CVE-2016-6128 in GD Graphics Library
Summary
by MITRE
The gdImageCropThreshold function in gd_crop.c in the GD Graphics Library (aka libgd) before 2.2.3, as used in PHP before 7.0.9, allows remote attackers to cause a denial of service (application crash) via an invalid color index.
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Analysis
by VulDB Data Team • 09/13/2022
The gd Graphics Library represents a fundamental component in many web applications and image processing systems, providing essential functionality for handling graphics operations in various programming environments including PHP. This vulnerability specifically targets the gdImageCropThreshold function within the gd_crop.c file, which is responsible for performing image cropping operations with threshold adjustments. The flaw exists in versions prior to 2.2.3 of the library and affects PHP installations before version 7.0.9, creating a significant attack surface for remote adversaries seeking to disrupt service availability. The vulnerability manifests when the function processes invalid color index values, leading to unpredictable behavior that can result in complete application crashes. This represents a classic buffer overread or invalid memory access condition that violates the fundamental principles of secure software design and can be exploited through carefully crafted image data submitted to applications utilizing the affected library.
The technical implementation of this vulnerability stems from inadequate input validation within the gdImageCropThreshold function, which fails to properly validate color index values before attempting to process them. When an attacker submits an image containing an invalid color index, the function proceeds with operations that ultimately lead to memory corruption or access violations. This flaw maps directly to CWE-125, which describes out-of-bounds read conditions, and CWE-787, which covers out-of-bounds write operations. The vulnerability operates at the intersection of multiple attack vectors described in the MITRE ATT&CK framework under T1499, specifically targeting the availability of services through denial of service attacks. The function's failure to implement proper bounds checking or input sanitization creates a path where malformed data can bypass normal execution flow and trigger system instability.
The operational impact of this vulnerability extends beyond simple application crashes to encompass broader service disruption scenarios that can affect entire web platforms. Remote attackers can exploit this weakness by uploading or submitting image files containing malicious color index values, causing the target application to crash and potentially leading to extended downtime. In web server environments where PHP applications process user-uploaded images, this vulnerability can be leveraged to create cascading failures that impact multiple services simultaneously. The denial of service nature of the exploit means that legitimate users may be unable to access services while the application recovers from crashes, potentially leading to business disruption and reputational damage. The vulnerability is particularly concerning because it requires no elevated privileges to exploit and can be triggered through normal web application workflows.
Mitigation strategies for this vulnerability must address both immediate patching requirements and broader architectural considerations. Organizations should prioritize updating their libgd installations to version 2.2.3 or later, while simultaneously upgrading PHP installations to version 7.0.9 or higher to eliminate the attack surface. Additionally, implementing input validation measures at the application level can provide defense-in-depth protection against malformed image data. Security practitioners should consider deploying web application firewalls that can detect and block suspicious image upload patterns, while also implementing proper error handling and application restart mechanisms to minimize service disruption. The vulnerability highlights the importance of maintaining up-to-date third-party libraries and demonstrates how seemingly minor flaws in graphics processing components can create significant security risks. Regular security assessments and vulnerability scanning should include checks for outdated graphics libraries, as these components often receive less attention than core system components despite their critical role in application security.