CVE-2021-40656 in libsixel
Summary
by MITRE • 04/08/2022
libsixel before 1.10 is vulnerable to Buffer Overflow in libsixel/src/quant.c:867.
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Analysis
by VulDB Data Team • 04/13/2022
The vulnerability identified as CVE-2021-40656 affects libsixel versions prior to 1.10 and represents a critical buffer overflow condition within the image processing library. This flaw exists in the quant.c source file at line 867, specifically within the color quantization functionality that processes image data for terminal display compatibility. The libsixel library serves as a crucial component for rendering graphics in terminal environments, enabling applications to display images through sixel graphics protocol which is particularly relevant in legacy terminal systems and specialized applications requiring graphical output within text-based interfaces.
The technical implementation of this vulnerability stems from inadequate input validation and memory management during the color quantization process. When processing certain malformed or specially crafted image files, the library fails to properly bounds-check array accesses, allowing an attacker to write beyond allocated memory buffers. This condition occurs during the processing of image color palettes where the code does not adequately verify the size of input data against the allocated buffer space. The flaw manifests as a classic stack-based buffer overflow, potentially enabling arbitrary code execution or system instability when the vulnerable library processes malicious input files. The vulnerability directly maps to CWE-121, which describes stack-based buffer overflow conditions, and represents a significant security risk in environments where libsixel processes untrusted image data.
The operational impact of this vulnerability extends across multiple domains where libsixel is integrated into applications and systems. Terminal-based applications that utilize sixel graphics for displaying charts, diagrams, or images face potential compromise when processing untrusted input. This includes specialized terminal emulators, system monitoring tools, and legacy applications that depend on sixel rendering capabilities. The vulnerability can be exploited through various attack vectors including email attachments, web downloads, or file transfers containing maliciously crafted image files. Systems running vulnerable versions of libsixel are at risk of complete compromise, as the buffer overflow could allow attackers to execute arbitrary code with the privileges of the affected application, potentially leading to privilege escalation or complete system control. The attack surface is particularly concerning given that many terminal-based applications do not perform extensive input sanitization, making them susceptible to exploitation.
Mitigation strategies for CVE-2021-40656 focus primarily on immediate version updates and comprehensive system hardening measures. Organizations should prioritize upgrading to libsixel version 1.10 or later, which includes patched implementations of the color quantization algorithm with proper bounds checking and memory management. Additionally, implementing input validation controls at application layers that utilize libsixel can provide defense-in-depth protection against malformed input processing. Security configurations should include disabling unnecessary sixel graphics processing capabilities where possible, implementing strict file type validation for image processing, and monitoring for unusual memory access patterns that might indicate exploitation attempts. System administrators should also consider implementing application whitelisting policies that restrict execution of vulnerable applications and establish network segmentation to limit potential lateral movement if exploitation occurs. The remediation process should include thorough testing of updated libraries in controlled environments before deployment to production systems, ensuring that the patch does not introduce compatibility issues with existing applications that depend on libsixel functionality.