---
title: "CVE-2026-58399: @acastellon/auth: Authentication bypass via spoofable headers in validateToken()"
url: https://erroneo.us/security/cve/CVE-2026-58399/
description: "CVE-2026-58399 (@acastellon/auth (npm)): @acastellon/auth is an authentication control system for microservices. Versions prior to 2.3.0 appear to allow an…"
type: vulnerability-record
section: "Security Digest"
date_published: 2026-07-01
date_modified: 2026-07-02
publisher: erroneo.us
author: erroneo.us Security Desk
cve: "CVE-2026-58399"
kev: false
cvss: 8.7
epss: 0.00973
license: All rights reserved. Quote with attribution and a link.
---

# CVE-2026-58399: @acastellon/auth: Authentication bypass via spoofable headers in validateToken()

**Signal:** Normal patch cycle. No exploitation signal; fix it in your next routine update if you are affected. (Computed from CISA KEV, EPSS and CVSS; not an editorial verdict.)

## Should you act?

- No confirmed exploitation. It is not in CISA’s KEV catalog and CISA’s SSVC assessment records no known exploitation. That can change quickly after a public write-up; re-check if you are affected.
- EPSS 1.0% , percentile 62: FIRST’s model estimates a 1.0% chance of exploitation activity in the next 30 days, higher than 62% of all scored CVEs (moderate). EPSS is a probability, not a verdict.
- Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
- Only applies if your dependency tree includes @acastellon/auth (npm) < 2.3.0, fixed in 2.3.0 . Check your lockfile, including transitive dependencies.

## What it is

@acastellon/auth is an authentication control system for microservices. Versions prior to 2.3.0 appear 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. A fix has been implemented in v2.3.0.

## Affected packages

- npm `@acastellon/auth` < 2.3.0 → fixed: 2.3.0

## Affected products (CVE record)

- antonio-castellon module-auth: < 2.3.0

## Severity

- GitHub_M CVSS 4.0: 8.7 `CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N`
- GitHub CVSS 4.0: 9.3 `CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N`

## Exploitation

- CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
- CISA SSVC assessment: exploitation none , automatable yes , technical impact total .
- FIRST EPSS (model of 2026-10-21): 0.97% probability of exploitation activity in the next 30 days; percentile 62.

## Sources

- [GitHub Advisory GHSA-gfj5-979r-92pw](https://github.com/advisories/GHSA-gfj5-979r-92pw)
- [CVE.org record CVE-2026-58399](https://www.cve.org/CVERecord?id=CVE-2026-58399)
- [NVD entry CVE-2026-58399](https://nvd.nist.gov/vuln/detail/CVE-2026-58399)
- [github.com](https://github.com/antonio-castellon/module-auth/security/advisories/GHSA-gfj5-979r-92pw)
- [github.com](https://github.com/antonio-castellon/module-auth/issues/6)
- [npmjs.com](https://www.npmjs.com/package/@acastellon/auth/v/2.3.0)

---
Source page: https://erroneo.us/security/cve/CVE-2026-58399/ · Security Digest: https://erroneo.us/security/ · More: https://erroneo.us/llms.txt