CVE-2015-6818 in FFmpeg
Summary
by MITRE
The decode_ihdr_chunk function in libavcodec/pngdec.c in FFmpeg before 2.7.2 does not enforce uniqueness of the IHDR (aka image header) chunk in a PNG image, which allows remote attackers to cause a denial of service (out-of-bounds array access) or possibly have unspecified other impact via a crafted image with two or more or these chunks.
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Analysis
by VulDB Data Team • 06/14/2022
The vulnerability identified as CVE-2015-6818 resides within the FFmpeg multimedia framework's handling of Portable Network Graphics files, specifically in the pngdec.c module where the decode_ihdr_chunk function fails to properly validate the uniqueness of IHDR chunks. This flaw represents a classic case of insufficient input validation that can be exploited to manipulate the parsing behavior of the library. The IHDR chunk in PNG format is critical as it contains essential image metadata including dimensions, color type, and compression method that the decoder relies upon for proper image reconstruction. When multiple IHDR chunks exist within a single PNG file, the decoder's failure to enforce chunk uniqueness creates a scenario where subsequent chunks overwrite or corrupt the initial header data, leading to unpredictable behavior in the parsing process.
The technical exploitation of this vulnerability occurs through the manipulation of PNG file structures to include duplicate IHDR chunks, which causes the decoder to process multiple header definitions for the same image. This condition leads to out-of-bounds array access when the decoder attempts to process the second IHDR chunk, as it has already allocated memory based on the first header's specifications. The improper handling of chunk ordering and validation creates a memory corruption scenario that can result in application crashes, denial of service conditions, or potentially more severe consequences depending on the execution environment. The vulnerability falls under CWE-129, which addresses insufficient validation of length of input buffers, and specifically relates to improper handling of duplicate or conflicting data structures within file format parsers.
From an operational impact perspective, this vulnerability affects any system that utilizes FFmpeg for PNG image processing, including web servers, media processing applications, and content management systems that accept user-uploaded images. Attackers can craft malicious PNG files that trigger the denial of service condition when processed by vulnerable FFmpeg versions, effectively causing service disruption without requiring authentication or complex exploitation techniques. The vulnerability's remote exploitation capability means that web applications processing PNG uploads are particularly at risk, as attackers can simply upload a malicious file to cause system instability. The unspecified other impacts mentioned in the description suggest that under certain conditions, this memory corruption could potentially lead to arbitrary code execution, though this would require additional exploitation vectors and is not guaranteed by the vulnerability alone.
Mitigation strategies for CVE-2015-6818 involve immediate patching of FFmpeg installations to version 2.7.2 or later, where the fix enforces proper validation of IHDR chunk uniqueness during PNG parsing. Organizations should also implement input validation measures at the application level, including PNG file format validation and content scanning before processing user uploads. Network-based defenses can include implementing file type checks and content filtering to prevent malicious PNG files from reaching systems that process them. The vulnerability demonstrates the importance of robust input validation in multimedia libraries and aligns with ATT&CK technique T1203, which covers exploitation of software vulnerabilities for denial of service, and T1068, which covers the use of privileges and capabilities for system compromise. System administrators should also consider implementing automated monitoring for unusual processing patterns or crashes that might indicate exploitation attempts, and maintain up-to-date security patches across all multimedia processing components to prevent similar issues in the future.