CVE-2021-43776: Cross-Site Scripting vulnerability in @backstage/plugin-auth-backend
Summary
CVE-2021-43776: Cross-Site Scripting vulnerability in @backstage/plugin-auth-backend. Affects @backstage/plugin-auth-backend versions < 0.4.9. Fixed in 0.4.9. How to fix ↓
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Background
The @backstage/plugin-auth-backend package is an npm package that provides authentication backend functionality for the Backstage framework. It is used to handle user authentication and authorization within Backstage applications. This package is vulnerable to a Cross-Site Scripting (XSS) attack.
Vulnerability Detail
The vulnerability allows a malicious actor to exploit a Cross-Site Scripting vulnerability in the @backstage/plugin-auth-backend package. By tricking a user into visiting a specially crafted URL, the attacker can execute arbitrary JavaScript code in the user's browser. This can lead to the exfiltration of access tokens or other sensitive information stored in the user's browser. Although the default Content Security Policy (CSP) helps mitigate this attack, some deployments may have disabled these policies, making them vulnerable.
How To Fix
Latest published version: 0.30.2 as of 22 Oct 2026
npm install @backstage/plugin-auth-backend@0.30.2Upgrading to it may include breaking changes; the version below is the minimum fixed release for this vulnerability.
If you need the minimum change
0.4.9
This is the first release that contains the fix.
npm install @backstage/plugin-auth-backend@0.4.9Tip — Not sure which version your app runs? Vulert can match this fix to it. Scan free →
Conclusion
The @backstage/plugin-auth-backend package is vulnerable to a Cross-Site Scripting (XSS) attack. To mitigate this vulnerability, it is recommended to upgrade to version 0.4.9 of the package. For further support on vulnerability remediation, please contact DevNack.com.
See every vulnerability recorded for @backstage/plugin-auth-backend →
FAQ
This vulnerability allows an attacker to execute arbitrary JavaScript code in a user's browser, potentially leading to the exfiltration of sensitive information.
To mitigate the risk, it is recommended to upgrade to version 0.4.9 of the @backstage/plugin-auth-backend package.
There are no known workarounds for this vulnerability. It is recommended to upgrade to the patched version.
Yes, the default CSP helps mitigate this vulnerability. However, some deployments may have disabled these policies, making them vulnerable.
For more information, you can open an issue in the Backstage repository or visit the Backstage chat.
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