CVE-2026-27120
📋 TL;DR
This vulnerability in Leafkit's htmlEscaped function allows attackers to bypass HTML escaping by using extended grapheme clusters containing special HTML characters. This can lead to cross-site scripting (XSS) attacks when user-controlled variables are used in HTML attributes. All applications using Leafkit versions before 1.4.1 with user input in templates are affected.
💻 Affected Systems
- Leafkit
- Vapor framework applications using Leafkit
📦 What is this software?
Leafkit by Vapor
⚠️ Risk & Real-World Impact
Worst Case
Full XSS exploitation leading to session hijacking, credential theft, or complete client-side compromise of users visiting vulnerable pages.
Likely Case
Limited XSS attacks allowing attackers to steal session cookies, perform actions on behalf of users, or deface websites.
If Mitigated
No impact if proper input validation and output encoding are already implemented, or if user input isn't used in vulnerable contexts.
🎯 Exploit Status
Exploitation requires user input to reach vulnerable htmlEscaped function in HTML attribute context.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 1.4.1
Vendor Advisory: https://github.com/vapor/leaf-kit/security/advisories/GHSA-4hfh-fch3-5q7p
Restart Required: Yes
Instructions:
1. Update leaf-kit dependency to version 1.4.1 or later. 2. Update package manifest (Package.swift or similar). 3. Run package update/resolve. 4. Rebuild and redeploy application. 5. Restart application services.
🔧 Temporary Workarounds
Manual HTML escaping
allImplement custom HTML escaping that properly handles extended grapheme clusters before passing user input to templates.
Input validation
allValidate and sanitize all user input before using in templates, rejecting inputs containing problematic grapheme clusters.
🧯 If You Can't Patch
- Implement Content Security Policy (CSP) headers to mitigate XSS impact
- Use additional output encoding layers beyond htmlEscaped function
🔍 How to Verify
Check if Vulnerable:
Check package manifest for leaf-kit dependency version. If version < 1.4.1 and application uses htmlEscaped with user input in HTML attributes, it's vulnerable.Check Version:
swift package show-dependencies | grep leaf-kit OR check Package.swift for .package(url: "https://github.com/vapor/leaf-kit.git", from: "X.X.X")Verify Fix Applied:
Verify leaf-kit version is 1.4.1 or later in package manifest and dependencies. Test with known bypass payloads to confirm proper escaping.📡 Detection & Monitoring
Log Indicators:
- Unusual grapheme cluster patterns in user input
- HTML attribute values containing mixed character sets
Network Indicators:
- Requests containing extended grapheme clusters in parameters
- Suspicious script tags in attribute contexts
SIEM Query:
source="web_logs" AND (uri_query="*%E2%80%8B*" OR uri_query="*%CC%*" OR user_agent="*<script*" AND uri="*.leaf*")