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attack-surface.md

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Attack Surface Analysis for ffmpegwasm/ffmpeg.wasm

Attack Surface: Codec Parsing Vulnerabilities

  • Description: Exploits targeting vulnerabilities in specific audio or video codec implementations within FFmpeg. These vulnerabilities often involve memory corruption (buffer overflows, use-after-free, etc.) during the decoding process. This is the most direct and likely attack vector against ffmpeg.wasm.
  • How ffmpeg.wasm Contributes: ffmpeg.wasm includes a vast array of codecs, each with its own complex parsing logic, increasing the likelihood of undiscovered vulnerabilities. The library itself is the source of the risk.
  • Example: A crafted H.264 video file with a malformed NAL unit triggers a buffer overflow in the H.264 decoder within ffmpeg.wasm, leading to arbitrary code execution within the WebAssembly sandbox. Another example: a specially designed FLAC audio file exploits a heap overflow in the FLAC decoder.
  • Impact: Potential for arbitrary code execution within the WebAssembly sandbox, leading to data breaches (of data within the sandbox), denial of service, or potentially further exploitation if combined with browser vulnerabilities.
  • Risk Severity: Critical
  • Mitigation Strategies:
    • Codec Whitelisting: Strictly limit the supported codecs to the absolute minimum required. Reject any input using unsupported codecs.
    • Input Validation: Perform preliminary checks on the input file's structure before passing it to ffmpeg.wasm.
    • Regular Updates: Keep ffmpeg.wasm updated to the latest version.
    • Fuzzing: Conduct regular fuzzing of the specific codecs used.
    • Memory Limits: Enforce strict memory limits on the WebAssembly module.

Attack Surface: Container Format Parsing Vulnerabilities

  • Description: Exploits targeting vulnerabilities in the parsing of container formats (e.g., MP4, AVI, MKV, WebM) used to encapsulate media streams. These vulnerabilities can also lead to memory corruption.
  • How ffmpeg.wasm Contributes: ffmpeg.wasm directly implements the parsing logic for numerous container formats, making it the source of this vulnerability class.
  • Example: A malformed MP4 file with an invalid atom structure triggers an out-of-bounds read in the MP4 demuxer within ffmpeg.wasm. Another example: a crafted AVI file with an oversized chunk causes a buffer overflow.
  • Impact: Similar to codec vulnerabilities: potential for arbitrary code execution within the WebAssembly sandbox, denial of service, or data exfiltration (within the sandbox).
  • Risk Severity: Critical
  • Mitigation Strategies:
    • Container Format Whitelisting: Strictly limit the supported container formats.
    • Input Validation: Perform basic structural checks on the container format before decoding.
    • Regular Updates: Keep ffmpeg.wasm updated.
    • Fuzzing: Fuzz the container format parsers.
    • Memory Limits: Enforce strict memory limits.

Attack Surface: Command String Manipulation (If Applicable - High Risk Subset)

  • Description: Specifically when the application allows user input to influence the FFmpeg command string and this input is used to construct file paths or protocol handlers within the ffmpeg.wasm virtual file system. This is a subset of the broader command string manipulation issue, focusing on the direct interaction with ffmpeg.wasm.
  • How ffmpeg.wasm Contributes: While ffmpeg.wasm doesn't execute shell commands, its internal file handling and protocol support can be abused if the application improperly handles user-provided filenames or paths. The vulnerability arises from the interaction between the application's (poor) input handling and ffmpeg.wasm's features.
  • Example: An application allows users to specify an output filename within the virtual file system. An attacker provides a filename like "../sensitive_data.txt" or "concat:file1|file2" (using FFmpeg's concat protocol) to attempt to access or manipulate files outside the intended output directory within the WebAssembly sandbox.
  • Impact: Potential for arbitrary file access within the WebAssembly virtual file system, leading to data leaks or modification of data within the sandbox.
  • Risk Severity: High
  • Mitigation Strategies:
    • Avoid Direct Command String Construction: Never directly construct FFmpeg command strings from user input.
    • Use API for Parameter Setting: Use the ffmpeg.wasm API to set options programmatically.
    • Strict Input Sanitization: Implement extremely strict sanitization and validation of any user input that affects file paths or protocol handlers. Whitelist allowed characters and patterns.
    • Virtual File System Isolation: Ensure that FFmpeg only has access to the intended files within the virtual file system. Use a dedicated, isolated directory for user-provided output.