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XenForo versions before 2.3.13 contain an uncontrolled recursion vulnerability in the BBCode parser. An authenticated user can submit a post containing BBCode tags nested deeply enough to exhaust PHP stack capacity during parsing or rendering. Because the crafted content is stored in a forum post, subsequent attempts to render the affected thread can repeatedly trigger fatal PHP errors and terminate PHP-FPM workers.
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1 valid exploit after Mallory filtered fakes, detection scripts, and README-only repos.
This three-file repository contains a standalone Python proof of concept for CVE-2026-73321, an uncontrolled-recursion denial of service in XenForo's BBCode processing prior to 2.3.13. poc.py accepts a target base URL and quote nesting depth (1-5,000), requires a --confirm-disruption flag, retrieves a guest CSRF token from /index.php?help/, and POSTs a nested [quote] payload to /index.php?editor/to-html. It handles HTTP and connection errors and prints a JSON result. The payload is intended to exhaust recursive parser/renderer stack space and recycle or crash a PHP worker, not to obtain code execution or access. README.md documents the affected version, observed worker crash behavior, usage, and references; .gitignore excludes Python bytecode/cache files.
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