Vulnerability in Xenforo
CVE-2026-73321
XenForo before 2.3.13 contains an uncontrolled recursion vulnerability in the BBCode parser that allows authenticated attackers to cause persistent denial of service by submitting a post with deeply nested BBCode tags. Attackers can craft a single malicious post with sufficient nesting depth to exceed PHP's stack limit, causing fatal errors that repeatedly terminate PHP-FPM workers for all visitors rendering the affected thread.
Published · last modified .
CVSS v3 metric
CVSS v3 base score 6.5 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H.
EPSS exploit prediction
EPSS: 0.006 (48.8th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 49th percentile — 48.8% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.006 · newest: 0.006 · change: 0.000
Affected products
Weakness classification (CWE)
Public proof-of-concept exploits
References
- Vendor Advisory (vendor-advisory, patch, Release Notes)
- Release Notes (release-notes, Release Notes)
- www.vulncheck.com/advisories/xenforo-uncontrolled-recursion-dos-via-bbcode-pars… (Third Party Advisory, third-party-advisory)
- Researcher Advisory (Exploit, technical-description, Third Party Advisory)
- Researcher PoC (Exploit, exploit, Third Party Advisory)
Frequently asked questions
- What is CVE-2026-73321?
- CVE-2026-73321 is a medium-severity vulnerability in Xenforo, classified under Uncontrolled Recursion. CVSS score: 6.5/10. Published 2026-09-08.
- How severe is CVE-2026-73321?
- Medium severity. CVSS v3 base score is 6.5 out of 10.
- Is CVE-2026-73321 known to be exploited?
- 1 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.