CVE-2019-20140 in libsixel
Summary
by MITRE
An issue was discovered in libsixel 1.8.4. There is a heap-based buffer overflow in the function gif_out_code at fromgif.c.
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Analysis
by VulDB Data Team • 03/18/2024
The vulnerability identified as CVE-2019-20140 represents a critical heap-based buffer overflow within the libsixel library version 1.8.4, specifically within the gif_out_code function located in the fromgif.c source file. This issue arises during the processing of gif image files and demonstrates a classic memory safety flaw that can lead to arbitrary code execution. The libsixel library serves as a graphics library for converting images to sixel graphics format, commonly used in terminal-based image display applications and legacy systems requiring character-based graphics rendering. The vulnerability manifests when the library processes malformed gif input files, creating conditions where attacker-controlled data can overwrite adjacent memory regions beyond the allocated buffer boundaries.
The technical flaw stems from inadequate input validation and buffer size management within the gif_out_code function, which handles the output code generation during gif file decoding. When processing gif data streams, the function fails to properly bounds-check the data being written to heap-allocated memory regions, allowing for memory corruption that can overwrite adjacent heap chunks. This heap-based overflow creates opportunities for attackers to manipulate program execution flow through controlled memory corruption, potentially leading to privilege escalation or remote code execution depending on the execution context. The vulnerability aligns with CWE-121, heap-based buffer overflow, and represents a direct violation of secure coding practices that emphasize proper memory management and input validation. The attack surface is particularly concerning as it affects applications that rely on libsixel for image processing, including terminal-based image viewers, legacy communication systems, and various graphics conversion utilities.
The operational impact of this vulnerability extends across multiple attack vectors and system configurations where libsixel is implemented. Applications using the affected library may experience crashes, data corruption, or complete system compromise when processing maliciously crafted gif files. The vulnerability is particularly dangerous in environments where users can upload or process untrusted image content, such as web applications, file sharing systems, or network services that handle image file processing. From an adversarial perspective, this flaw maps to several ATT&CK techniques including TA0002 (Execution) and TA0005 (Defense Evasion) as attackers can leverage the overflow to execute arbitrary code and potentially bypass security controls. The exploitability of this vulnerability increases significantly in contexts where the library is used in server applications or systems that automatically process user-provided content without proper sanitization.
Mitigation strategies for CVE-2019-20140 require immediate patching of the libsixel library to version 1.8.5 or later, which contains the necessary fixes for the buffer overflow condition. System administrators should implement comprehensive input validation for all gif file processing operations and deploy sandboxing mechanisms to isolate image processing functions from core system processes. Additional protective measures include enabling heap protection mechanisms such as stack canaries, address space layout randomization, and data execution prevention features to reduce exploit reliability. Organizations should conduct thorough vulnerability assessments to identify all systems utilizing affected versions of libsixel and implement monitoring for suspicious file processing activities. The fix addresses the root cause by implementing proper bounds checking and memory allocation validation within the gif_out_code function, ensuring that all data written to heap memory remains within properly allocated boundaries. Regular security updates and dependency management practices should be enforced to prevent similar vulnerabilities from emerging in other components of the software supply chain.