CVE-2026-77615: Opencast: Stored XSS in Paella player via WebVTT/DFXP caption cue text

The Opencast Paella player renders caption cue text into innerHTML without escaping. The captions canvas clears _captionsContainer.innerHTML and then appends each active cue with _captionsContainer.innerHTML += cue, so HTML inside a WebVTT or DFXP cue becomes live DOM and executes in the Opencast origin.

The caption track is read from any media package element with a captions/* flavor and is served, with the player manifest, to anonymous viewers through /search/episode.json. The caption plugins that consume it are enabled in the default player configuration, the “Subtitles” upload that produces a captions/source track is active by default, and no caption processing step escapes the cue text.

A user who can upload a subtitle to an event and publish it stores the payload in the published caption file. Any viewer who opens the event and turns captions on runs the script.

Result: a non-admin content author stores JavaScript in a subtitle cue that executes in the browser session of every viewer who enables captions on that event, including anonymous viewers and authenticated staff.