CVE-2020-18778 in libav
Summary
by MITRE • 08/24/2021
In Libav 12.3, there is a heap-based buffer over-read in vc1_decode_p_mb_intfi in vc1_block.c that allows an attacker to cause denial-of-service via a crafted file.
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Analysis
by VulDB Data Team • 09/10/2025
The vulnerability CVE-2020-18778 represents a critical heap-based buffer over-read flaw within the Libav multimedia framework version 12.3. This issue specifically affects the vc1_decode_p_mb_intfi function located in the vc1_block.c source file, which is responsible for decoding VC-1 video frames during playback operations. The flaw arises from insufficient bounds checking when processing malformed VC-1 video streams, creating a scenario where the decoder attempts to read memory beyond the allocated buffer boundaries. This type of vulnerability falls under the Common Weakness Enumeration category CWE-125, which describes out-of-bounds read conditions that can lead to unpredictable behavior and system instability.
The technical execution of this vulnerability occurs when an attacker crafts a malicious VC-1 video file that contains malformed data structures within the video frame headers or block definitions. During normal video decoding operations, the vc1_decode_p_mb_intfi function processes macroblock data and performs interlaced frame interpolation calculations. However, due to the lack of proper input validation and buffer boundary checks, the function accesses memory locations that extend beyond the intended buffer limits. This over-read condition can result in the decoder reading uninitialized memory, previously freed memory regions, or data belonging to other processes running within the same memory space. The vulnerability is particularly dangerous because it can be triggered through standard media playback operations without requiring any special privileges or user interaction beyond opening the malicious file.
The operational impact of CVE-2020-18778 extends beyond simple denial-of-service conditions to potentially enable more sophisticated attacks depending on the execution environment. When exploited, the buffer over-read causes the Libav-based application to crash or become unresponsive, effectively rendering the media player or streaming application unusable. This denial-of-service scenario can be particularly impactful in server environments where media processing applications handle multiple concurrent streams, as a single malicious file can bring down entire media processing pipelines. The vulnerability affects a wide range of applications that utilize Libav for video decoding, including media players, streaming servers, content management systems, and digital signage solutions. In the context of the ATT&CK framework, this vulnerability maps to the T1203 technique for legitimate credential use and potentially to T1499 for network infiltration through compromised media processing systems.
Mitigation strategies for CVE-2020-18778 should focus on immediate patching of affected Libav installations and implementation of robust input validation measures. Organizations utilizing Libav should prioritize updating to versions that contain the patched vc1_decode_p_mb_intfi function with proper buffer boundary checks and input sanitization. Additionally, implementing proper sandboxing techniques for media processing applications can help contain the impact of potential exploitation attempts. Network-level defenses should include content filtering mechanisms that scan media files for known malicious patterns before processing, while application-level protections can involve memory corruption detection tools and runtime monitoring systems. The vulnerability also underscores the importance of proper software supply chain security, as this issue demonstrates how multimedia libraries can become attack vectors when not properly hardened against malformed input data. Security teams should also consider implementing automated vulnerability scanning tools that can detect and flag potentially malicious media files before they are processed by vulnerable applications.