SECURITY: Authentication bypass in @acastellon/auth v2.2.0 via spoofable headers in validateToken() (mTLS fix in 2.3.0+)

[antonio-castellon]

Credit: hackchang for discovering and reporting this vulnerability.

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OFFICIAL GITHUB SECURITY ADVISORY DRAFT CONTENT

Copy and paste the sections below into the GitHub "New draft security advisory" form.

Summary (Title):
Authentication bypass in @acastellon/auth v2.2.0 via spoofable headers in validateToken() (fixed in v2.3.0)

Description:

@acastellon/auth v2.2.0 appears to allow an unauthenticated authentication bypass in validateToken() through spoofable auth-user and Host request headers.

The validateToken middleware contains a service-to-service bypass for auth-user: service-brother when req.get('host').startsWith(getHostName()). Both values involved in the check can be influenced by an unauthenticated HTTP client: auth-user is a request header, and Host is also client-controlled. As a result, a remote unauthenticated attacker can send a request with crafted headers and bypass token validation before the normal legacy/JWT/OIDC validation logic runs.

Impact:
An attacker may be able to access routes protected by validateToken() without a valid token. In deployments where downstream services trust auth-user or is-* headers, this may also lead to privilege escalation.

Affected package:
@acastellon/auth v2.2.0

Affected code:
auth.js, validateToken()
The issue is related to the service-brother bypass and getHostName() check.

Example request:

GET /protected HTTP/1.1
Host: <configured CNAME or hostname>
auth-user: service-brother
is-admin: true

Expected behavior:
The request should require a valid authentication token.

Actual behavior:
The middleware calls next() before token validation.

Fix implemented in v2.3.0+:

• Removed the spoofable bypass.
• Always sanitize incoming auth-user and is-* headers.
• Added mTLS client certificate based service auth (with optional TRUSTED_MTLS_SERVICES allowlist).
• Updated consumers (rest, graphql, dns-client) for mTLS support.
• Unit tests added for sanitization + mTLS path.

References:

• GitHub issue: SECURITY: Authentication bypass in @acastellon/auth v2.2.0 via spoofable headers in validateToken() (mTLS fix in 2.3.0+) #6
• Fixed in release: https://www.npmjs.com/package/@acastellon/auth/v/2.3.0

Credits:

• hackchang (person who found the bug)

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Instructions to create the advisory:

1. Go to https://github.com/antonio-castellon/module-auth/security/advisories
2. Click "New draft security advisory"
3. Paste the Summary into the title field.
4. Paste the Description into the description field.
5. Set Ecosystem: npm
6. Package: @acastellon/auth
7. Affected versions: < 2.3.0
8. Patched versions: >= 2.3.0
9. Severity: High
10. In the Credits section, add "hackchang"
11. Add references and publish as draft.

This issue is closed as the fix is released in v2.3.0.