CVE-2006-1991 in PHP
Summary
by MITRE
The substr_compare function in string.c in PHP 5.1.2 allows context-dependent attackers to cause a denial of service (memory access violation) via an out-of-bounds offset argument.
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Analysis
by VulDB Data Team • 06/18/2019
The CVE-2006-1991 vulnerability represents a critical memory access violation flaw within the PHP 5.1.2 runtime environment that specifically targets the substr_compare function implementation in the string.c source file. This vulnerability emerges from inadequate input validation mechanisms that fail to properly handle out-of-bounds offset arguments, creating a scenario where malicious actors can exploit the function's behavior to trigger unexpected memory access patterns. The flaw operates within the context of PHP's string manipulation capabilities, where the substr_compare function is designed to compare portions of strings based on specified offsets and lengths, making it a fundamental component in many web applications that process user input.
The technical execution of this vulnerability relies on the function's failure to validate offset parameters against the bounds of the target string, allowing attackers to provide negative values or values exceeding the string length. When such invalid offset arguments are processed, the substr_compare function attempts to access memory locations that are either outside the allocated string buffer or in unallocated memory regions, resulting in segmentation faults or memory access violations. This type of vulnerability falls under the CWE-129 category of "Improper Validation of Array Index" and can be classified as a buffer overflow condition that specifically affects string handling functions within the PHP interpreter. The vulnerability demonstrates characteristics consistent with the ATT&CK technique T1499.004 which involves network denial of service attacks through resource exhaustion or memory corruption.
The operational impact of this vulnerability extends beyond simple denial of service conditions, as it can potentially be leveraged in more sophisticated attack chains where attackers first establish a denial of service condition before attempting to exploit additional vulnerabilities or escalate their privileges. Web applications that rely heavily on user input processing and string manipulation functions are particularly at risk, as the vulnerability can be triggered through various input vectors including form submissions, URL parameters, or file uploads that pass through PHP's string comparison functions. The memory access violation can cause the PHP process to crash or terminate unexpectedly, leading to service disruption for legitimate users while potentially providing attackers with opportunities to conduct further reconnaissance or attempt additional attacks against the affected system.
Effective mitigation strategies for CVE-2006-1991 require immediate patching of affected PHP installations to version 5.1.3 or later, which contains the necessary input validation fixes for the substr_compare function. System administrators should implement comprehensive input validation at multiple layers of the application stack, ensuring that all string manipulation functions properly validate offset and length parameters before processing user-supplied data. Network-level defenses such as web application firewalls and intrusion detection systems can provide additional protection by monitoring for suspicious parameter patterns that may indicate attempts to exploit this vulnerability. Regular security assessments and code reviews should focus on identifying similar input validation gaps in custom PHP applications, as the vulnerability demonstrates how seemingly benign string handling functions can become attack vectors when proper parameter validation is absent. The fix implemented in subsequent PHP versions addresses the root cause by introducing proper bounds checking mechanisms that prevent out-of-bounds memory access while maintaining the function's intended operational behavior for legitimate use cases.