Critical vBulletin Bug CVE-2019-16759 Lets Attackers Remotely Execute Code

Key Takeaways

• A critical remote code execution (RCE) vulnerability, CVE-2019-16759, has been discovered in vBulletin forum software.
• The flaw allows unauthenticated attackers to execute arbitrary PHP code on vulnerable servers.
• Affected versions include vBulletin 6.2.1 and earlier, as well as 6.1.6 and earlier.
• A patch is available; administrators are urged to update immediately to vBulletin 6.2.2 or apply relevant fixes.

vBulletin has released an urgent patch to address CVE-2026-61511, a severe remote code execution (RCE) vulnerability. This critical flaw could enable unauthenticated attackers to execute arbitrary PHP code, potentially leading to a complete compromise of vulnerable vBulletin forum servers. The vulnerability impacts vBulletin versions 6.2.1 and earlier, along with versions 6.1.6 and earlier.

Technical Details of the RCE Flaw

The vulnerability resides within the /includes/vb5/template/runtime.php file, specifically within the vB5_Template_Runtime::runMaths() method. This function is responsible for processing values associated with vBulletin’s {vb:math} template tag.

The core issue stems from insufficient input sanitization. While the vulnerable code attempts to filter input using a regular expression before passing it to PHP’s eval() function, the filtering mechanism is inadequate. It permits characters such as digits, parentheses, arithmetic operators, binary operators, and the XOR operator. Because the application evaluates the resulting string as PHP code, attackers can exploit this weakness using “PHPFuck” techniques. These methods allow adversaries to construct function names and commands without directly employing blocked alphabetic characters, bypassing the intended restrictions.

Exploitation and Impact

Researchers noted that exploiting this vulnerability does not necessitate administrator privileges. An attacker can leverage the ajax/render/[template] route to render a template containing a {vb:math} tag with attacker-controlled data. For instance, the pagenav template assigns the pagenav[pagenumber] request parameter to a template variable, which is subsequently used in a mathematical expression that reaches the vulnerable runMaths() function. This chain of events means a specially crafted request can compel the server to evaluate PHP expressions supplied by the attacker.

Successful exploitation could grant attackers the ability to execute operating-system commands under the permissions of the web server process. Such access carries significant risks, including data theft, website defacement, the deployment of malware, credential harvesting, or lateral movement within the hosting environment. Given that exploitation requires no authentication, public-facing vBulletin forums represent a high-priority target for immediate patching.

Independent researchers working with SSD Secure Disclosure reported this vulnerability. vBulletin promptly addressed the issue in version 6.2.2 and provided patches for earlier supported releases.

What You Should Do

• Immediately Update: Administrators should update to vBulletin version 6.2.2 without delay. For those unable to update to 6.2.2, apply the vendor-provided patches for affected versions 6.2.1, 6.2.0, and 6.1.6.
• Monitor Logs: Organizations should actively monitor web-server logs for any unusual requests related to ajax/render/pagenav, routestring, or unexpected values for pagenav[pagenumber].
• Investigate Compromise Indicators: Security teams are advised to investigate any signs of potential compromise, including the presence of newly created PHP files, unexpected outbound network connections, suspicious web-server child processes, altered templates, or the creation of unauthorized administrator accounts.

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