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CVE-2026-73321— XenForo < 2.3.13 Uncontrolled Recursion DoS via BBCode Parser

Quick assessment

Affected
XenForo XenForo
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

XenForo 2.3.13 之前的版本中，BBCode 解析器存在一个不受控递归漏洞。经过身份验证的攻击者可以通过提交包含深度嵌套 BBCode 标签的帖子来触发该漏洞，从而造成持久的拒绝服务（DoS）。攻击者可以构造一条包含足够嵌套层级的恶意帖子，使其超过 PHP 的栈大小限制，从而引发致命错误，导致 PHP-FPM 工作进程反复终止，影响所有正在渲染该线程的访问者。

CVSS 6.5 · Medium

Possible ATT&CK Techniques 1 AI

T1499 · Endpoint Denial of Service

Affected Version Matrix 1

VendorProduct Version RangeStatus
XenForo XenForo < 2.3.13 affected
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I. Basic Information for CVE-2026-73321

Vulnerability Information
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Vulnerability Title
XenForo < 2.3.13 Uncontrolled Recursion DoS via BBCode Parser
Source: CVE Program / CVE List V5
Vulnerability 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.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
Source: CVE Program / CVE List V5
Vulnerability Type
未经控制的递归
Source: CVE Program / CVE List V5

Affected Products

Vendor Product Affected Versions CPE Subscribe
XenForo XenForo 0 ~ 2.3.13 -

II. Public POCs for CVE-2026-73321

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III. Intelligence Information for CVE-2026-73321

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Vendor Advisories for CVE-2026-73321 (1)

Exploits & Public PoCs for CVE-2026-73321 (1)

Security Blog Posts for CVE-2026-73321 (1)

Same Patch Batch · XenForo · 2026-09-08 · 14 CVEs total

CVE-2026-73315 8.6 HIGH XenForo < 2.3.13 SSRF via PayPal REST Webhook Handler
CVE-2026-73314 7.5 HIGH XenForo < 2.3.13 Signature Verification Bypass via PayPal REST Webhook
CVE-2026-73316 7.5 HIGH XenForo < 2.3.13 Payment Replay via PayPal REST Payment Provider
CVE-2026-73311 7.4 HIGH XenForo < 2.3.13 OAuth2 Authorization Code Reuse
CVE-2026-73309 7.4 HIGH XenForo < 2.3.13 Authentication Bypass via OAuth2 Token Endpoint
CVE-2026-73312 7.4 HIGH XenForo < 2.3.13 Refresh Token Replay via Expired Access Token
CVE-2026-74239 7.2 HIGH XenForo < 2.3.13 Path Traversal via Style Archive Importer on Windows
CVE-2026-73313 6.8 MEDIUM XenForo < 2.3.13 MFA Bypass via Passkey TFA Provider
CVE-2026-73320 6.1 MEDIUM XenForo < 2.3.13 Unauthenticated Information Disclosure via Unfurl Endpoint
CVE-2026-73319 6.1 MEDIUM XenForo < 2.3.13 XSS via Dynamic Redirect Handler
CVE-2026-73310 5.9 MEDIUM XenForo < 2.3.13 OAuth2 Authorization Code Token Theft via redirect_uri Bypass
CVE-2026-73318 3.8 LOW XenForo < 2.3.13 Missing Authorization via force-agreement Controller
CVE-2026-73317 2.7 LOW XenForo < 2.3.13 Missing Authorization via ACP Cache-Rebuild Dispatcher

IV. Related Vulnerabilities

Same product: XenForo
Same vendor: XenForo
Same weakness: CWE-674

V. Comments for CVE-2026-73321

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