Streamlink is a CLI utility which pipes video streams from various services into a video player. Prior to 8.4.0, Streamlink's HLS and DASH parsers do not validate the URI scheme of segment entries and other resources. A remote .m3u8 HLS playlist or .mpd DASH manifest can list file:///path/to/file as a segment, and streamlink will read that local file and write its contents to the output stream. This vulnerability is fixed in 8.4.0.