CVE-2015-4026 in PHP
Summary
by MITRE
The pcntl_exec implementation in PHP before 5.4.41, 5.5.x before 5.5.25, and 5.6.x before 5.6.9 truncates a pathname upon encountering a \x00 character, which might allow remote attackers to bypass intended extension restrictions and execute files with unexpected names via a crafted first argument. NOTE: this vulnerability exists because of an incomplete fix for CVE-2006-7243.
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Analysis
by VulDB Data Team • 05/19/2022
The vulnerability described in CVE-2015-4026 represents a critical security flaw in PHP's pcntl_exec function implementation that affects multiple PHP versions including 5.4.x before 5.4.41, 5.5.x before 5.5.25, and 5.6.x before 5.6.9. This issue stems from an incomplete remediation of a previously identified vulnerability CVE-2006-7243, creating a persistent security weakness that allows attackers to manipulate file execution paths through strategic insertion of null characters within file paths. The core technical flaw manifests when the pcntl_exec function processes a pathname containing a null character, causing the implementation to truncate the path at that specific character position rather than properly handling or rejecting the malformed input.
The operational impact of this vulnerability is significant as it enables remote attackers to bypass intended file extension restrictions and execute files with unexpected names through carefully crafted first arguments. This occurs because the truncation mechanism effectively removes portions of the intended file path, potentially allowing malicious actors to substitute a legitimate-looking file extension with a different one, thereby executing arbitrary code. The vulnerability specifically targets the pcntl_exec function which is used to execute a program in a new process, making it particularly dangerous in web applications that may use this function to execute system commands based on user input. Attackers can exploit this by injecting null characters into file paths that are then processed by pcntl_exec, potentially executing files in unexpected locations or with different extensions than originally intended.
This security weakness maps directly to CWE-170, which addresses improper null termination or truncation of strings, and aligns with ATT&CK technique T1059.007 for Command and Scripting Interpreter - PowerShell. The vulnerability demonstrates how incomplete security fixes can create persistent risks in software implementations, particularly when dealing with system-level functions that interact with file paths and process execution. The issue affects PHP applications that rely on pcntl_exec for executing external programs, making it a concern for web applications that may use this functionality to process user-supplied data or execute system commands based on dynamic inputs. Organizations using affected PHP versions face the risk of arbitrary code execution, privilege escalation, and potential system compromise through exploitation of this path truncation vulnerability.
The recommended mitigation strategy involves upgrading to PHP versions that contain the complete fix for this vulnerability, specifically PHP 5.4.41, 5.5.25, or 5.6.9 and later. Additionally, developers should implement proper input validation and sanitization for any user-supplied data that might be passed to pcntl_exec functions, ensuring that null characters and other potentially dangerous characters are properly filtered or escaped before processing. Organizations should also consider implementing additional security controls such as input validation at multiple layers, including web application firewalls and runtime monitoring systems, to detect and prevent exploitation attempts targeting this specific vulnerability. The remediation process should include thorough testing to ensure that the upgrade does not introduce regressions in existing functionality while maintaining the security posture against this and related path manipulation attacks.