CVE-2020-21677 in libsixel
Summary
by MITRE • 08/11/2021
A heap-based buffer overflow in the sixel_encoder_output_without_macro function in encoder.c of Libsixel 1.8.4 allows attackers to cause a denial of service (DOS) via converting a crafted PNG file into Sixel format.
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Analysis
by VulDB Data Team • 08/15/2021
The heap-based buffer overflow vulnerability identified as CVE-2020-21677 resides within the Libsixel library version 1.8.4, specifically in the sixel_encoder_output_without_macro function located in the encoder.c source file. This vulnerability represents a critical security flaw that can be exploited through improper input validation during image format conversion processes. The Libsixel library serves as a graphics library designed to convert images into Sixel format, which is commonly used for terminal-based image display in legacy systems and certain specialized applications. When processing a specially crafted PNG file, the library fails to properly validate buffer boundaries, leading to memory corruption that can result in application instability and system compromise.
The technical flaw manifests as a heap-based buffer overflow occurring during the conversion of PNG images to Sixel format, where the sixel_encoder_output_without_macro function does not adequately check the size of input data against allocated buffer space. This function processes image data without proper bounds checking, allowing attackers to supply malicious input that exceeds the allocated memory buffer. The vulnerability stems from inadequate memory management practices and lacks proper input sanitization mechanisms. According to CWE classification, this corresponds to CWE-121, which describes heap-based buffer overflow conditions where insufficient bounds checking allows memory corruption through buffer overruns. The overflow occurs in heap memory allocation, making it particularly dangerous as it can lead to arbitrary code execution or complete system crashes.
The operational impact of this vulnerability extends beyond simple denial of service, as it can potentially enable more sophisticated attacks depending on the execution environment. When exploited, the buffer overflow can cause the target application to crash, resulting in service disruption and denial of service conditions for legitimate users. In environments where Libsixel is integrated into critical systems or embedded devices, this vulnerability could provide attackers with a pathway for persistent access or further exploitation. The vulnerability affects systems that utilize Libsixel for image processing, including terminal emulators, legacy system interfaces, and specialized display applications that rely on Sixel graphics output. From an ATT&CK framework perspective, this vulnerability aligns with techniques involving denial of service and privilege escalation through memory corruption, potentially enabling adversaries to gain unauthorized access to systems or disrupt critical services.
Mitigation strategies for CVE-2020-21677 should focus on 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 conduct comprehensive vulnerability assessments to identify all instances where Libsixel is installed and in use, particularly in environments where it processes untrusted input from external sources. Additional protective measures include implementing input validation controls, deploying application sandboxing mechanisms, and establishing monitoring procedures to detect potential exploitation attempts. Organizations should also consider implementing network segmentation and access controls to limit exposure of systems that utilize the vulnerable library. The fix implemented in the patched versions addresses the root cause by introducing proper bounds checking and memory allocation validation within the sixel_encoder_output_without_macro function, ensuring that input data cannot exceed allocated buffer sizes. Regular security updates and patch management procedures should be enforced to prevent similar vulnerabilities from being introduced through outdated library versions.