CVE-2017-8906 in x265
Summary
by MITRE
An integer underflow vulnerability exists in pixel-a.asm, the x86 assembly code for planeClipAndMax() in MulticoreWare x265 through 2.4, as used by the x265_encoder_encode dependency in libbpg and other products. A small picture can cause an integer underflow, which leads to a Denial of Service in the process of encoding.
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Analysis
by VulDB Data Team • 12/06/2022
The integer underflow vulnerability in CVE-2017-8906 represents a critical flaw within the x265 video encoding library that affects multiple downstream applications including libbpg. This vulnerability specifically manifests in the pixel-a.asm assembly file where the planeClipAndMax() function processes video frame data during the encoding process. The flaw occurs when handling extremely small picture dimensions that trigger an integer underflow condition during arithmetic operations. Such conditions typically arise when a signed integer variable is decremented below its minimum representable value, causing it to wrap around to a large positive number. The affected version range through 2.4 indicates this vulnerability has persisted across multiple releases, suggesting a fundamental design flaw in the boundary condition handling for frame processing operations.
The technical implementation of this vulnerability stems from inadequate input validation within the assembly code that processes video frame clipping operations. When the encoding process encounters a picture with dimensions that are below the expected minimum thresholds, the integer underflow causes the system to allocate memory or perform operations using invalid parameters. This particular flaw operates at the assembly level where the x86 instructions handle pixel data processing, making it particularly dangerous as it bypasses higher-level language protections. The underflow condition in the planeClipAndMax() function specifically impacts how the system calculates boundaries and clipping parameters for video frames, leading to unpredictable memory access patterns. According to CWE-191, this represents an integer underflow condition where an integer is decremented below its minimum value, creating a scenario where subsequent operations may access memory outside allocated bounds.
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. In the context of the x265 encoder, this vulnerability can cause processes to crash or become unresponsive when processing malformed video inputs, effectively rendering the encoding service unavailable to legitimate users. The vulnerability is particularly concerning because it affects widely used video processing libraries, meaning that any application relying on x265 for video encoding could be compromised. From an attacker perspective, this vulnerability aligns with ATT&CK technique T1499.004 which involves network denial of service attacks, though in this case it operates as a local denial of service through malformed input processing. The DoS condition can be triggered by sending specially crafted video files with dimensions that cause the integer underflow, making it a straightforward exploitation vector that requires minimal technical expertise.
Mitigation strategies for this vulnerability should focus on both immediate patching and defensive programming practices. The primary solution involves updating to versions of x265 that have addressed this integer underflow issue, as the original vulnerability was resolved through proper input validation and boundary checking. Organizations should implement strict input validation for video encoding processes, ensuring that all frame dimensions are checked against acceptable ranges before processing begins. Additionally, defensive programming techniques such as using unsigned integers where appropriate, implementing proper bounds checking, and employing static analysis tools to identify similar patterns in assembly code should be implemented. The vulnerability highlights the importance of thorough testing with edge cases in multimedia processing libraries, particularly those implemented in assembly code where traditional software protections may not apply. System administrators should also consider implementing monitoring for unusual encoding process behavior and establish incident response procedures for handling potential exploitation attempts targeting this class of vulnerability.