CVE-2026-61511 is a critical eval injection vulnerability affecting vBulletin versions 5.x through 5.7.5 and 6.x through 6.2.1. The flaw resides in the vB5_Template_Runtime::runMaths() method within the template runtime engine. Unauthenticated remote attackers can exploit this vulnerability by supplying crafted input via the pagenav[pagenumber] parameter. The insufficiently restrictive regex filter can be bypassed using phpfuck-style encoding with permitted characters, allowing arbitrary PHP code injection. Exploitation occurs through the unauthenticated ajax/render template route, requiring no credentials. This makes the vulnerability particularly dangerous as it is remotely and freely exploitable. Patches have been released by vBulletin for affected versions including 6.2.0, 6.2.1, and 6.1.6, with version 6.2.2 also made available. Multiple security researchers and disclosure sources have documented this vulnerability.