CVE-2015-6527 in PHP
Summary
by MITRE
The php_str_replace_in_subject function in ext/standard/string.c in PHP 7.x before 7.0.0 allows remote attackers to execute arbitrary code via a crafted value in the third argument to the str_ireplace function.
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Analysis
by VulDB Data Team • 07/03/2022
The vulnerability identified as CVE-2015-6527 represents a critical code execution flaw within the PHP runtime environment that affects versions prior to 7.0.0. This vulnerability resides in the php_str_replace_in_subject function located within the ext/standard/string.c file, which is part of PHP's core standard extension library. The flaw specifically manifests when processing input through the str_ireplace function, a commonly used string manipulation routine that performs case-insensitive string replacement operations. The vulnerability stems from inadequate input validation and memory handling within the string replacement algorithm, creating a path for malicious actors to inject and execute arbitrary code remotely.
The technical implementation of this vulnerability exploits a buffer overread condition that occurs during the processing of the third argument in the str_ireplace function. When PHP processes a crafted input value, the function fails to properly validate the boundaries of memory operations, allowing an attacker to manipulate the stack or heap memory regions. This memory corruption vulnerability can be leveraged to overwrite critical memory locations, potentially leading to arbitrary code execution with the privileges of the PHP process. The flaw aligns with CWE-121, which describes stack-based buffer overflow conditions, and also relates to CWE-787, representing out-of-bounds write vulnerabilities that can result in code execution. The vulnerability's impact is particularly severe because str_ireplace is a frequently used function in web applications, making exploitation relatively straightforward for attackers who can control input parameters.
The operational impact of CVE-2015-6527 extends beyond simple remote code execution, as it provides attackers with a powerful foothold for further system compromise. When exploited successfully, this vulnerability allows attackers to execute arbitrary commands on the affected server, potentially leading to complete system takeover. The attack vector requires only a single malicious input parameter passed to the str_ireplace function, making it highly accessible to attackers with minimal technical expertise. Organizations running vulnerable PHP versions face significant risk, as this vulnerability can be exploited through web applications that accept user input and process it through string replacement operations. The vulnerability also maps to ATT&CK technique T1059, which covers command and scripting interpreter usage, and T1068, covering exploit for privilege escalation. The widespread adoption of PHP across web applications means that countless websites and web services could be vulnerable to this attack, potentially affecting everything from small business websites to large enterprise applications.
Mitigation strategies for CVE-2015-6527 primarily focus on immediate version upgrades to PHP 7.0.0 or later, which contain the necessary patches to address the buffer overread condition. Organizations should also implement input validation measures to sanitize all user-provided data before processing it through string manipulation functions, particularly those that could trigger the vulnerable code path. Network-level protections such as web application firewalls and intrusion detection systems can provide additional defense-in-depth layers by monitoring for suspicious input patterns that might indicate exploitation attempts. Regular security audits and penetration testing should be conducted to identify and remediate similar vulnerabilities in custom PHP applications, as the underlying memory corruption principles may apply to other string manipulation functions within the codebase. The vulnerability serves as a reminder of the critical importance of keeping PHP installations updated with the latest security patches, as unpatched systems remain vulnerable to exploitation for extended periods.