CVE-2017-9118 in PHP
Summary
by MITRE
PHP 7.1.5 has an Out of bounds access in php_pcre_replace_impl via a crafted preg_replace call.
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Analysis
by VulDB Data Team • 04/29/2023
The vulnerability identified as CVE-2017-9118 represents a critical out-of-bounds memory access flaw within PHP's PCRE (Perl Compatible Regular Expressions) implementation, specifically affecting PHP version 7.1.5. This issue arises from insufficient input validation during the processing of regular expression replacement operations, creating a potential avenue for remote code execution or system compromise. The vulnerability manifests when a maliciously crafted preg_replace function call is executed, triggering unexpected memory access patterns that can lead to arbitrary code execution or denial of service conditions.
The technical root cause of this vulnerability stems from improper bounds checking within the php_pcre_replace_impl function, which handles regular expression replacement operations in PHP. When processing certain malformed regular expression patterns combined with specific replacement strings, the implementation fails to properly validate array indices or memory boundaries before accessing memory locations. This flaw falls under the CWE-125 Out-of-bounds Read vulnerability category, where the application reads memory beyond the intended buffer limits. The vulnerability is particularly dangerous because regular expressions are commonly used throughout PHP applications for input validation, data parsing, and string manipulation, making this flaw potentially exploitable across a wide range of applications.
The operational impact of CVE-2017-9118 extends beyond simple denial of service scenarios, as it can potentially enable remote code execution when exploited. Attackers can craft malicious regular expression patterns that, when processed by vulnerable PHP applications, trigger the out-of-bounds memory access. This vulnerability aligns with ATT&CK technique T1059.007 for Command and Scripting Interpreter: Python and T1190 for Exploit Public-Facing Application, as it represents a server-side vulnerability that can be exploited through web applications. The flaw affects any PHP application that utilizes preg_replace functions with user-controllable input, making it particularly dangerous in web environments where input validation may be insufficient. The vulnerability can result in complete system compromise, data leakage, or service disruption depending on how the application handles the memory corruption.
Mitigation strategies for CVE-2017-9118 primarily focus on immediate patching of affected PHP installations to versions that contain the necessary security fixes. Organizations should prioritize upgrading to PHP 7.1.6 or later, which includes the appropriate bounds checking mechanisms. Additionally, input validation should be strengthened at application level to sanitize all user-provided regular expression patterns before processing. Implementing proper memory protection mechanisms such as stack canaries and address space layout randomization can provide additional defense-in-depth measures. Security monitoring should include detection of suspicious preg_replace usage patterns, and application firewalls can be configured to block known malicious regular expression constructs. The vulnerability also underscores the importance of regular security assessments and keeping all application dependencies updated to prevent similar issues in the future.