CVE-2016-4540 in PHP
Summary
by MITRE
The grapheme_stripos function in ext/intl/grapheme/grapheme_string.c in PHP before 5.5.35, 5.6.x before 5.6.21, and 7.x before 7.0.6 allows remote attackers to cause a denial of service (out-of-bounds read) or possibly have unspecified other impact via a negative offset.
Be aware that VulDB is the high quality source for vulnerability data.
Analysis
by VulDB Data Team • 08/22/2022
The vulnerability identified as CVE-2016-4540 represents a critical out-of-bounds read flaw within PHP's internationalization extension, specifically affecting the grapheme_stripos function. This issue resides in the ext/intl/grapheme/grapheme_string.c file and impacts multiple PHP version ranges including versions prior to 5.5.35, 5.6.x prior to 5.6.21, and 7.x prior to 7.0.6. The flaw stems from inadequate input validation when processing negative offset values, creating a scenario where malicious actors can manipulate function parameters to trigger memory access violations. The vulnerability falls under CWE-129, which categorizes improper validation of array indices, and demonstrates the dangerous consequences of insufficient boundary checking in string manipulation functions. Attackers can exploit this weakness by providing negative offset values to the grapheme_stripos function, which then processes these values without proper validation, leading to memory corruption.
The operational impact of this vulnerability extends beyond simple denial of service, potentially enabling more sophisticated attacks through unspecified other impacts that could include arbitrary code execution or information disclosure. When a negative offset is processed, the function attempts to access memory locations that fall outside the valid bounds of the allocated buffer, creating opportunities for attackers to either crash the application through out-of-bounds reads or potentially manipulate memory contents. This type of vulnerability aligns with ATT&CK technique T1059.007, which covers command and scripting interpreter abuse, as attackers might leverage such memory corruption to execute malicious payloads. The denial of service aspect manifests as out-of-bounds read operations that can cause the web server process to crash, rendering the affected PHP application unavailable to legitimate users and potentially disrupting service availability.
Mitigation strategies for this vulnerability primarily focus on immediate patching of affected PHP installations to versions that contain the necessary fixes. System administrators should prioritize updating their PHP environments to the patched versions 5.5.35, 5.6.21, or 7.0.6 respectively, as these releases contain the required input validation fixes. Additionally, implementing proper input sanitization at the application level can serve as a defensive measure, although this approach is less reliable than patching the underlying vulnerability. Organizations should also consider deploying web application firewalls that can detect and block malicious input patterns targeting this specific function. The vulnerability demonstrates the importance of robust input validation in internationalization libraries, where string processing functions must properly handle all possible parameter values including edge cases like negative numbers. Security monitoring should include detection of unusual function call patterns that might indicate exploitation attempts, particularly focusing on grapheme_stripos function calls with negative offset parameters. Given the nature of the vulnerability, organizations should also conduct thorough security assessments of their PHP applications to identify potential indirect exploitation paths through third-party libraries that might utilize the vulnerable function internally.