Process crash via unguarded content-model recursion
Published Mar 16, 2026 · Updated Mar 16, 2026
Uncontrolled recursion in CPython’s pyexpat module allows remote authenticated users to crash a process with a deeply nested inline DTD. The conv_content_model function recursively converts attacker-controlled XML_Content child nodes without entering CPython’s recursion guard, exhausting the C stack. Exploitation requires an application to register ElementDeclHandler and pass untrusted XML containing an inline document type definition.
Summary
What happened
Uncontrolled recursion in CPython’s pyexpat module allows remote authenticated users to crash a process with a deeply nested inline DTD. The conv_content_model function recursively converts attacker-controlled XML_Content child nodes without entering CPython’s recursion guard, exhausting the C stack. Exploitation requires an application to register ElementDeclHandler and pass untrusted XML containing an inline document type definition.
The record
- CVE
- CVE-2026-4224
- Published
- Mar 16, 2026
- Updated
- Mar 16, 2026
- Vendor
- Python Software Foundation
- Product
- CPython
- Classifications
- CWE-674, T1499.004
- Attack vector
- network
- Privileges
- authenticated
Timeline
How it unfolded
- Mar 16, 2026CVE publishedPublication date reported by the CVE source.
- Mar 16, 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 <3.15.0
What conditions does exploitation require?
What is affected?
Published CVSS scores
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
- CPython deeply nested content-model regression testproof of concept · demonstrated
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Technologies
Your stack
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Your stack
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