CVE-2025-15265
📋 TL;DR
This vulnerability allows attackers to execute arbitrary JavaScript in users' browsers by injecting malicious keys into Svelte's async hydration process. When exploited, it enables cross-site scripting (XSS) attacks that can steal sessions, compromise accounts, or perform actions on behalf of users. This affects any application using vulnerable versions of Svelte with SSR (server-side rendering) and async hydration features.
💻 Affected Systems
- Svelte
📦 What is this software?
Svelte by Svelte
⚠️ Risk & Real-World Impact
Worst Case
Complete account takeover, session theft, data exfiltration, and unauthorized actions performed on behalf of authenticated users across the entire application.
Likely Case
Session hijacking leading to unauthorized access, data theft, and potential privilege escalation within affected applications.
If Mitigated
Limited impact with proper input validation and output encoding, though the core vulnerability remains until patched.
🎯 Exploit Status
Exploitation requires attacker to control keys passed to hydratable components, which could occur through user input, API responses, or other data sources.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 5.46.3
Vendor Advisory: https://github.com/sveltejs/svelte/security/advisories/GHSA-6738-r8g5-qwp3
Restart Required: No
Instructions:
1. Update Svelte package to version 5.46.3 or later. 2. Run 'npm update svelte' or 'yarn upgrade svelte'. 3. Rebuild and redeploy your application. 4. Test that async hydration still functions correctly.
🔧 Temporary Workarounds
Input Validation and Sanitization
allImplement strict validation and sanitization of all keys passed to hydratable components to prevent injection of malicious content.
Content Security Policy (CSP)
allImplement strict CSP headers to mitigate impact by restricting script execution sources.
🧯 If You Can't Patch
- Implement strict input validation and sanitization for all user-controlled data passed to hydratable components
- Deploy a Web Application Firewall (WAF) with XSS protection rules and monitor for exploitation attempts
🔍 How to Verify
Check if Vulnerable:
Check package.json or run 'npm list svelte' to see if Svelte version is between 5.46.0 and 5.46.2 (inclusive).Check Version:
npm list svelte | grep svelteVerify Fix Applied:
Confirm Svelte version is 5.46.3 or later using 'npm list svelte' and test async hydration functionality.📡 Detection & Monitoring
Log Indicators:
- Unusual script tags in rendered HTML
- Suspicious key values in hydration data
- JavaScript errors related to script parsing
Network Indicators:
- Unexpected script loads from unusual domains
- Suspicious POST/GET parameters containing script tags
SIEM Query:
web.logs | where url contains "hydrat" and (body contains "</script>" or body contains "javascript:")