ApostropheCMS: Mutation-XSS / allowedTags bypass via literal `</textarea/>` solidus close

### Summary A mutation-XSS / allowedTags bypass: when `textarea` (or `xmp`) is included in `allowedTags`, an input containing a literal `</textarea/>` (a solidus right after the RCDATA end-tag name) lets non-allowed markup such as `<img src=x onerror=…>` pass through `sanitizeHtml()` **live and unescaped**, even though `img`/`onerror` are not in the allowlist. A spec-compliant browser executes the surviving handler — XSS. This is a literal-solidus variant that bypasses the two most recent fixes in this code area (CVE-2026-40186, CVE-2026-44990), both already applied in 2.17.5. The default configuration is not affected. ### Details `sanitize-html` emits the text content of HTML raw-text elements (`textarea`, `xmp`) without escaping. Two things combine: - **Parser differential:** on input, htmlparser2 does NOT recognize `</textarea/>` (solidus after the RCDATA end-tag name) as a close tag; it emits `</textarea/><img …>` as a single raw-text node. - **Unescaped passthrough:** the `ontext` handler (`index.js` ~575-583) appends `textarea`/`xmp` content with `result += text` (no `escapeHtml`), assuming it is "already properly encoded" — true for entity-decoded content (what CVE-2026-40186 fixed) but false for this mis-tokenized literal close tag. A spec browser treats `</textarea/>` as a valid `textarea` close, so the following `<img onerror>` is parsed as a live element. The recent fixes addressed entity-encoding (CVE-2026-40186) and the `xmp` default (CVE-2026-44990); neither covers the literal-solidus mis-tokenization, so the raw passthrough still leaks. ### PoC ```js // npm i [email protected] parse5 && node poc.js const sanitizeHtml = require('sanitize-html'); const input = '<textarea></textarea/><img src=x onerror="alert(document.domain)">'; const opts = { allowedTags: sanitizeHtml.defaults.allowedTags.concat(['textarea']) }; // img NOT allowed console.log(sanitizeHtml(input, opts)); // => <textarea></textarea/><img src=x onerror="alert(document.domain)"></textarea> // the <img onerror> survives live and unescaped console.log(sanitizeHtml(input)); // default config (no textarea allowed) => "" (safe) ``` Re-parsing the sanitized OUTPUT with **parse5** (the WHATWG HTML parser browsers/jsdom use) yields a **live `<img src=x onerror=alert(document.domain)>`** at `body` level (it escaped the textarea RCDATA, not inert text) → the `onerror` fires in a browser. Confirmed on 2.17.5 (Node v24). A canonical `poc.js` is attached. <img width="650" height="118" alt="image" src="https://github.com/user-attachments/assets/d63bb5b7-ba3e-4b0e-a821-86a453ea0352" /> ### Impact Cross-site scripting (CWE-79). Requires `textarea` (or `xmp`) in `allowedTags` — a benign-looking, common addition in form builders, CMS, and rich-text editors. Adding a harmless tag that then enables XSS via *non-allowed* `img`/`onerror` breaks the sanitizer's core contract; the maintainers have fixed this class before (e.g. GHSA-9mrh). An attacker who can submit content rendered through such a configuration achieves stored/reflected XSS (cookie theft, session hijack). Severity Medium (default config is safe; user interaction to view the page). **Suggested fix:** route `textarea`/`xmp` content through `escapeHtml` instead of the raw passthrough, and/or fix the htmlparser2 `</tag/>` RCDATA end-tag tokenization to match the WHATWG spec.

Source published

3 Sep 2026 20:07:16 UTC

Source modified

3 Sep 2026 20:15:05 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:N/UI:R/S:C/C:L/I:L/A:N

Publisher rating · MODERATE