CVE-2018-20199 in Freeware Advanced Audio Decoder
Summary
by MITRE
A NULL pointer dereference was discovered in ifilter_bank of libfaad/filtbank.c in Freeware Advanced Audio Decoder 2 (FAAD2) 2.8.8. The vulnerability causes a segmentation fault and application crash, which leads to denial of service because adding to windowed output is mishandled in the ONLY_LONG_SEQUENCE case.
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Analysis
by VulDB Data Team • 08/27/2025
The vulnerability identified as CVE-2018-20199 represents a critical NULL pointer dereference flaw within the Freeware Advanced Audio Decoder 2 version 2.8.8 implementation. This issue specifically manifests in the ifilter_bank function located within the libfaad/filtbank.c source file, where the decoder fails to properly validate pointer references during audio processing operations. The flaw exists in the handling of audio frame data when processing specific sequence types, particularly in the ONLY_LONG_SEQUENCE case which governs how audio samples are windowed and processed for output. The vulnerability demonstrates characteristics consistent with CWE-476, which describes NULL pointer dereference conditions that can lead to application instability and system crashes. This type of vulnerability falls under the ATT&CK technique T1499.004 for network denial of service attacks, where adversaries can exploit software flaws to disrupt service availability through controlled input manipulation.
The technical implementation of this vulnerability stems from inadequate input validation within the audio decoding pipeline where the decoder attempts to access memory locations through null pointers during the windowed output processing phase. When the decoder encounters audio data that triggers the ONLY_LONG_SEQUENCE processing path, it fails to properly initialize or verify pointer values before attempting memory access operations. This results in a segmentation fault that terminates the application process and causes a complete denial of service condition. The flaw is particularly dangerous because it can be triggered through malformed audio input files that contain specific bit patterns or sequence configurations that force the decoder into the vulnerable code path. The windowed output addition operations become invalid when the pointer reference remains uninitialized, leading directly to memory access violations that crash the entire decoding process.
The operational impact of CVE-2018-20199 extends beyond simple application crashes to create significant security and availability concerns for systems relying on FAAD2 for audio processing. Any application or service that utilizes FAAD2 as a backend audio decoder becomes vulnerable to remote denial of service attacks when processing untrusted audio content. This includes media servers, streaming platforms, audio processing applications, and embedded systems that handle multimedia content. The vulnerability can be exploited by attackers who craft malicious audio files designed to trigger the specific sequence conditions that lead to the NULL pointer dereference. The effects are immediate and severe, as the application crashes and must be restarted to resume normal operation, potentially disrupting services for extended periods. This type of vulnerability particularly affects systems where audio processing is critical for user experience and where service availability is paramount for business operations.
Mitigation strategies for CVE-2018-20199 should prioritize immediate software updates to FAAD2 version 2.8.9 or later, which contain the necessary patches to address the NULL pointer dereference issue. Organizations should implement input validation measures that sanitize all audio content before processing, including checking for malformed sequences and implementing robust error handling routines that prevent the decoder from entering vulnerable code paths. Network-based defenses should include content filtering mechanisms that identify and block suspicious audio file patterns that could trigger the vulnerability. System administrators should also consider implementing application sandboxing and process isolation techniques to limit the impact of potential exploitation attempts. The fix implemented in the patched version addresses the core issue by ensuring proper pointer initialization and validation before memory access operations, specifically within the ONLY_LONG_SEQUENCE handling code. Security monitoring should include detection of application crashes and abnormal termination patterns that could indicate exploitation attempts, with logging and alerting mechanisms configured to track such incidents. Additionally, regular vulnerability assessments should be conducted to identify similar patterns in other audio processing libraries and ensure comprehensive protection against related denial of service threats.