CVE-2026-27120: Leaf-kit html escaping does not work on characters that are part of extended grapheme cluster

Severity: medium (CVSS 6.1 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.3% probability of exploitation in the next 30 days
Public exploit or detection template: yes
Published: 2026-02-20 · updated 2026-06-17
Weakness: CWE-75

Description

Leafkit is a templating language with Swift-inspired syntax. Prior to 1.4.1, htmlEscaped in leaf-kit will only escape html special characters if the extended grapheme clusters match, which allows bypassing escaping by using an extended grapheme cluster containing both the special html character and some additional characters. In the case of html attributes, this can lead to XSS if there is a leaf variable in the attribute that is user controlled. This vulnerability is fixed in 1.4.1.

Affected products

vapor Leafkit: before 1.14.1 — fixed in 1.14.1

How to fix

Upgrade to a fixed version: vapor Leafkit 1.14.1. Check the vendor advisory for mitigations.

References

Get alerted when CVE-2026-27120 or the next vulnerability affects your stack