CVE-2015-9253 in PHP
Summary
by MITRE
An issue was discovered in PHP through 7.2.2. The php-fpm master process restarts a child process in an endless loop when using program execution functions (e.g., passthru, exec, shell_exec, or system) with a non-blocking STDIN stream, causing this master process to consume 100% of the CPU, and consume disk space with a large volume of error logs, as demonstrated by an attack by a customer of a shared-hosting facility.
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Analysis
by VulDB Data Team • 02/08/2023
The vulnerability identified as CVE-2015-9253 represents a critical resource exhaustion flaw affecting PHP versions through 7.2.2, specifically within the PHP-FPM (FastCGI Process Manager) implementation. This issue manifests when the master process attempts to restart child processes in an infinite loop while handling program execution functions such as passthru, exec, shell_exec, or system. The flaw exploits a fundamental weakness in how PHP-FPM manages process lifecycle when these functions are invoked with non-blocking STDIN streams, creating a condition where the master process becomes trapped in continuous restart cycles.
The technical mechanism behind this vulnerability involves the interaction between PHP-FPM's process management and the standard input handling of executed programs. When a child process executes a program through one of the affected functions while STDIN is configured as non-blocking, the process management logic fails to properly handle the stream state, leading to an infinite restart loop. This behavior occurs because the master process cannot distinguish between legitimate process termination and abnormal termination caused by the non-blocking stream condition, resulting in continuous process recreation attempts.
The operational impact of CVE-2015-9253 is severe and multifaceted, particularly in shared hosting environments where multiple users operate on the same infrastructure. The master process consumes 100% of available CPU resources, effectively rendering the web server unresponsive to legitimate requests while simultaneously generating massive volumes of error logs that rapidly consume disk space. This creates a denial of service condition that can compromise the entire hosting environment and potentially affect other customers on the same server. The vulnerability is particularly dangerous in shared hosting facilities where attackers can leverage this issue to disrupt services without requiring elevated privileges.
From a cybersecurity perspective, this vulnerability aligns with CWE-400, which addresses "Uncontrolled Resource Consumption" and maps to ATT&CK technique T1499.1, "OS File and Directory Permissions Modification" through resource exhaustion attacks. The flaw demonstrates how seemingly benign functionality can be exploited to create significant system instability, making it a prime example of how process management vulnerabilities can be weaponized. The attack vector is particularly relevant in multi-tenant environments where a single compromised user account can potentially affect the entire hosting infrastructure.
Mitigation strategies for CVE-2015-9253 focus on both immediate patching and operational hardening measures. The most effective solution involves upgrading to PHP versions 7.2.3 or later, where the underlying process management logic has been corrected to properly handle non-blocking STDIN streams. Organizations should also implement process monitoring to detect unusual CPU consumption patterns and configure appropriate resource limits on PHP-FPM processes. Additionally, administrators should consider disabling or restricting the use of program execution functions in shared hosting environments, and implement proper input validation to prevent malicious use of these functions. The vulnerability highlights the importance of thorough testing of process management logic in web server environments and the need for robust error handling in concurrent execution scenarios.