CVE-2026-50159 — Mermaid allows CSS injection applying to sibling elements of the diagram

Affected: mermaid-js mermaid

Mermaid is a JavaScript tool that uses Markdown-inspired text to create and modify diagrams and charts. Prior to 10.9.8 and 11.16.1, Mermaid is vulnerable to CSS injection via sibling combinator selectors generated from diagram-supplied class or id names. An attacker who can supply diagram text can inject arbitrary CSS into the rendered page, potentially altering the appearance or behavior of unrelated page elements. This issue is fixed in versions 10.9.8 and 11.16.1.

GHSA-55q2-fjhq-7xh7 — DOMPurify: IN_PLACE hook removal leaves a detached subtree executable, causing XSS

Affected: npm: dompurify

### Summary

During `IN_PLACE` sanitization, a hook that removes an element can leave that element's detached descendants executable. A descendant image can retain its attacker-provided `onload` handler and fire after `sanitize()` returns, even though the returned root is clean and the image remains disconnected from the document.

### Details

In DOMPurify 3.4.12, `_sanitizeElements()` in `src/purify.ts:1862-1904` runs the `beforeSanitizeElements` or `uponSanitizeElement` hook and returns immediately when the hook detached the current node. The return does not call `_neutralizeSubtree(currentNode)`.

The detached subtree is not added to `DOMPurify.removed`, so the post-walk `IN_PLACE` neutralization cannot reach it. If the browser queued a resource event while the application constructed the detached dirty root, a descendant can therefore retain its handler and execute after sanitization.

The hook only rejects the containing element and does not add or approve the event handler. DOMPurify's ordinary removal path de-arms the same queued event; only the hook-detachment early return skips the existing subtree neutralization.

### PoC

Load the published `dompurify@3.4.12` `dist/purify.js` before this script in Chromium:

```html <div id="result">not fired</div> <script> const root = document.createElement('div'); root.innerHTML = ` <footer> <img src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" onload="result.textContent = 'XS…

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CVE-2026-50159 — Mermaid allows CSS injection applying to sibling elements of the diagram

GHSA-55q2-fjhq-7xh7 — DOMPurify: IN_PLACE hook removal leaves a detached subtree executable, causing XSS