CVE-2019-3573 in libsixel
Summary
by MITRE
In libsixel v1.8.2, there is an infinite loop in the function sixel_decode_raw_impl() in the file fromsixel.c, as demonstrated by sixel2png.
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Analysis
by VulDB Data Team • 08/25/2025
The vulnerability identified as CVE-2019-3573 represents a critical denial of service flaw within the libsixel library version 1.8.2. This library serves as a crucial component for handling sixel graphics format conversions, particularly in terminal environments where graphical content is rendered using character-based methods. The issue manifests specifically within the sixel_decode_raw_impl() function located in the fromsixel.c source file, which processes raw sixel data streams for conversion to other image formats. The vulnerability becomes apparent when the sixel2png utility attempts to process malformed or specially crafted sixel input data, triggering an exploitable condition that leads to system resource exhaustion.
The technical root cause of this vulnerability lies in inadequate input validation and loop termination logic within the sixel_decode_raw_impl() function. When processing certain malformed sixel data sequences, the decoding algorithm enters an infinite loop where control flow never progresses beyond a specific iteration boundary. This occurs because the function fails to properly validate the data structure or implement proper loop bounds checking during the parsing of sixel graphics parameters. The flaw is classified under CWE-835, which specifically addresses the issue of loops with insufficient termination conditions, making it particularly dangerous in environments where the library processes untrusted input from external sources.
The operational impact of this vulnerability extends beyond simple resource consumption, presenting significant risks to system stability and availability. An attacker could exploit this weakness by providing malicious sixel data to any application leveraging libsixel v1.8.2, including terminal emulators, image conversion tools, or any software that utilizes the library for sixel graphics processing. The infinite loop causes continuous CPU utilization, effectively rendering the target application unresponsive and potentially leading to system-wide resource exhaustion. This makes the vulnerability particularly dangerous in server environments or applications that process user-provided content, where such denial of service attacks could be used to disrupt services or consume excessive computational resources.
From a cybersecurity perspective, this vulnerability aligns with ATT&CK technique T1499.004, which covers resource exhaustion via infinite loops and similar conditions that cause system instability. The flaw demonstrates how seemingly minor input validation deficiencies can be escalated into significant security concerns, particularly in widely-used libraries that form the foundation of numerous applications. Organizations relying on libsixel for graphics processing should consider this vulnerability as a critical risk requiring immediate attention, as it represents a potential attack vector that could be exploited in various contexts where sixel graphics processing occurs. The vulnerability's exploitation requires minimal input crafting effort, making it particularly concerning for applications that process untrusted sixel data from external sources or user uploads.
Mitigation strategies should prioritize immediate library version updates to versions that have addressed this specific infinite loop condition through proper input validation and loop termination mechanisms. System administrators should also implement input sanitization measures for any applications that process sixel graphics, particularly those accepting external or user-provided data. Additionally, monitoring systems should be configured to detect unusual CPU utilization patterns that could indicate exploitation attempts. The fix implemented in subsequent library versions typically involves adding proper bounds checking and validation of sixel data parameters before entering decoding loops, ensuring that all input sequences are properly validated against expected data structures and maximum permissible values.