Plate: Media embed provider metadata can bypass URL sanitization and execute iframe JavaScript

Summary

The media embed renderer trusts serialized provider or sourceUrl metadata and skips the URL protocol validation that normally blocks unsafe media embed URLs. A crafted Plate document can set a known video provider while keeping url as a javascript: iframe source. When a victim opens that document in an app using the registry media embed component, the component renders the attacker URL directly as an iframe src.

Impact

An attacker who can create or share Plate document content with another user can execute JavaScript in the victim browser context when the victim opens the document. This can expose application data available to the page and perform actions as the victim, depending on the host application's session model.

Reproduction

Use a Plate editor or viewer that renders media embeds with the registry MediaEmbedElement and load a document containing this node:

{
  "type": "media_embed",
  "provider": "vimeo",
  "sourceUrl": "https://vimeo.com/1",
  "url": "javascript:parent.postMessage('plate-media-xss','*')",
  "children": [{ "text": "" }]
}

When rendered, the node is treated as a Vimeo media embed because provider is present. The iframe receives the unvalidated url value as its src.

Root Cause / Technical Details

parseMediaUrl is the intended hardening point for media embed URLs and rejects parser output unless the URL protocol is http: or https:. However, useMediaState had a fast path for serialized nodes that already contain provider or sourceUrl. That fast path returned { id, provider, sourceUrl, url } directly and did not call parseMediaUrl or otherwise validate the stored url.

The registry MediaEmbedElement then checks that embed.provider is one of the video providers and renders non-YouTube video providers with <iframe src={embed.url}>. Because provider is attacker-controlled serialized document metadata, it can be set to while remains .