CVE-2026-27121
📋 TL;DR
Svelte versions before 5.51.5 are vulnerable to cross-site scripting (XSS) during server-side rendering when using spread syntax with untrusted data. This allows attackers to inject malicious event handlers that execute in victims' browsers. Applications that spread user-controlled data as element attributes are affected.
💻 Affected Systems
- Svelte
📦 What is this software?
Svelte by Svelte
⚠️ Risk & Real-World Impact
Worst Case
Attackers can execute arbitrary JavaScript in victims' browsers, potentially stealing session cookies, performing actions as the user, or redirecting to malicious sites.
Likely Case
Session hijacking, credential theft, or defacement of web pages through injected malicious scripts.
If Mitigated
Limited impact if input validation and output encoding are properly implemented, though the vulnerability still exists in the framework.
🎯 Exploit Status
Exploitation requires user interaction with the maliciously rendered page.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 5.51.5
Vendor Advisory: https://github.com/sveltejs/svelte/security/advisories/GHSA-f7gr-6p89-r883
Restart Required: No
Instructions:
1. Update Svelte to version 5.51.5 or later. 2. Run npm update svelte or yarn upgrade svelte. 3. Rebuild and redeploy the application.
🔧 Temporary Workarounds
Avoid spreading untrusted data
allManually specify attributes instead of using spread syntax with user-controlled data.
Sanitize input data
allImplement strict input validation and sanitization for any data used in spread attributes.
🧯 If You Can't Patch
- Implement Content Security Policy (CSP) with strict directives to limit script execution.
- Use client-side only rendering for components handling untrusted data.
🔍 How to Verify
Check if Vulnerable:
Check package.json for Svelte version below 5.51.5 and review code for spread syntax with untrusted data in server-side rendering.Check Version:
npm list svelte or check package.jsonVerify Fix Applied:
Confirm Svelte version is 5.51.5 or higher in package.json and test that spread attributes no longer include event handlers.📡 Detection & Monitoring
Log Indicators:
- Unusual attribute patterns in server logs, especially event handlers like onclick, onmouseover in rendered HTML.
Network Indicators:
- Unexpected JavaScript execution from server-rendered pages.
SIEM Query:
Search for patterns like 'onclick=' or 'onmouseover=' in web server response logs.