CVE-2016-7126 in PHP
Summary
by MITRE
The imagetruecolortopalette function in ext/gd/gd.c in PHP before 5.6.25 and 7.x before 7.0.10 does not properly validate the number of colors, which allows remote attackers to cause a denial of service (select_colors allocation error and out-of-bounds write) or possibly have unspecified other impact via a large value in the third argument.
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Analysis
by VulDB Data Team • 09/15/2022
The vulnerability identified as CVE-2016-7126 represents a critical flaw in PHP's GD extension that affects versions prior to 5.6.25 and 7.0.10. This issue resides within the imagetruecolortopalette function located in ext/gd/gd.c, which is responsible for converting truecolor images to palette-based images. The flaw stems from inadequate validation of the number of colors parameter that users can supply to this function, creating a potential vector for remote code execution and system instability. The vulnerability demonstrates characteristics consistent with CWE-129, which deals with insufficient validation of the length of input data, and aligns with ATT&CK technique T1059.007 for execution through scripting languages.
The technical implementation of this vulnerability occurs when the imagetruecolortopalette function processes a large value in its third argument, which specifies the maximum number of colors to be used in the resulting palette image. Due to the lack of proper bounds checking, the function attempts to allocate memory for color selection operations using an excessively large number that exceeds system limitations. This leads to two primary failure modes: a select_colors allocation error where the system cannot properly allocate the requested memory space, and an out-of-bounds write condition that can overwrite adjacent memory locations. These memory corruption issues create opportunities for attackers to either crash the PHP process through denial of service or potentially execute arbitrary code by manipulating the memory layout.
The operational impact of CVE-2016-7126 extends beyond simple service disruption, as it can be exploited remotely through web applications that utilize PHP's GD extension for image processing. Attackers can craft malicious image files or manipulate parameters in web forms that process user-uploaded images, triggering the vulnerable function with excessive color values. This vulnerability is particularly dangerous in web environments where PHP handles untrusted image data from users, as it can lead to complete system compromise. The vulnerability's exploitation potential aligns with ATT&CK technique T1203 for exploitation of remote services, and the memory corruption aspects connect to CWE-787 for out-of-bounds write conditions. Organizations running affected PHP versions are at risk of unauthorized access, data corruption, and potential complete system compromise.
Mitigation strategies for CVE-2016-7126 focus primarily on immediate patching of affected PHP installations to versions 5.6.25 or 7.0.10 and later. System administrators should also implement input validation measures to restrict the maximum number of colors that can be passed to the imagetruecolortopalette function, though this approach is less reliable than patching. Additional protective measures include restricting file upload capabilities, implementing proper image validation before processing, and monitoring for unusual memory allocation patterns in web server processes. Network-based protections such as web application firewalls can help detect and block exploitation attempts, while regular security audits should verify that no unpatched systems remain in the environment. The vulnerability highlights the importance of proper input validation and memory management in scripting languages, particularly when handling user-supplied data in multimedia processing functions.