CVE-2011-0753 in PHP
Summary
by MITRE
Race condition in the PCNTL extension in PHP before 5.3.4, when a user-defined signal handler exists, might allow context-dependent attackers to cause a denial of service (memory corruption) via a large number of concurrent signals.
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Analysis
by VulDB Data Team • 10/13/2021
The vulnerability identified as CVE-2011-0753 represents a critical race condition within the PCNTL extension of PHP versions prior to 5.3.4, fundamentally compromising system stability and security integrity. This flaw specifically manifests when user-defined signal handlers are present within the PHP runtime environment, creating a dangerous scenario where concurrent signal processing can lead to unpredictable memory corruption patterns. The race condition occurs during the handling of multiple simultaneous signals, exploiting a timing vulnerability in how PHP manages signal delivery and processing within its process control extension.
The technical implementation of this vulnerability stems from inadequate synchronization mechanisms within the PCNTL extension's signal handling architecture. When multiple signals arrive concurrently while a user-defined signal handler is active, the extension fails to properly serialize access to shared memory structures used for signal state management. This results in memory corruption that can manifest as heap corruption, stack corruption, or other forms of memory instability that fundamentally compromise the PHP process integrity. The vulnerability is context-dependent, meaning it requires the presence of user-defined signal handlers to be exploitable, making it particularly insidious as it can be triggered through legitimate signal processing operations within applications.
The operational impact of this vulnerability extends beyond simple denial of service conditions, potentially enabling more sophisticated attacks that leverage memory corruption for privilege escalation or arbitrary code execution. Attackers can exploit this weakness by flooding the target system with a large number of concurrent signals, causing the PHP process to crash or behave unpredictably. The memory corruption effects can be particularly severe in web server environments where PHP processes handle multiple concurrent requests, potentially allowing attackers to destabilize entire application stacks. This vulnerability directly maps to CWE-362, which describes a race condition in concurrent programming, and aligns with ATT&CK technique T1499.004 for network denial of service attacks that leverage system instability.
Mitigation strategies for CVE-2011-0753 primarily focus on immediate version upgrading to PHP 5.3.4 or later, where the race condition has been addressed through improved signal handling synchronization. System administrators should also implement signal rate limiting mechanisms to prevent excessive concurrent signal delivery to PHP processes, though this represents a partial workaround rather than a complete solution. Additionally, applications should be reviewed to minimize the use of user-defined signal handlers where possible, and proper signal handling patterns should be implemented to reduce the attack surface. Security monitoring should be enhanced to detect unusual signal processing patterns that might indicate exploitation attempts, while regular security audits should verify that PHP installations are running patched versions to prevent this vulnerability from being exploited in production environments.