CVE-2026-63670

### 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)">