CVE-2021-21836 in Advanced Content
Summary
by MITRE • 08/25/2021
An exploitable integer overflow vulnerability exists within the MPEG-4 decoding functionality of the GPAC Project on Advanced Content library v1.0.1. A specially crafted MPEG-4 input using the “ctts” FOURCC code can cause an integer overflow due to unchecked arithmetic resulting in a heap-based buffer overflow that causes memory corruption. An attacker can convince a user to open a video to trigger this vulnerability.
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Analysis
by VulDB Data Team • 08/29/2021
The CVE-2021-21836 vulnerability represents a critical security flaw within the GPAC Project Advanced Content library version 1.0.1, specifically affecting MPEG-4 decoding operations. This vulnerability resides in the handling of the ctts FOURCC code within MPEG-4 container files, which serves as a crucial component for timestamp information in video streams. The flaw manifests when the library processes malformed video files that contain specially crafted ctts atom structures, creating a scenario where integer arithmetic operations exceed their defined boundaries without proper validation mechanisms.
The technical implementation of this vulnerability stems from an integer overflow condition that occurs during the processing of timestamp data within the MPEG-4 file structure. When the GPAC library encounters a malformed ctts atom, it performs arithmetic operations on unsigned integers without adequate bounds checking, leading to a situation where the calculated buffer size exceeds the maximum representable value for the integer type. This overflow results in a heap-based buffer overflow condition where subsequent memory operations write data beyond the allocated buffer boundaries, corrupting adjacent memory regions and potentially allowing arbitrary code execution.
The operational impact of this vulnerability extends beyond simple memory corruption, as it creates a potential attack vector for remote code execution within applications that utilize the GPAC library for multimedia processing. The attack requires a victim to open a specially crafted malicious video file, making this a user-initiated exploit that could be delivered through various attack vectors including phishing campaigns, malicious websites, or compromised media sharing platforms. The vulnerability affects any application or system that relies on the GPAC library for video processing, including media players, content management systems, and streaming applications.
Security researchers have classified this vulnerability under CWE-190, which specifically addresses integer overflow conditions, and it aligns with ATT&CK technique T1203, representing exploitation of software vulnerabilities through malicious file delivery. The flaw demonstrates a classic example of insufficient input validation in multimedia processing libraries, where the assumption that input files conform to expected standards fails to account for malformed or maliciously crafted content. The integer overflow occurs in the context of buffer management within the library's memory allocation routines, where the computed buffer size becomes negative or exceeds maximum limits, causing subsequent heap operations to write beyond allocated memory regions.
Mitigation strategies for CVE-2021-21836 require immediate patching of the GPAC library to version 1.0.2 or later, which includes proper bounds checking and integer overflow protection mechanisms. Organizations should also implement defensive measures such as sandboxing multimedia processing applications, restricting file type handling, and deploying network-based intrusion detection systems to monitor for suspicious file transfers. Additionally, user education regarding the risks of opening untrusted multimedia files remains crucial, as the vulnerability requires user interaction to be exploited. The fix implemented by the GPAC project includes enhanced input validation for atom sizes and proper overflow detection in arithmetic operations, addressing the root cause of the vulnerability while maintaining compatibility with legitimate MPEG-4 file formats.