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CVE-2026-67399 – Unauthenticated Remote Code Execution – WHMCS 8.0.0–8.13.6 and 9.0.0–9.0.7

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Summary

CVE-2026-67399 is a critical deserialization-of-untrusted-data vulnerability (CWE-502) in WHMCS, a widely used web hosting billing and automation platform. The flaw allows an unauthenticated remote attacker to submit a forged payload that WHMCS deserializes without adequate validation, leading to arbitrary code execution on the host. WHMCS rates this issue Critical, and it affects both the 8.x and 9.x release lines.

Technical details

  • Root cause: WHMCS deserializes attacker-supplied data without sufficient validation or restriction on the object types/payloads accepted (CWE-502, Deserialization of Untrusted Data).
  • Trigger conditions: The vendor advisory states the issue involves "submission of forged payloads without adequate restrictions" and that, "under specific conditions," this can be leveraged to achieve code execution. No authentication is required to trigger the flaw.
  • Attack vector: Network — the vulnerability is remotely exploitable over the web application without any user interaction or privileges (AV:N/AC:L/AT:N/PR:N/UI:N per the vendor’s CVSS 4.0 vector).
  • Impact: Successful exploitation results in arbitrary code execution on the WHMCS server, which the vendor describes as leading to "full compromise of the installation and its data" — including confidentiality, integrity, and availability impact.

Affected software

  • WHMCS 9.0.0 up to (but not including) 9.0.8
  • WHMCS 8.0.0 up to (but not including) 8.13.7

Severity

  • CVSS Score: 9.3 (Critical)
  • Vector (CVSS 4.0): CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N

Mitigation and recommended actions

  • Immediate: Upgrade to WHMCS 9.0.8 (for the 9.x line) or 8.13.7 (for the 8.x line), or later, as soon as possible. These are the versions in which the vendor states the issue is fixed.
  • If patching is not immediately possible: Restrict and monitor network access to the WHMCS admin and client-facing endpoints where feasible, and review server logs for unexpected or malformed payload submissions. No official vendor-sanctioned workaround has been published beyond upgrading; organizations unable to patch immediately should treat the instance as high risk and prioritize the update.

References

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