Paella Player: Stored XSS via caption cue text

CISA KEV is observed exploitation evidence. FIRST EPSS is a dated prediction of exploitation in the next 30 days. Missing scores are unknown. This collector requests predictions only for explicit CVE aliases in the public NuGet archive; Microsoft release views retain their existing prediction scope.

No collected risk observation for these identifiers. This does not establish absence of exploitation or a zero probability.

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.

Source published

17 Sep 2026 20:51:26 UTC

Source modified

22 Sep 2026 01:50:02 UTC

Observed here

23 Oct 2026 15:48:27 UTC

Source terms

CVE Program Terms of Use

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: cve-program · CVE Program Terms of Use. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 3.1 · 8.7 CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:H/I:H/A:N

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.

Source published

Sep 17, 2026, 8:51:26 PM (timezone not stated)

Source modified

Sep 22, 2026, 1:50:02 AM (timezone not stated)

Observed here

11 Nov 2026 16:09:08 UTC

Source terms

ENISA EUVD · source terms and attribution apply

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: euvd · ENISA EUVD · source terms and attribution apply. Source text is retained; field grouping and change labels are provided by ChangeIntel.

EUVD CVSS 3.1 · 8.7 CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:H/I:H/A:N

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.

Source published

17 Sep 2026 21:17:38 UTC

Source modified

24 Sep 2026 21:25:27 UTC

Observed here

25 Oct 2026 16:19:04 UTC

Source terms

NIST NVD usage terms; underlying source attribution retained

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: nvd · NIST NVD usage terms; underlying source attribution retained. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 3.1 · [email protected] · Secondary · 8.7 CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:H/I:H/A:N

## 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`. On Opencast 20.0 (build d919405, Paella 8), the served core bundle contains the identical `innerHTML += cue` sink and the player loads the cue raw; the sink executes JavaScript in the engage origin when fed the player's own published caption file. ## Impact - JavaScript execution in the Opencast origin in the session of any viewer who enables captions on the event. - Anonymous viewers and authenticated staff are equally affected; an instructor or admin viewer exposes that session context to the script. - Session and CSRF-token theft, actions performed as the victim against the Opencast REST API. - Stored by a non-admin content author, triggered by viewing with captions on, no attacker authentication at view time. ## Credit Jan Kahmen, [turingpoint](https://www.turingpoint.de) ([email protected])

Source published

18 Sep 2026 13:11:59 UTC

Source modified

18 Sep 2026 13:30:05 UTC

Observed here

24 Oct 2026 13:17:17 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V3 · CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:H/I:H/A:N

Publisher rating · HIGH