GHSA-6jj5-j4j8-8473 (CVE-2026-28499)

Details

Summary

LeafKit HTML-escaping is not working correctly when a template prints a collection (Array / Dictionary) via #(value). This can result in XSS, allowing potentially untrusted input to be rendered unescaped.

Details

LeafKit attempts to escape expressions during serialization, but due to LeafData.htmlEscaped()'s implementation, when the escaped type's conversion to String is marked as .ambiguous (as it is the case for Arrays and Dictionaries), an unescaped self is returned.

Note: I recommend first looking at the POC, before taking a look at the details below, as it is simple. In the detailed, verbose analysis below, I explored the functions involved in more detail, in hopes that it will help you understand and locate this issue.

The issue's detailed analysis:

Leaf expression serialization eventually reaches LeafSerializer's serialize private function below. This is where the leafData is .htmlEscaped(), and then serialized.

https://github.com/vapor/leaf-kit/blob/8ff06839d8b3ddf74032d2ade01e3453eb556d30/Sources/LeafKit/LeafSerialize/LeafSerializer.swift#L60-L66

The LeafData.htmlEscaped() method uses the LeafData.string computed property to convert itself to a string. Then, it calls the htmlEscaped() method on it. However, if the string conversion fails, notice that an unescaped, unsafe self is returned (line 324 below):

https://github.com/vapor/leaf-kit/blob/8ff06839d8b3ddf74032d2ade01e3453eb556d30/Sources/LeafKit/LeafData/LeafData.swift#L321-L328