{"id":"CVE-2026-63670","aliases":["GHSA-jxwj-j7wr-gfrw"],"url":"https://o3.security/vulnerability/CVE-2026-63670","summary":"ApostropheCMS: Mutation-XSS / allowedTags bypass via literal `</textarea/>` solidus close","details":"### Summary\nA 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.\n\n### Details\n`sanitize-html` emits the text content of HTML raw-text elements (`textarea`, `xmp`) without escaping. Two things combine:\n- **Parser differential:** on input, htmlparser2 does NOT recognize `</textarea/>` (solidus after the RCDATA\n  end-tag name) as a close tag; it emits `</textarea/><img …>` as a single raw-text node.\n- **Unescaped passthrough:** the `ontext` handler (`index.js` ~575-583) appends `textarea`/`xmp` content with\n`result += text` (no `escapeHtml`), assuming it is \"already properly encoded\" — true for entity-decoded\n  content (what CVE-2026-40186 fixed) but false for this mis-tokenized literal close tag.\nA 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.\n\n### PoC\n```js\n// npm i sanitize-html@2.17.5 parse5 && node poc.js\nconst sanitizeHtml = require('sanitize-html');\nconst input = '<textarea></textarea/><img src=x onerror=\"alert(document.domain)\">';\nconst opts  = { allowedTags: sanitizeHtml.defaults.allowedTags.concat(['textarea']) };  // img NOT allowed\nconsole.log(sanitizeHtml(input, opts));\n// => <textarea></textarea/><img src=x onerror=\"alert(document.domain)\"></textarea>\n//    the <img onerror> survives live and unescaped\n\nconsole.log(sanitizeHtml(input));   // default config (no textarea allowed) => \"\"  (safe)\n```\nRe-parsing the sanitized OUTPUT with **parse5** (the WHATWG HTML parser browsers/jsdom use) yields a **live\n`<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.\n<img width=\"650\" height=\"118\" alt=\"image\" src=\"https://github.com/user-attachments/assets/d63bb5b7-ba3e-4b0e-a821-86a453ea0352\" />\n\n### Impact\nCross-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.","published":"2026-08-17T19:49:51.903Z","modified":"2026-09-11T03:30:44.639781696Z","cvss":{"score":6.1,"severity":"MEDIUM","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N"},"epss":null,"cisaKev":null,"exploitsKnown":null,"affectedPackages":[{"ecosystem":"npm","name":"sanitize-html","fixedVersion":"2.17.6"}],"fix":{"url":"https://github.com/apostrophecms/apostrophe/commit/eae1fb2b72ec5d1c27d0977509c6482a0408f725","label":"apostrophecms/apostrophe@eae1fb2"},"references":[{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/63xxx/CVE-2026-63670.json"},{"type":"ADVISORY","url":"https://github.com/apostrophecms/apostrophe/security/advisories/GHSA-jxwj-j7wr-gfrw"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-63670"},{"type":"FIX","url":"https://github.com/apostrophecms/apostrophe/commit/eae1fb2b72ec5d1c27d0977509c6482a0408f725"},{"type":"FIX","url":"https://github.com/apostrophecms/apostrophe/pull/5501"},{"type":"PACKAGE","url":"https://github.com/apostrophecms/apostrophe"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-09-11T03:30:44.639781696Z"}}