CVE-2026-58399
CVE-2026-58399 is a Improper Authentication vulnerability in @acastellon/auth. O3 Security confirms whether CVE-2026-58399 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
@acastellon/auth: Authentication bypass via spoofable headers in validateToken()
Exploitation Status
No confirmed exploitation observed yet
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
- A successful exploit gives an attacker total control of the affected component, not partial access.
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-58399.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
Real-World Exposure
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.
@acastellon/authnpmDescription
@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.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | @acastellon/auth | all versions | 2.3.0 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for @acastellon/auth. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update @acastellon/auth to 2.3.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-58399 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether CVE-2026-58399 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to CVE-2026-58399. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2026-58399 in your dependencies?
O3 detects CVE-2026-58399 across npm dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.