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HIGH severity

CVE-2021-43776 — @backstage/plugin-auth-ba…

HIGH

CVE-2021-43776 is a high-severity (CVSS 7.4) Cross-site Scripting (XSS) vulnerability in @backstage/plugin-auth-backend. A fix is available for @backstage/plugin-auth-backend — see the affected versions and patch details below.

Cross-Site Scripting vulnerability in @backstage/plugin-auth-backend

Published
Updated
Affected
1 pkg
Patched
1 / 1
Exploits
None indexed
Exploitation data as of Nov 7, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗
0.7%probability of exploitation in next 30 days
Lower Risk0.00%
Lower risk than most CVEs53th percentile — riskier than 53% of all scored CVEsHighest risk
0.00%0.39%0.79%1.18%0.3%0.7%May 26Sep 26Nov 26

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

1 pkg affected

How broadly this vulnerability is actually deployed: weekly install volume shows current usage, and reverse-dependency count shows how many other packages break if it stays unpatched.

10other npm packages depend on this — each one inherits the vulnerability until it's patched upstream
@backstage/plugin-auth-backendnpm
210Kdownloads / week

Description

Impact

This vulnerability allows a malicious actor to trick another user into visiting a vulnerable URL that executes an XSS attack. This attack can potentially allow the attacker to exfiltrate access tokens or other secrets from the user's browser. The default CSP does prevent this attack, but it is expected that some deployments have these policies disabled due to incompatibilities.

Patches

This is vulnerability is patched in version 0.4.9 of @backstage/plugin-auth-backend.

For more information

If you have any questions or comments about this advisory:

Affected Packages

1 total 1 fixed
EcosystemPackageVulnerable rangeFix
📦npm@backstage/plugin-auth-backendall versions0.4.9npm install @backstage/plugin-auth-backend@0.4.9

Affected Products

1 product · 1 configurations
Application
auth backendlinuxfoundation
< 0.4.9
range

Detection & mitigation playbook

Open-source dependency

  1. Detect

    Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for @backstage/plugin-auth-backend, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

  2. Fix

    Update @backstage/plugin-auth-backend to 0.4.9 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-43776 is resolved across your whole dependency graph.

  3. Workarounds

    Escape or sanitise the affected output on the server side rather than relying on client-side filtering, and add a Content-Security-Policy that blocks inline script execution so injected markup cannot run even if it reaches the page.

Frequently Asked Questions

### Impact This vulnerability allows a malicious actor to trick another user into visiting a vulnerable URL that executes an XSS attack. This attack can potentially allow the attacker to exfiltrate access tokens or other secrets from the user's browser. The default CSP does prevent this attack, but it is expected that some deployments have these policies disabled due to incompatibilities. ### Patches This is vulnerability is patched in version `0.4.9` of `@backstage/plugin-auth-backend`. ### For more information If you have any questions or comments about this advisory: * Open an issue in t
O3 Security · Impact-Aware SCA

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CVE-2021-43776: @backstage/plugin-auth — Fixed in 0.4.9