Impact

Active Storage's VideoPreviewer, VideoAnalyzer and AudioAnalyzer run ffmpeg and ffprobe on a blob's contents without restricting the input format, so FFmpeg selects a demuxer by probing the file. A blob created through a direct upload keeps the content type the client declared, so a client can store an HLS playlist under a video/* or audio/* type. When that blob is previewed or analyzed, the hls demuxer opens the file:// segments that the playlist names, and the preview image or the stored metadata could disclose a local media file that the application process can read and whose path the attacker knows.

Applications that accept uploads from untrusted users and generate video previews, or analyze video or audio attachments with FFmpeg, are affected.

Details

The fixed releases limit the formats FFmpeg may open to common audio and video containers, configured by the new config.active_storage.ffmpeg_allowed_formats setting. Files in other formats no longer preview or receive media metadata; the Active Storage CHANGELOG names some of them and shows how to add a format to the list.

Workarounds

Applications that cannot upgrade can stop Active Storage from running FFmpeg on uploaded files by removing ActiveStorage::Previewer::VideoPreviewer from config.active_storage.previewers, and ActiveStorage::Analyzer::VideoAnalyzer and ActiveStorage::Analyzer::AudioAnalyzer from config.active_storage.analyzers.

Releases

The fixed releases are available at the normal locations.

Patches for those who cannot upgrade at once:

• 8.1: https://github.com/rails/rails/commit/3b1f2627538dc597cd973530c08514a26c2b0282.patch
• 8.0: https://github.com/rails/rails/commit/69aff80d115bbb9d1e82fd6c6f2fd3ddeafb958b.patch

Credit

This issue was responsibly reported by HackerOne researcher w3rmut.