Persistent denial of service via nested BBCode
Published Sep 8, 2026 · Updated Sep 10, 2026
Uncontrolled recursion in XenForo before 2.3.13 allows remote authenticated users to terminate PHP-FPM workers via nested BBCode. The BBCode parser recursively renders attacker-controlled nested tags without enforcing a depth limit, so a crafted post exceeds PHP's stack limit. Any user able to submit a persistent post can make affected threads terminate workers whenever visitors render them, interrupting forum service.
Summary
What happened
Uncontrolled recursion in XenForo before 2.3.13 allows remote authenticated users to terminate PHP-FPM workers via nested BBCode. The BBCode parser recursively renders attacker-controlled nested tags without enforcing a depth limit, so a crafted post exceeds PHP's stack limit. Any user able to submit a persistent post can make affected threads terminate workers whenever visitors render them, interrupting forum service.
The record
- CVE
- CVE-2026-73321
- Published
- Sep 8, 2026
- Updated
- Sep 10, 2026
- Vendor
- XenForo Ltd.
- Product
- XenForo
- Classifications
- T1499.004
- Attack vector
- network
- Privileges
- authenticated
Timeline
How it unfolded
- Sep 8, 2026CVE publishedPublication date reported by the CVE source.
- Sep 10, 2026Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
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Is a vulnerable build present?
Compare these published version ranges with your installed build and any vendor patches.
- Affected versionversion=0 <2.3.13
What conditions does exploitation require?
What is affected?
Published CVSS scores
No CVSS assessment is available in this record.
CVSS describes severity. EPSS estimates exploitation probability.
Attacks
What attackers are doing with it
Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.
Weakness, pattern, technique
Public exploit references
No public exploit references are available in this record.
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Technologies
Your stack
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Your stack
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