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CVE-2026-121491 – Unauthenticated Code Injection / RCE – Fiber Admin WordPress Plugin 3.1.0 to befor

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Summary

CVE-2026-121491 is a code injection vulnerability (CWE-94) in the Fiber Admin WordPress plugin. It can let an unauthenticated remote attacker execute arbitrary code on the server. It is rated Critical (CVSS 9.0), and exploitation requires specific conditions (see below).

Technical details

  • Root cause: The plugin does not validate or escape a request header before writing it into a PHP file that it regenerates. The regeneration runs on an unprotected admin hook.
  • Trigger conditions: The plugin’s custom database error page must be enabled. The attacker must also win a race against an administrator saving the plugin’s settings.
  • Attack vector: Network. No authentication or user interaction is required.
  • Impact: Arbitrary code execution on the WordPress server, with high impact on confidentiality, integrity and availability. The CVSS scope is Changed.

Affected software

  • Fiber Admin WordPress plugin versions 3.1.0 up to, but not including, 4.0.1

Severity

  • CVSS v3.1 base score: 9.0 (Critical)
  • Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:H/A:H

Mitigation and recommended actions

  • Immediate: Upgrade Fiber Admin to version 4.0.1 or later.
  • If you cannot patch immediately: The vulnerability description lists enabling the custom database error page as a precondition. Disabling that feature or deactivating the plugin removes that condition. This follows from the description and is not an official vendor workaround. Also review the PHP files the plugin generates for unexpected content.
  • Credit: Naoki Kawahigashi (finder); coordinated by WPScan.

References

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