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CVE-2026-56020 – Authentication Bypass – Webmin (MiniServ) prior to 2.641

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Summary

CVE-2026-56020 is a high-severity authentication bypass vulnerability (CWE-290: Authentication Bypass by Spoofing) in the Webmin HTTP server component, miniserv.pl, affecting all versions prior to 2.641. An unauthenticated remote attacker can send a forged HTTP header to impersonate any user configured to authenticate via SSL client certificates — including administrative accounts — without possessing a valid certificate. The vulnerability carries a CVSS v3.1 score of 8.1 (HIGH) and a CVSS v4.0 score of 9.2 (CRITICAL).

Technical details

  • Root cause: miniserv.pl (Webmin’s built-in HTTP server) improperly trusts a user-supplied HTTP header value to determine the identity of a connecting SSL client certificate holder, rather than relying solely on the verified certificate presented during the TLS handshake. This allows an attacker to spoof arbitrary certificate Distinguished Names (DNs) via a crafted request header.
  • Trigger condition: The vulnerability is exploitable only when SSL client certificate authentication is enabled in Webmin’s configuration — a non-default setting. Environments with this configuration active are directly at risk.
  • Attack vector: A remote, unauthenticated attacker sends a single crafted HTTP request containing a forged header to the Webmin listener (default port 10000). No valid certificate, prior session, or credentials are required.
  • Impact: Full authentication bypass, allowing the attacker to impersonate any Webmin user, including root. Webmin is a system administration panel that typically operates with root-level privileges, meaning a successful exploit results in complete server compromise — full confidentiality, integrity, and availability loss (C:H/I:H/A:H).

Affected software

  • Webmin (MiniServ HTTP server) — all versions prior to 2.641

Severity

  • CVSS v3.1 Base Score: 8.1 (HIGH)
  • Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
  • CVSS v4.0 Base Score: 9.2 (CRITICAL)
  • Vector: CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N

Mitigation and recommended actions

  • Immediate: Upgrade to Webmin 2.641 or later, which contains the vendor-provided fix.
  • If immediate patching is not possible:
    • Disable SSL client certificate authentication if it is not operationally required.
    • Restrict access to the Webmin port (default: TCP 10000) at the network perimeter to trusted IP ranges only, blocking unauthenticated internet access to the management interface.

IONIX Status

The IONIX research team is tracking ongoing exploitation attempts and recommends immediate patching. Potentially affected assets are outlined in this post.

References

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