CVE-2013-3735 in PHP
Summary
by MITRE
** DISPUTED ** The Zend Engine in PHP before 5.4.16 RC1, and 5.5.0 before RC2, does not properly determine whether a parser error occurred, which allows context-dependent attackers to cause a denial of service (memory consumption and application crash) via a crafted function definition, as demonstrated by an attack within a shared web-hosting environment. NOTE: the vendor s http://php.net/security-note.php page says "for critical security situations you should be using OS-level security by running multiple web servers each as their own user id."
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Analysis
by VulDB Data Team • 01/17/2025
The vulnerability identified as CVE-2013-3735 represents a critical parsing error within the Zend Engine that powers PHP applications. This flaw exists in PHP versions prior to 5.4.16 RC1 and 5.5.0 RC2, where the engine fails to properly detect parser errors during function definition processing. The vulnerability is particularly concerning in shared hosting environments where multiple users execute code on the same server infrastructure. The issue stems from inadequate error handling mechanisms within the parser component that allows malformed function definitions to bypass normal validation checks, leading to unpredictable behavior in the execution environment.
The technical exploitation of this vulnerability occurs when context-dependent attackers craft specific function definitions that trigger the parser error detection failure. When such malformed code is processed by the vulnerable PHP engine, it causes the application to consume excessive memory resources and may ultimately result in application crashes. This behavior manifests as a denial of service condition that can affect the entire hosting environment. The vulnerability operates at the parser level within the Zend Engine, which is responsible for converting PHP source code into executable bytecode, making it a fundamental component of the application stack.
From an operational impact perspective, this vulnerability creates significant risks in shared hosting scenarios where multiple tenants operate on the same server infrastructure. An attacker could potentially consume all available memory resources or cause application crashes that affect other users on the same hosting platform. The vulnerability's classification under CWE-129 indicates improper input validation issues within the parser component, while its exploitation patterns align with ATT&CK technique T1499.004 for network denial of service attacks. The vendor's security note emphasizes that this issue is particularly critical in shared hosting environments, suggesting that OS-level security measures such as running multiple web servers with separate user IDs should be implemented as a mitigation strategy.
The remediation approach for this vulnerability requires immediate patching of PHP installations to versions 5.4.16 RC1 or later, and 5.5.0 RC2 or later. Organizations should also implement additional security controls such as operating system-level isolation between web server processes, implementing resource limits and monitoring for unusual memory consumption patterns, and deploying web application firewalls to detect and block malicious code submissions. The vulnerability demonstrates the importance of proper error handling in parser implementations and highlights the need for comprehensive testing of edge cases in language interpreters. Security practitioners should also consider implementing code review processes that specifically test for malformed function definitions and other parser edge cases that could trigger similar vulnerabilities in the future.