CVE-2018-7557 in FFmpeg
Summary
by MITRE
The decode_init function in libavcodec/utvideodec.c in FFmpeg through 3.4.2 allows remote attackers to cause a denial of service (Out of array read) via an AVI file with crafted dimensions within chroma subsampling data.
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Analysis
by VulDB Data Team • 02/16/2023
The vulnerability identified as CVE-2018-7557 represents a critical out-of-bounds memory read flaw within the FFmpeg multimedia framework, specifically affecting versions through 3.4.2. This issue resides in the decode_init function located within the libavcodec/utvideodec.c source file, which handles video decoding operations for various formats including AVI files. The vulnerability manifests when processing AVI containers that contain maliciously crafted chroma subsampling data with manipulated dimension parameters, creating a scenario where the software attempts to access memory locations beyond the allocated array boundaries. Such flaws in multimedia processing libraries pose significant risks as they can be exploited through routine media file handling operations, making them particularly dangerous in environments where users might encounter untrusted content.
The technical execution of this vulnerability involves the manipulation of chroma subsampling parameters within AVI file headers, specifically targeting the dimension fields that define how color information is sampled and stored in video frames. When FFmpeg's utvideodec.c module processes these crafted dimensions, the decode_init function fails to properly validate the input parameters against the actual allocated memory structures. This leads to an array bounds violation where the software attempts to read data from memory locations that do not correspond to the expected array elements. The flaw is classified as a CWE-129 weakness, representing an insufficient validation of array index values, and falls under the broader category of CWE-787, which encompasses out-of-bounds read conditions that can result in information disclosure or system instability.
From an operational perspective, this vulnerability creates a significant denial of service risk that can be exploited by remote attackers through the simple act of distributing malicious AVI files. The impact extends beyond mere service disruption as the out-of-bounds read can potentially lead to system crashes, application instability, or in more severe cases, information leakage from adjacent memory regions. The attack surface is particularly broad since AVI files are widely used across various platforms and applications, making this vulnerability applicable to numerous software systems that rely on FFmpeg for media processing. The vulnerability aligns with ATT&CK technique T1203, which involves the exploitation of software vulnerabilities for denial of service, and represents a classic example of how multimedia processing libraries can become attack vectors when proper input validation is absent.
Mitigation strategies for CVE-2018-7557 require immediate patching of affected FFmpeg installations to version 3.4.3 or later, which contains the necessary fixes to properly validate chroma subsampling dimensions before processing. Organizations should implement comprehensive media file validation procedures that include signature checks and automated scanning of incoming content for malformed structures. Network security measures should incorporate deep packet inspection capabilities to identify and block suspicious AVI file transfers, while application-level defenses should include memory protection mechanisms such as stack canaries and address space layout randomization. System administrators should also consider implementing sandboxing techniques for media processing operations and regularly update their FFmpeg installations to ensure protection against similar vulnerabilities. The fix implemented in the patched versions addresses the root cause by adding proper bounds checking and input validation to ensure that chroma subsampling parameters fall within acceptable ranges before being processed by the decode_init function.