CVE-2026-73321
XenForo < 2.3.13 Uncontrolled Recursion DoS via BBCode Parser
Description
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.
INFO
Published Date :
Sept. 8, 2026, 2:17 p.m.
Last Modified :
Sept. 11, 2026, 8:30 p.m.
Remotely Exploit :
Yes !
Source :
[email protected]
CVSS Scores
| Score | Version | Severity | Vector | Exploitability Score | Impact Score | Source |
|---|---|---|---|---|---|---|
| CVSS | 134c704f-9b21-4f2e-91b3-4a467353bcc0 | |||||
| CVSS 3.1 | MEDIUM | 83251b91-4cc7-4094-a5c7-464a1b83ea10 | ||||
| CVSS 3.1 | MEDIUM | [email protected] | ||||
| CVSS 4.0 | HIGH | 83251b91-4cc7-4094-a5c7-464a1b83ea10 | ||||
| CVSS 4.0 | HIGH | [email protected] |
Solution
- Update XenForo to 2.3.13 or a newer version.
- Apply vendor-supplied patches for the BBCode parser.
- Limit BBCode nesting depth server-side.
References to Advisories, Solutions, and Tools
Here, you will find a curated list of external links that provide in-depth
information, practical solutions, and valuable tools related to
CVE-2026-73321.
| URL | Resource |
|---|---|
| https://bombobombone.github.io/posts/cve-2026-73321/ | Exploit Third Party Advisory |
| https://github.com/BomboBombone/CVE-2026-73321 | Exploit Third Party Advisory |
| https://www.vulncheck.com/advisories/xenforo-uncontrolled-recursion-dos-via-bbcode-parser | Third Party Advisory |
| https://xenforo.com/community/threads/security-fixes-released-for-all-xenforo-and-media-gallery-versions-2-2-0-2-3-12.239856/ | Release Notes |
| https://xenforo.com/community/threads/xenforo-2-3-13-and-add-ons-released-includes-security-fixes.239857/ | Release Notes |
CWE - Common Weakness Enumeration
While CVE identifies
specific instances of vulnerabilities, CWE categorizes the common flaws or
weaknesses that can lead to vulnerabilities. CVE-2026-73321 is
associated with the following CWEs:
Common Attack Pattern Enumeration and Classification (CAPEC)
Common Attack Pattern Enumeration and Classification
(CAPEC)
stores attack patterns, which are descriptions of the common attributes and
approaches employed by adversaries to exploit the CVE-2026-73321
weaknesses.
We scan GitHub repositories to detect new proof-of-concept exploits. Following list is a collection of public exploits and proof-of-concepts, which have been published on GitHub (sorted by the most recently updated).
Results are limited to the first 15 repositories due to potential performance issues.
The following list is the news that have been mention
CVE-2026-73321 vulnerability anywhere in the article.
The following table lists the changes that have been made to the
CVE-2026-73321 vulnerability over time.
Vulnerability history details can be useful for understanding the evolution of a vulnerability, and for identifying the most recent changes that may impact the vulnerability's severity, exploitability, or other characteristics.
-
Initial Analysis by [email protected]
Sep. 11, 2026
Action Type Old Value New Value Added CPE Configuration OR *cpe:2.3:a:xenforo:xenforo:*:*:*:*:*:*:*:* versions up to (excluding) 2.3.13 Added Reference Type VulnCheck: https://bombobombone.github.io/posts/cve-2026-73321/ Types: Exploit, Third Party Advisory Added Reference Type VulnCheck: https://github.com/BomboBombone/CVE-2026-73321 Types: Exploit, Third Party Advisory Added Reference Type VulnCheck: https://www.vulncheck.com/advisories/xenforo-uncontrolled-recursion-dos-via-bbcode-parser Types: Third Party Advisory Added Reference Type VulnCheck: https://xenforo.com/community/threads/security-fixes-released-for-all-xenforo-and-media-gallery-versions-2-2-0-2-3-12.239856/ Types: Release Notes Added Reference Type VulnCheck: https://xenforo.com/community/threads/xenforo-2-3-13-and-add-ons-released-includes-security-fixes.239857/ Types: Release Notes -
CVE Modified by 134c704f-9b21-4f2e-91b3-4a467353bcc0
Sep. 10, 2026
Action Type Old Value New Value Added SSVC {'id': 'CVE-2026-73321', 'role': 'CISA Coordinator', 'options': [{'exploitation': 'poc'}, {'automatable': 'no'}, {'technicalImpact': 'partial'}], 'version': '2.0.3', 'timestamp': '2026-09-10T13:49:59.215010Z'} -
CVE Modified by [email protected]
Sep. 09, 2026
Action Type Old Value New Value Added Reference https://bombobombone.github.io/posts/cve-2026-73321/ Added Reference https://github.com/BomboBombone/CVE-2026-73321 -
New CVE Received by [email protected]
Sep. 08, 2026
Action Type Old Value New Value Added Description 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. Added CVSS V4.0 AV:N/AC:L/AT:N/PR:L/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X Added CVSS V3.1 AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H Added CWE CWE-674 Added Affected New affected value received. <a href="https://github.com/CVEProject/cvelistV5/blob/main/cves/2026/73xxx/CVE-2026-73321.json">CVE-2026-73321</a> Added Reference https://www.vulncheck.com/advisories/xenforo-uncontrolled-recursion-dos-via-bbcode-parser Added Reference https://xenforo.com/community/threads/security-fixes-released-for-all-xenforo-and-media-gallery-versions-2-2-0-2-3-12.239856/ Added Reference https://xenforo.com/community/threads/xenforo-2-3-13-and-add-ons-released-includes-security-fixes.239857/