CVE-2021-40145 in GD Graphics Library
Summary
by MITRE • 08/26/2021
** DISPUTED ** gdImageGd2Ptr in gd_gd2.c in the GD Graphics Library (aka LibGD) through 2.3.2 has a double free. NOTE: the vendor's position is "The GD2 image format is a proprietary image format of libgd. It has to be regarded as being obsolete, and should only be used for development and testing purposes."
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Analysis
by VulDB Data Team • 12/16/2025
The vulnerability CVE-2021-40145 represents a critical double free condition within the GD Graphics Library version 2.3.2 and earlier, specifically affecting the gdImageGd2Ptr function in the gd_gd2.c source file. This flaw occurs during the processing of GD2 image format files, which despite being proprietary to libgd, remain in use within various applications and systems. The double free vulnerability manifests when the library attempts to release memory that has already been freed, creating a potential exploitation vector for attackers seeking to manipulate memory management operations. The affected function processes GD2 image data structures and handles memory allocation and deallocation in a manner that permits the same memory block to be freed twice, leading to undefined behavior and potential system compromise.
The technical implementation of this vulnerability stems from improper memory management within the GD2 format parser, where the library fails to maintain proper tracking of memory allocation states during image processing operations. When parsing malformed GD2 files, the gdImageGd2Ptr function executes code paths that result in duplicate free operations on heap-allocated memory blocks. This condition aligns with CWE-415, which specifically addresses double free vulnerabilities in memory management operations. The flaw exists because the library's internal memory tracking mechanisms do not adequately prevent the same memory pointer from being passed to the free function multiple times during the processing lifecycle of GD2 image data structures.
From an operational perspective, this vulnerability presents significant risks to systems that process untrusted GD2 image files, particularly in web applications, content management systems, and image processing services. Attackers could potentially craft malicious GD2 files that trigger the double free condition when processed by vulnerable applications, leading to memory corruption that might allow for arbitrary code execution or denial of service conditions. The impact extends beyond simple application crashes to potentially enable more sophisticated exploitation techniques, as the memory corruption could be leveraged to manipulate heap metadata or overwrite critical program structures. This vulnerability particularly affects environments where GD2 format support is enabled for user-uploaded content or automated image processing workflows.
The vendor's position regarding the GD2 format as obsolete and limited to development and testing purposes does not diminish the security implications of this vulnerability within production environments where the format remains enabled. Organizations using libgd for image processing must consider the broader security implications, as many applications continue to support GD2 format processing despite its deprecated status. The vulnerability demonstrates the importance of maintaining security hygiene even for legacy or deprecated features, as these components often remain in production systems and can serve as attack vectors. Mitigation strategies should focus on disabling GD2 format support in production environments where it is not required, updating to patched versions of libgd, and implementing proper input validation and sandboxing measures for any image processing functionality that continues to support GD2 format handling. The vulnerability also highlights the need for comprehensive security testing of all image format parsers, as the complexity of image format specifications often leads to subtle memory management issues that can be exploited by threat actors.