WHMCS 8.13.3 Security Update

8.13.3 (Maintenance) Security Fixes

WHMCS 8.13.3 is a security maintenance release. It bundles a number of security hardening changes. Below is an explanation of what each fix addresses, along with the affected versions and the update path.

Client Area Authorization Bypass (CVE-2026-29204)

CVE-2026-29204 is an authorization bypass (an Insecure Direct Object Reference, CWE-639) in the Client Area. clientarea.php fails to verify that an addonId submitted in a request actually belongs to the logged-in user. As a result, an authenticated client can submit requests using another user’s addon ID and act in the context of the victim’s account, including accessing services and cPanel resources they do not own.

The flaw affects WHMCS 7.4.0 and later, and is fixed in 8.13.3 (8.x) and 9.0.4 (9.x). It scores 9.1 (Critical) on CVSS 3.1. Exploitation requires a valid, authenticated session, but any registered client account is enough, and the barrier to obtaining one is effectively zero. If you cannot update immediately, block requests to clientarea.php that carry an addonId parameter as a stopgap.

Affected Versions and Remediation

Apply the update to the latest patch release of your WHMCS branch. WHMCS supports updating in place through the Admin Area (Utilities > Update WHMCS) or by uploading the release package. Back up both your WHMCS files and database before updating.