CVE-2009-4018 in PHP
Summary
by MITRE
The proc_open function in ext/standard/proc_open.c in PHP before 5.2.11 and 5.3.x before 5.3.1 does not enforce the (1) safe_mode_allowed_env_vars and (2) safe_mode_protected_env_vars directives, which allows context-dependent attackers to execute programs with an arbitrary environment via the env parameter, as demonstrated by a crafted value of the LD_LIBRARY_PATH environment variable.
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Analysis
by VulDB Data Team • 06/27/2025
The vulnerability identified as CVE-2009-4018 represents a critical security flaw in PHP's process execution mechanism that undermines the intended security boundaries of the safe_mode protection system. This vulnerability specifically affects PHP versions prior to 5.2.11 and 5.3.1, creating a significant risk for web applications that rely on the proc_open function for executing external programs. The flaw exists within the ext/standard/proc_open.c source file where the function fails to properly validate environment variable restrictions that are typically enforced by PHP's safe_mode configuration directives. This oversight allows attackers to bypass the security controls designed to limit which environment variables can be passed to executed processes, fundamentally weakening the sandboxing protections that safe_mode was intended to provide.
The technical exploitation of this vulnerability occurs through the manipulation of the env parameter in the proc_open function call, which accepts an array of environment variables to be passed to the executed program. Attackers can craft malicious values for environment variables such as LD_LIBRARY_PATH, which controls the dynamic linker's search path for shared libraries. When the proc_open function fails to enforce the safe_mode_allowed_env_vars and safe_mode_protected_env_vars directives, it permits the inclusion of arbitrary environment variables that can be used to influence program execution behavior. This includes the ability to inject malicious libraries or modify program paths, potentially leading to privilege escalation or code execution attacks. The vulnerability falls under CWE-250, which addresses "Execute Code with Unusual or Unconventional Pathname" and represents a direct violation of the principle of least privilege in system security design.
The operational impact of this vulnerability extends beyond simple code execution, as it can enable attackers to manipulate the execution environment of external programs in ways that may not be immediately apparent. When attackers can control environment variables like LD_LIBRARY_PATH, they can potentially cause the system to load malicious shared libraries from unexpected locations, effectively performing a form of library injection attack. This capability is particularly dangerous in web applications where user input may be processed through the proc_open function, as it allows for the exploitation of the vulnerability without requiring direct system access. The attack vector is context-dependent, meaning that exploitation requires a scenario where the vulnerable PHP application accepts user input and passes it to the proc_open function with an unfiltered environment array. This vulnerability aligns with ATT&CK technique T1059.007 for "Command and Scripting Interpreter: PowerShell" and T1548.002 for "Abuse Elevation Control Mechanism: Bypass User Account Control," as it represents a method for bypassing system security controls through environment variable manipulation.
Organizations affected by this vulnerability should prioritize immediate patching of their PHP installations to versions 5.2.11 or 5.3.1 and later, which contain the necessary fixes to properly enforce the safe_mode environment variable restrictions. Additionally, administrators should implement strict input validation and sanitization practices for any user-provided data that may be used in proc_open function calls, particularly when constructing environment variable arrays. The mitigation strategy should also include monitoring for unusual process execution patterns and implementing network-level controls to prevent unauthorized access to systems running vulnerable PHP versions. Security teams should conduct comprehensive code reviews to identify other potential uses of the proc_open function throughout their applications and ensure that proper environment variable validation is implemented. The vulnerability demonstrates the importance of maintaining proper separation between different security mechanisms and highlights the need for thorough testing of security controls in web application environments where process execution is involved.