CVE-2019-16759

CVE-2019-16759 is a critical (CVSS 9.8) unauthenticated remote code execution flaw in vBulletin 5.x through 5.5.4 (CWE-94, code injection) in the PHP widget rendering component. An attacker triggers it by sending a crafted request to the 'ajax/render/widget_php' routestring with a malicious PHP payload in the widgetConfig[code] parameter, requiring no credentials or user interaction. Successful exploitation yields remote command execution on the web server as the web application user, enabling full compromise of the forum, theft or modification of its user database, and a foothold for further network access. Any site running vBulletin 5.x through 5.5.4 is affected, and exploitation is confirmed in the wild: the flaw was attacked as a zero-day after public exploits appeared, with headline-reported compromises including the Comodo Forums breach affecting about 245,000 users and the ZoneAlarm forum hack, and botnets observed exploiting the flaw. It is listed in CISA KEV (added 2021-11-03) with a 99.7% EPSS score, indicating near-certain near-term exploitation probability.

What to do: Upgrade vBulletin to a release newer than 5.5.4 (the patched 5.5.x version per vendor instructions), as required by the CISA KEV action. As an interim mitigation, block or filter requests to the 'ajax/render/widget_php' routestring (or strip the widgetConfig[code] parameter) at the web server or WAF. Because exploitation requires no authentication and public PoC exploits are widely available, assume compromise and check logs for requests to ajax/render/widget_php containing widgetConfig[code], and investigate any forum for signs of data theft or web shell implantation.