CVE-2026-27125
📋 TL;DR
This vulnerability in Svelte's server-side rendering allows attribute spreading on elements to enumerate inherited properties from an object's prototype chain when Object.prototype pollution exists. This can cause unexpected attributes in SSR output or cause SSR errors, potentially leading to server-side issues or information disclosure. Only applications using Svelte for server-side rendering with polluted Object.prototype environments are affected.
💻 Affected Systems
- Svelte
📦 What is this software?
Svelte by Svelte
⚠️ Risk & Real-World Impact
Worst Case
Server-side rendering fails completely, causing application downtime, or sensitive information leaks through unexpected attributes in rendered HTML.
Likely Case
Minor rendering anomalies or errors in SSR output when Object.prototype pollution exists in the environment.
If Mitigated
No impact if Object.prototype is not polluted or if client-side rendering only is used.
🎯 Exploit Status
Exploitation requires Object.prototype pollution, which is typically achieved through other vulnerabilities or misconfigurations.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 5.51.5
Vendor Advisory: https://github.com/sveltejs/svelte/security/advisories/GHSA-crpf-4hrx-3jrp
Restart Required: Yes
Instructions:
1. Update Svelte package to version 5.51.5 or later. 2. Run npm update svelte or yarn upgrade svelte. 3. Restart your application server. 4. Verify the update with npm list svelte or yarn list svelte.
🔧 Temporary Workarounds
Disable SSR
allUse client-side rendering only to avoid the vulnerability
Object.prototype Sanitization
allEnsure Object.prototype is not polluted in your environment
🧯 If You Can't Patch
- Implement strict input validation to prevent Object.prototype pollution
- Monitor SSR output for unexpected attributes and implement error handling
🔍 How to Verify
Check if Vulnerable:
Check if using Svelte version <5.51.5 and if using server-side rendering with attribute spreadingCheck Version:
npm list svelte | grep svelte or check package.jsonVerify Fix Applied:
Confirm Svelte version is 5.51.5 or later and test SSR with attribute spreading📡 Detection & Monitoring
Log Indicators:
- SSR errors related to attribute enumeration
- Unexpected HTML attributes in server logs
Network Indicators:
- HTML responses containing unexpected attributes from SSR
SIEM Query:
Search for application errors containing 'Svelte', 'SSR', or 'attribute spreading' in error messages