CVE-2017-13666 in x265
Summary
by MITRE
An integer underflow vulnerability exists in pixel-a.asm, the x86 assembly code for planeClipAndMax() in MulticoreWare x265 through 2.5, as used in libbpg and other products. A small height value can cause an integer underflow, which leads to a crash. This is a different vulnerability than CVE-2017-8906.
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Analysis
by VulDB Data Team • 12/16/2022
The vulnerability identified as CVE-2017-13666 represents a critical integer underflow flaw within the x265 video encoding library's assembly code implementation. This issue specifically affects the pixel-a.asm file which contains the x86 assembly implementation of the planeClipAndMax() function. The vulnerability was discovered in MulticoreWare x265 versions through 2.5 and subsequently impacted other products including libbpg that utilize this encoding library. The flaw manifests when processing video frames with unusually small height values, creating a scenario where integer arithmetic operations result in unexpected underflow conditions that ultimately cause application crashes.
The technical root cause of this vulnerability lies in the improper handling of integer values during the plane clipping and maximum value computation operations within the assembly code. When the height parameter of a video frame falls below certain thresholds, the arithmetic operations involved in the planeClipAndMax() function fail to properly validate input parameters, leading to integer underflow conditions. This type of vulnerability falls under the CWE-191 category of Integer Underflow (Wrap or Wraparound) as defined by the Common Weakness Enumeration framework. The underflow condition occurs when a signed integer operation results in a value that cannot be represented within the integer's range, causing the value to wrap around to a negative number or zero, which then propagates through subsequent operations.
The operational impact of this vulnerability is significant as it can be exploited to cause denial of service conditions through controlled crashes of applications that rely on the x265 library for video processing. Attackers can craft specially formatted video files with minimal height values to trigger the integer underflow, resulting in application instability and potential system crashes. This vulnerability is particularly concerning in environments where video processing applications are exposed to untrusted input, such as web applications, media processing servers, or content management systems that handle user-uploaded video content. The vulnerability affects a wide range of products that utilize x265 as their underlying video encoding engine, making it a widespread concern across multiple software ecosystems. From an adversarial perspective, this vulnerability maps to the ATT&CK technique T1499.004 for Network Denial of Service, as it can be used to disrupt video processing services through controlled application crashes.
The remediation approach for this vulnerability requires immediate patching of affected x265 library versions to address the integer underflow condition in the planeClipAndMax() function. Developers should implement proper input validation to ensure that height parameters are within expected ranges before performing arithmetic operations. The fix involves adding bounds checking to prevent the underflow condition from occurring, typically by validating that input parameters are positive integers before proceeding with calculations. Additionally, code review processes should be enhanced to identify similar patterns in other assembly code components that might be susceptible to similar integer overflow or underflow conditions. Organizations should also implement proper error handling and crash recovery mechanisms to mitigate the impact of any remaining vulnerabilities in their video processing pipelines. This vulnerability highlights the importance of rigorous testing for integer arithmetic operations in low-level assembly code, particularly in security-critical applications where input validation and boundary checking are essential for maintaining system stability and preventing exploitation.