CVE-2017-16803 in libav
Summary
by MITRE
In Libav through 11.11 and 12.x through 12.1, the smacker_decode_tree function in libavcodec/smacker.c does not properly restrict tree recursion, which allows remote attackers to cause a denial of service (bitstream.c:build_table() out-of-bounds read and application crash) via a crafted Smacker stream.
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Analysis
by VulDB Data Team • 01/10/2023
The vulnerability identified as CVE-2017-16803 represents a critical denial of service flaw within the Libav multimedia framework, specifically affecting versions through 11.11 and 12.x through 12.1. This issue resides in the smacker_decode_tree function located in libavcodec/smacker.c, where improper tree recursion handling creates exploitable conditions that can be triggered remotely through maliciously crafted Smacker video streams. The flaw manifests as an out-of-bounds read condition within the bitstream.c:build_table() function, ultimately leading to application crashes that effectively render the affected systems unavailable to legitimate users.
The technical implementation of this vulnerability stems from insufficient validation of recursive tree structures during Smacker stream decoding processes. When a maliciously constructed Smacker stream is processed, the smacker_decode_tree function fails to properly limit the depth or complexity of the recursive tree traversal, allowing attackers to craft input data that causes excessive recursion patterns. This recursive behavior eventually leads to buffer overflows and memory access violations within the build_table() function, where the application attempts to read beyond allocated memory boundaries. The vulnerability operates at the codec level, making it particularly dangerous as it can be triggered during normal media playback operations without requiring special privileges or user interaction beyond loading the malicious content.
From an operational perspective, this vulnerability poses significant risks to systems that utilize Libav for multimedia processing, including media servers, content delivery networks, and applications handling user-generated media content. The remote exploitation capability means that attackers can trigger denial of service conditions from anywhere on the network, potentially affecting large-scale deployments where multiple users might be simultaneously impacted. The out-of-bounds read condition creates a deterministic crash scenario that can be repeatedly exploited, making it an effective tool for service disruption attacks. According to CWE classification, this vulnerability maps to CWE-121, heap-based buffer overflow, and CWE-674, uncontrolled recursion, while the ATT&CK framework categorizes this under T1499.004, Network Denial of Service, and T1059.007, Command and Scripting Interpreter, through the exploitation of media processing components.
Mitigation strategies for CVE-2017-16803 should focus on immediate version updates to Libav 12.2 or later, which contain the necessary patches to address the recursive tree handling issue. Organizations should implement input validation measures that limit the complexity and depth of Smacker stream processing, particularly in environments where untrusted media content is processed. Network segmentation and content filtering mechanisms can help prevent malicious streams from reaching vulnerable systems, while application-level sandboxing can limit the impact if exploitation occurs. Security monitoring should include detection of unusual memory access patterns and recursive function calls that may indicate exploitation attempts. Additionally, regular vulnerability assessments of multimedia processing components and adherence to secure coding practices for recursive algorithms should be implemented to prevent similar issues in future development cycles.