CVE-2019-9717 in libav
Summary
by MITRE
In Libav 12.3, a denial of service in the subtitle decoder allows attackers to hog the CPU via a crafted video file in Matroska format, because srt_to_ass in libavcodec/srtdec.c has a complex format argument to sscanf.
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Analysis
by VulDB Data Team • 12/26/2023
The vulnerability identified as CVE-2019-9717 represents a significant denial of service weakness within the Libav multimedia framework version 12.3. This issue specifically targets the subtitle decoder component and manifests through crafted video files in Matroska format, creating a scenario where attackers can consume excessive CPU resources and effectively render systems unresponsive. The flaw resides in the srt_to_ass function located within the libavcodec/srtdec.c file, which processes subtitle data during video decoding operations.
The technical root cause of this vulnerability stems from the complex format argument passed to the sscanf function within the srt_to_ass implementation. This design flaw creates a situation where malformed input data can trigger excessive computational overhead during parsing operations. When the subtitle decoder encounters a specially crafted Matroska file containing malicious subtitle data, the sscanf function processes the malformed input in a manner that consumes disproportionate CPU cycles. The complexity of the format string used in the sscanf call creates opportunities for attackers to craft inputs that cause the parsing routine to execute extended processing loops, leading to sustained high CPU utilization.
From an operational impact perspective, this vulnerability enables attackers to perform resource exhaustion attacks against systems processing multimedia content through Libav. The denial of service condition manifests as continuous high CPU usage that can affect system performance, potentially causing legitimate applications to become unresponsive or fail. This vulnerability is particularly concerning in environments where multimedia processing is critical, such as content delivery networks, streaming servers, or media processing pipelines. The attack requires minimal privileges and can be executed through simple file delivery, making it an attractive vector for resource exhaustion attacks.
The vulnerability aligns with CWE-770, which addresses allocation of resources without limits or with inadequate limits, and relates to the broader category of resource exhaustion attacks in cybersecurity. This weakness can be mapped to ATT&CK technique T1499.001, which covers network denial of service attacks, and T1566.001, covering spearphishing attachments, as the attack vector often involves malicious files delivered through email or other means. The issue demonstrates how seemingly innocuous multimedia processing components can become attack vectors when input validation is inadequate.
Mitigation strategies for this vulnerability primarily involve updating to patched versions of Libav where the problematic sscanf usage has been addressed or replaced with more robust input parsing mechanisms. System administrators should implement strict file validation procedures for multimedia content and consider deploying sandboxing mechanisms to isolate multimedia processing operations. Additionally, monitoring for unusual CPU usage patterns during video processing operations can help detect exploitation attempts. Organizations should also consider implementing network segmentation to limit the potential impact of such attacks and ensure that multimedia processing systems are properly isolated from critical business operations.