Fix available:** Yes
- **Published:** 13 Aug 2026
- **Updated:** 13 Aug 2026
- **Also known as:** GHSA-38hq-7x33-php4

## What is CVE-2026-73563?

CVE-2026-73563 is a medium-severity vulnerability in backstage, published on 13 Aug 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.

### Official description

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.

Weakness: CWE-601 (URL Redirection to Untrusted Site)

## Which versions are affected by CVE-2026-73563?

2 open source packages ship an affected version, including @backstage/plugin-auth-backend. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `@backstage/plugin-auth-backend` | npm | All versions | 0.29.2 |
| `@backstage/plugin-auth-backend` | npm | All versions | 0.29.2 |

Products: Linuxfoundation Backstage\/Plugin-Auth-Backend.

## How do I fix CVE-2026-73563?

Upgrade @backstage/plugin-auth-backend to 0.29.2 or later.

## Is CVE-2026-73563 being exploited?

There is no known exploitation of CVE-2026-73563 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-73563 and fix it if it is affected.

What it is: CVE-2026-73563 is a medium-severity vulnerability in backstage, published on 13 Aug 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Exploitation: There is no known exploitation of CVE-2026-73563 in the wild and no public exploit code.

Affected packages and fixed versions:
- @backstage/plugin-auth-backend (npm): affected all versions; fixed in 0.29.2
- @backstage/plugin-auth-backend (npm): affected all versions; fixed in 0.29.2

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-73563/ (Markdown: https://insights.sbomify.com/CVE-2026-73563/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-73563
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-73563
- OSV: https://osv.dev/vulnerability/CVE-2026-73563

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-73563/
Summaries are generated and may contain mistakes; the official description above is the record.