CVE-2026-27120: Leafkit Templating XSS Vulnerability

CVE-2026-27120 Overview

CVE-2026-27120 is a Cross-Site Scripting (XSS) vulnerability in Leafkit, a templating language with Swift-inspired syntax used in the Vapor web framework ecosystem. Prior to version 1.4.1, the htmlEscaped() function in leaf-kit fails to properly escape HTML special characters when they appear within extended grapheme clusters. This flaw allows attackers to bypass HTML escaping by using specially crafted extended grapheme clusters containing both the special HTML character and additional characters, potentially leading to XSS attacks when user-controlled data is rendered in HTML attributes.

Critical Impact

Attackers can inject malicious scripts by exploiting the Unicode grapheme cluster handling flaw in Leafkit's HTML escaping mechanism, leading to Cross-Site Scripting attacks in applications using user-controlled Leaf template variables within HTML attributes.

Affected Products

• Leafkit versions prior to 1.4.1
• Vapor web applications using vulnerable leaf-kit templating
• Swift server-side applications utilizing Leafkit for HTML rendering

Discovery Timeline

• 2026-02-20 - CVE CVE-2026-27120 published to NVD
• 2026-02-23 - Last updated in NVD database

Technical Details for CVE-2026-27120

Vulnerability Analysis

The vulnerability exists in the htmlEscaped() function within Sources/LeafKit/String+HTMLEscape.swift. The original implementation uses Swift's string replacement methods (replacing() or replacingOccurrences(of:with:)) which operate on extended grapheme clusters rather than individual Unicode scalars. In Swift, an extended grapheme cluster is a sequence of one or more Unicode scalars that produce a single human-readable character.

When an attacker crafts input containing HTML special characters (such as <, >, ", ', or &) combined with other Unicode scalars to form extended grapheme clusters, the string matching fails because the grapheme cluster as a whole does not match the single-character pattern. This allows the dangerous HTML characters to pass through unescaped, enabling injection of arbitrary HTML and JavaScript code.

The weakness falls under CWE-75 (Failure to Sanitize Special Elements into a Different Plane), as the escaping logic fails to account for Unicode composition mechanisms.

Root Cause

The root cause is the use of grapheme cluster-based string matching for HTML character escaping. Swift's String type treats strings as collections of extended grapheme clusters by default. When the replacing() or replacingOccurrences(of:with:) methods search for a single character like <, they look for an exact grapheme cluster match. If the < character is combined with other Unicode scalars (such as combining marks), it forms a different grapheme cluster that no longer matches the simple < pattern, causing the escape logic to skip it entirely.

Attack Vector

The attack vector is network-based and requires user interaction. An attacker can exploit this vulnerability by injecting specially crafted Unicode strings into user-controlled template variables that are rendered in HTML attribute contexts. When the Leafkit template engine processes the input and applies HTML escaping, the malicious characters survive due to the grapheme cluster bypass. The resulting HTML output contains unescaped special characters, enabling JavaScript execution in the victim's browser when they view the page.

For example, an attacker could submit input containing a quote character combined with a Unicode combining mark, which would bypass the escaping and break out of an HTML attribute to inject event handlers or script tags.

The security fix addresses this by operating on individual Unicode scalars rather than grapheme clusters:

-import Foundation
-
 extension String {
     /// Escapes HTML entities in a `String`.
     public func htmlEscaped() -> String {
-        if #available(macOS 13.0, iOS 16.0, watchOS 9.0, tvOS 16.0, *) {
-            self
-                .replacing("&", with: "&")
-                .replacing("\"", with: """)
-                .replacing("'", with: "'")
-                .replacing("<", with: "<")
-                .replacing(">", with: ">")
-        } else {
-            self
-                .replacingOccurrences(of: "&", with: "&")
-                .replacingOccurrences(of: "\"", with: """)
-                .replacingOccurrences(of: "'", with: "'")
-                .replacingOccurrences(of: "<", with: "<")
-                .replacingOccurrences(of: ">", with: ">")
+        self.unicodeScalars.reduce(into: "") { result, scalar in
+            switch scalar {
+            case "&": result += "&"
+            case "\"": result += """
+            case "'": result += "'"
+            case "<": result += "<"
+            case ">": result += ">"
+            default: result.unicodeScalars.append(scalar)
+            }
         }
     }

Source: GitHub Commit

Detection Methods for CVE-2026-27120

Indicators of Compromise

• Unusual Unicode sequences in user input fields, particularly combining characters alongside HTML special characters
• Web application logs showing input containing extended grapheme clusters with embedded <, >, ", ', or & characters
• Client-side JavaScript errors or unexpected script execution reports from users

Detection Strategies

• Implement Web Application Firewall (WAF) rules to detect and block input containing suspicious Unicode combining sequences adjacent to HTML special characters
• Monitor application logs for requests containing non-ASCII characters in form fields that are rendered in HTML attribute contexts
• Deploy Content Security Policy (CSP) headers to detect and report XSS attempts through policy violation reports

Monitoring Recommendations

• Enable verbose logging for Leafkit template rendering to capture input values before and after HTML escaping
• Set up alerts for CSP violation reports indicating inline script execution attempts
• Monitor for anomalous patterns in user-submitted data containing Unicode combining marks or zero-width characters

How to Mitigate CVE-2026-27120

Immediate Actions Required

• Upgrade Leafkit to version 1.4.1 or later immediately
• Audit all Leaf templates to identify user-controlled variables rendered within HTML attributes
• Implement Content Security Policy headers to mitigate potential XSS impact while patching
• Consider additional server-side input validation for Unicode content

Patch Information

The vulnerability is fixed in Leafkit version 1.4.1. The fix modifies the htmlEscaped() function to iterate over individual Unicode scalars using self.unicodeScalars instead of operating on grapheme clusters. This ensures that HTML special characters are properly escaped regardless of whether they are combined with other Unicode scalars. The patch is available via the GitHub commit and through the official GitHub Security Advisory.

Workarounds

• Implement additional input sanitization at the application layer before passing data to Leaf templates, stripping or rejecting Unicode combining characters
• Avoid placing user-controlled variables directly in HTML attribute contexts; use safer contexts or additional encoding
• Deploy strict Content Security Policy headers with script-src 'self' to prevent inline script execution from XSS payloads

# Update Leafkit via Swift Package Manager
# In Package.swift, update the dependency:
.package(url: "https://github.com/vapor/leaf-kit.git", from: "1.4.1")

# Then run:
swift package update