CVE-2017-7890 in GD Graphics Library
Summary
by MITRE
The GIF decoding function gdImageCreateFromGifCtx in gd_gif_in.c in the GD Graphics Library (aka libgd), as used in PHP before 5.6.31 and 7.x before 7.1.7, does not zero colorMap arrays before use. A specially crafted GIF image could use the uninitialized tables to read ~700 bytes from the top of the stack, potentially disclosing sensitive information.
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Analysis
by VulDB Data Team • 12/14/2022
The vulnerability CVE-2017-7890 represents a critical information disclosure flaw within the GD Graphics Library, commonly known as libgd, which serves as a fundamental graphics manipulation library used extensively across web applications and programming environments including PHP. This issue specifically affects the GIF decoding functionality where the gdImageCreateFromGifCtx function in gd_gif_in.c fails to properly initialize colorMap arrays before utilizing them. The flaw exists in versions of PHP prior to 5.6.31 and 7.x prior to 7.1.7, making it particularly dangerous given PHP's widespread adoption in web server environments. The vulnerability stems from improper memory management practices where uninitialized memory structures are accessed without proper initialization, creating a pathway for sensitive data leakage.
The technical exploitation of this vulnerability occurs through carefully crafted malicious GIF images that manipulate the colorMap arrays during the decoding process. When the GD library processes such malformed GIF files, it attempts to read from uninitialized memory locations that happen to contain data from the program stack. This particular flaw allows an attacker to potentially extract approximately 700 bytes of data from the top of the stack, which may contain sensitive information such as stack canaries, memory addresses, cryptographic keys, or other confidential data structures. The uninitialized nature of these arrays creates a scenario where memory contents from previous operations or program state persist in memory locations that are subsequently accessed during GIF processing. This type of vulnerability aligns with CWE-457: Use of Uninitialized Variable, which specifically addresses the dangers of accessing variables that have not been properly initialized, and can be categorized under the broader class of information disclosure vulnerabilities in the CWE taxonomy.
The operational impact of CVE-2017-7890 extends beyond simple information disclosure, as it can serve as a stepping stone for more sophisticated attacks. An attacker who successfully exploits this vulnerability gains access to potentially sensitive stack data that could reveal memory layout information, helping in bypassing security protections such as stack canaries or address space layout randomization. The vulnerability's exploitation is particularly concerning in web environments where users can upload or otherwise provide content that gets processed by the affected libraries. This creates a potential attack surface where malicious GIF files could be uploaded to web servers and processed, leading to information leakage from the server's memory. The attack vector is straightforward yet effective, as it requires only the ability to upload or otherwise provide a specially crafted GIF file to the vulnerable system. The vulnerability's classification under ATT&CK technique T1059.007 (Command and Scripting Interpreter: PowerShell) is relevant when considering how such information disclosure could be leveraged for further attacks, though the vulnerability itself is primarily a memory corruption issue.
Mitigation strategies for CVE-2017-7890 focus on immediate patching and implementation of input validation controls. The most effective solution involves upgrading PHP installations to versions 5.6.31 or 7.1.7 and later, where the library has been patched to properly initialize the colorMap arrays before use. Organizations should implement comprehensive patch management processes to ensure all affected systems are updated promptly. Additionally, implementing strict input validation and sanitization for all uploaded files, particularly image files, can provide defense in depth against exploitation attempts. Web application firewalls and content filtering systems can be configured to detect and block suspicious GIF file patterns that might indicate malicious intent. The vulnerability also underscores the importance of secure coding practices and thorough code review processes, particularly when dealing with memory management and initialization of data structures. Regular security assessments and penetration testing should include evaluation of graphics processing libraries and their handling of user-provided content to identify similar uninitialized variable issues that could potentially lead to information disclosure or other security vulnerabilities.