dbcve.org
Auth BackendApplication · Linuxfoundation

CVE-2021-43776

MEDIUM · 6.1 CVSS v3.1 Published 2021-11-26
Fix available
A fix is available. Upgrade to 0.4.9 or later.
See remediation →
68/100
Remediation priority · Elevated
Remotely reachable No privileges

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
Backstage is an open platform for building developer portals. In affected versions the auth-backend plugin 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. This is vulnerability is patched in version `0.4.9` of `@backstage/plugin-auth-backend`.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

Backstage's auth-backend plugin contains a reflected XSS vulnerability where a malicious actor can craft a URL that, when visited by a target user, executes JavaScript in the user's browser. This can allow exfiltration of access tokens or other secrets stored in the browser. The default Content Security Policy (CSP) blocks this attack, but deployments with CSP disabled are vulnerable.

MitigationUpgrade @backstage/plugin-auth-backend to version 0.4.9 or later. If upgrading is not immediately possible, ensure Content Security Policy headers are properly configured to prevent XSS attacks.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
Auth BackendApplication
Affected:< 0.4.9

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
Required
Scope
Changed
Confidentiality
Low
Integrity
Low
Availability
None

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

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Check the installed version of @backstage/plugin-auth-backend
    Examine your package.json file or run npm list @backstage/plugin-auth-backend to identify the currently installed version
    Affected if The version listed is less than 0.4.9
  2. Verify Content Security Policy is enabled
    Inspect your web server, reverse proxy, or Backstage app configuration to determine whether CSP headers are being served
    Affected if CSP headers are not configured or explicitly disabled in the deployment
  3. Confirm the auth-backend plugin is active
    Check that the @backstage/plugin-auth-backend plugin is included in your Backstage application and that authentication routes are exposed
    Affected if The plugin is running and handles auth requests from users
  4. Identify exposed auth endpoints
    Review your deployed authentication routes (typically under /auth/) to confirm they are reachable and can accept user-supplied parameters
    Affected if Untrusted parameters can be passed through auth URLs without sanitization

You are affected if your @backstage/plugin-auth-backend version is below 0.4.9 AND Content Security Policy is not enforced in your deployment.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to 0.4.9 or later
Fixed in 0.4.9
Interim mitigation

Upgrade @backstage/plugin-auth-backend to version 0.4.9 or later. If upgrading is not immediately possible, ensure Content Security Policy headers are properly configured to prevent XSS attacks.

Recommended fix High confidence

@backstage/[email protected]

  1. Identify the package.json or lock file in your Backstage project to locate the current version of @backstage/plugin-auth-backend
  2. Update the @backstage/plugin-auth-backend dependency in package.json to version 0.4.9 or later
  3. Run 'yarn install' or 'npm install' to fetch the updated package
  4. Verify the installed version matches the target: yarn list @backstage/plugin-auth-backend or npm list @backstage/plugin-auth-backend
  5. Test the application to ensure authentication functionality works correctly after the upgrade
Caveat Review the @backstage/plugin-auth-backend changelog between your current version and 0.4.9 for any breaking changes specific to your implementation

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Auth Backend Scoped from the published advisory
  • Consultation2.0 h
  • Implementation2.0 h
  • Testing4.0 h
  • Review / QA2.0 h
10.0 hours of engineering $1,720
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $2,752.

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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2021-43776 in production — separate from our analysis above.

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data