CVE-2026-77615

Paella Player is a set of libraries to create a multi stream video player. Prior to Paella Player 2.12.11 (as used in Opencast prior to 19.7 and 20.2), there is a potential XSS attack though closed captions cue text. This vulnerability is fixed in 2.12.11.

Paella Player is a set of libraries to create a multi stream video player. Prior to Paella Player 2.12.11 (as used in Opencast prior to 19.7 and 20.2), there is a potential XSS attack though closed captions cue text. This vulnerability is fixed in 2.12.11.

Summary

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.

Affected

opencast/opencast, engage-paella-player module. Supported release lines 19.x and 20.x are affected (and 18.x). Live-confirmed on 18.8 (Paella 7, paella-core 1.50.2) and 20.0 (Paella 8, paella-core 1.50.4); 19.5 ships the code-identical caption path (paella-core 1.50.4, same EpisodeConversor and default plugin config as 20.0). The captions canvas uses the same innerHTML += cue sink across these versions. Default configuration: the WebVTT and DFXP caption plugins are enabled: true in etc/ui-config/mh_default_org/paella7/config.json, the "Subtitles" upload option (captions/source, .vtt) is active in etc/listproviders/event.upload.asset.options.properties, and the fast workflow publishes captions/* to the engage player. Condition: an event with a caption track published to the engage player. No non-default flag required.

Root cause

The captions canvas appends each cue to _captionsContainer.innerHTML in the bundled paella-core (served at /paellaN/ui/paella-player.js / the 20.x core chunk), so markup in a cue becomes live DOM. The caption entry is built from any media package element whose flavor matches captions/* at modules/engage-paella-player-7/src/js/EpisodeConversor.js:392, and /search/episode.json serves the manifest and the caption file to anonymous clients. The WebVTT and DFXP plugins that consume it are enabled by default at etc/ui-config/mh_default_org/paella7/config.json:571 and :574. The cue text is not HTML-escaped before assignment to innerHTML, and partial-process-uploaded-captions only cuts and tags the file, it never sanitizes it. Opencast sets neither a Content-Security-Policy nor an X-Content-Type-Options header, so the injected script runs without restriction.

Reproduction

Default config, default caption plugins enabled, a non-admin user with ROLE_API_EVENTS_CREATE, ROLE_API_EVENTS_TRACK_EDIT, and ROLE_UI_TASKS_CREATE (no ROLE_ADMIN).

1. As the non-admin user, create an event, then upload a WebVTT subtitle as captions/source whose cue text is an XSS payload, and publish the event.

WEBVTT

00:00:00.000 --> 00:00:30.000
<img src=x onerror=document.title=window.__xss=document.domain>

2. The anonymous search manifest then exposes the caption and serves the cue raw.

GET /search/episode.json?id=<event>
"type":"captions/source", "url":".../static/.../x.vtt"
GET .../static/.../x.vtt  ->  cue text returned verbatim

3. Open the event in the player as an anonymous viewer, open the captions menu, and select the track; the cue is written to innerHTML and the onerror handler runs.

Live-verified: on Opencast 18.8 (build 8705223) in Chrome, a non-admin author published the subtitle and an anonymous viewer enabled captions, rendering the cue as a live <img> node and setting window.__xss and document.title to document.domain.…

Paella Player is a set of libraries to create a multi stream video player. Prior to Paella Player 2.12.11 (as used in Opencast prior to 19.7 and 20.2), there is a potential XSS attack though closed captions cue text. This vulnerability is fixed in 2.12.11.

Paella Player is a set of libraries to create a multi stream video player. Prior to Paella Player 2.12.11 (as used in Opencast prior to 19.7 and 20.2), there is a potential XSS attack though closed captions cue text. This vulnerability is fixed in 2.12.11.

Summary

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.

Affected

opencast/opencast, engage-paella-player module. Supported release lines 19.x and 20.x are affected (and 18.x). Live-confirmed on 18.8 (Paella 7, paella-core 1.50.2) and 20.0 (Paella 8, paella-core 1.50.4); 19.5 ships the code-identical caption path (paella-core 1.50.4, same EpisodeConversor and default plugin config as 20.0). The captions canvas uses the same innerHTML += cue sink across these versions. Default configuration: the WebVTT and DFXP caption plugins are enabled: true in etc/ui-config/mh_default_org/paella7/config.json, the "Subtitles" upload option (captions/source, .vtt) is active in etc/listproviders/event.upload.asset.options.properties, and the fast workflow publishes captions/* to the engage player. Condition: an event with a caption track published to the engage player. No non-default flag required.

Root cause

The captions canvas appends each cue to _captionsContainer.innerHTML in the bundled paella-core (served at /paellaN/ui/paella-player.js / the 20.x core chunk), so markup in a cue becomes live DOM. The caption entry is built from any media package element whose flavor matches captions/* at modules/engage-paella-player-7/src/js/EpisodeConversor.js:392, and /search/episode.json serves the manifest and the caption file to anonymous clients. The WebVTT and DFXP plugins that consume it are enabled by default at etc/ui-config/mh_default_org/paella7/config.json:571 and :574. The cue text is not HTML-escaped before assignment to innerHTML, and partial-process-uploaded-captions only cuts and tags the file, it never sanitizes it. Opencast sets neither a Content-Security-Policy nor an X-Content-Type-Options header, so the injected script runs without restriction.

Reproduction

Default config, default caption plugins enabled, a non-admin user with ROLE_API_EVENTS_CREATE, ROLE_API_EVENTS_TRACK_EDIT, and ROLE_UI_TASKS_CREATE (no ROLE_ADMIN).

1. As the non-admin user, create an event, then upload a WebVTT subtitle as captions/source whose cue text is an XSS payload, and publish the event.

WEBVTT

00:00:00.000 --> 00:00:30.000
<img src=x onerror=document.title=window.__xss=document.domain>

2. The anonymous search manifest then exposes the caption and serves the cue raw.

GET /search/episode.json?id=<event>
"type":"captions/source", "url":".../static/.../x.vtt"
GET .../static/.../x.vtt  ->  cue text returned verbatim

3. Open the event in the player as an anonymous viewer, open the captions menu, and select the track; the cue is written to innerHTML and the onerror handler runs.

Live-verified: on Opencast 18.8 (build 8705223) in Chrome, a non-admin author published the subtitle and an anonymous viewer enabled captions, rendering the cue as a live <img> node and setting window.__xss and document.title to document.domain.…