CVE-2009-3557 in PHP
Summary
by MITRE
The tempnam function in ext/standard/file.c in PHP before 5.2.12 and 5.3.x before 5.3.1 allows context-dependent attackers to bypass safe_mode restrictions, and create files in group-writable or world-writable directories, via the dir and prefix arguments.
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Analysis
by VulDB Data Team • 08/28/2021
The vulnerability identified as CVE-2009-3557 represents a critical security flaw in PHP's tempnam function implementation that existed in versions prior to 5.2.12 and 5.3.1. This issue specifically affects the way PHP handles temporary file creation within its standard library, creating a pathway for malicious actors to circumvent the safe_mode protection mechanisms that are designed to restrict file system access. The vulnerability is particularly concerning because it allows attackers to create temporary files in directories that should normally be restricted, effectively undermining the security controls that protect against unauthorized file system operations.
The technical flaw resides in the improper validation of directory paths and prefixes when the tempnam function is invoked. When PHP processes a call to tempnam, it should verify that the specified directory argument falls within allowed boundaries and that the prefix argument does not introduce security vulnerabilities. However, in vulnerable versions, the function failed to properly sanitize these parameters, allowing context-dependent attackers to manipulate the directory path and prefix values to force file creation in insecure locations. This occurs because the implementation does not adequately check whether the target directory is writable by the current user or whether it contains group-writable or world-writable permissions that could be exploited.
The operational impact of this vulnerability extends beyond simple file creation, as it enables attackers to bypass fundamental security restrictions that are crucial for maintaining system integrity. When safe_mode is enabled, PHP should restrict file operations to prevent unauthorized access to sensitive system resources, but this vulnerability allows attackers to create temporary files in locations where they might not otherwise have write permissions. This could potentially lead to privilege escalation scenarios where an attacker can place malicious files in directories that are accessible to other users or system processes, creating opportunities for further exploitation. The vulnerability particularly affects web applications that rely on PHP's temporary file handling mechanisms and use safe_mode for additional protection.
The security implications of this vulnerability align with CWE-22, which describes improper limitation of a pathname to a restricted directory, and can be categorized under ATT&CK technique T1059.007 for scripting languages, as it enables attackers to manipulate temporary file creation processes. Organizations running vulnerable PHP versions face significant risks when deploying web applications that depend on safe_mode protection, as this vulnerability essentially renders the safe_mode configuration ineffective for temporary file operations. The exploitation of this vulnerability typically requires an attacker to have some level of control over the input parameters passed to the tempnam function, which is often achievable through web application input fields or parameter manipulation.
Mitigation strategies for CVE-2009-3557 primarily involve upgrading to PHP versions 5.2.12 or 5.3.1 and later, which contain the necessary patches to properly validate directory paths and prefix arguments. System administrators should also implement additional monitoring to detect unusual temporary file creation patterns and review directory permissions to ensure that group-writable and world-writable directories are minimized. The patch addresses the root cause by implementing proper validation of the dir and prefix parameters, ensuring that file creation operations occur only in secure directories that meet the necessary permission requirements. Organizations should also consider implementing web application firewalls and input validation mechanisms to prevent exploitation attempts, as well as conducting regular security assessments to identify other potential vulnerabilities in their PHP-based applications.