TI-2026-21572 Aliases CVE-2026-27120 GHSA-4hfh-fch3-5q7p

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

MEDIUM 60CWE-75Published 2026-02-20 · Updated 2026-02-24

Key facts

Attack vector: Network
Privileges required: None
User interaction: Required
Attack complexity: Low
Impact type: Cross-site scripting
Confidentiality · integrity · availability: Low · Low · None
Fix status: Fixed

Attack profile from CVSS 3.1 · CNA. Fix status from CNA and GitHub. Impact type from CWE-79.

Summary

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.

Why it matters now

Exploitation Proof of concept CISA SSVC · not in CISA KEV

EPSS 0.00266 18th percentileScored 2026-11-07

Public exploit Yes 1 exploit referenceTagged by NVD

Fix Fixed leaf-kit (vapor): 1.4.1github.com/vapor/leaf-kit (SwiftURL): 1.4.1

SSVC decision points

CISA ADP · SSVC 2.0.3

Exploitation: Poc
Automatable: No
Technical impact: Partial

The decision (Track, Attend, Act) also depends on your mission and well-being impact; see CISA's SSVC guide.

Affected products

Vendor	Product	Affected versions	Fixed in	Status	Source
vapor	leaf-kit	before 1.4.1	1.4.1	Patch available	CNA
—	github.com/vapor/leaf-kit (SwiftURL)	before 1.4.1	1.4.1	Patch available	GHSA-4hfh-fch3-5q7p

leaf-kit Vendorvapor Affected before 1.4.1 Fixed in 1.4.1 Patch available CNA
github.com/vapor/leaf-kit (SwiftURL) Vendor— Affected before 1.4.1 Fixed in 1.4.1 Patch available GHSA-4hfh-fch3-5q7p

CPE configurations NVD · 1

CPE	Versions	Role	Vendor
cpe:2.3:a:vapor:leafkit::::::::	before 1.14.1	Vulnerable	vapor

cpe:2.3:a:vapor:leafkit:*:*:*:*:*:*:*:* Versions before 1.14.1 Vulnerable ·vapor

Remediation

Fix · CNA leaf-kit (vapor) Fixed in 1.4.1
Fix · GHSA-4hfh-fch3-5q7p github.com/vapor/leaf-kit (SwiftURL) Fixed in 1.4.1 Advisory →

Timeline

2026-10-02 Threatist record created from CVE-2026-27120
2026-10-03 Alias added: GHSA-4hfh-fch3-5q7p

References

TI-2026-22762 LeafKit's HTML escaping may be skipped for Collection values, enabling XSS CVSS 4.0MEDIUM 6.9
TI-2021-15509 LeafKit allows XSS with untrusted user input CVSS 3.1HIGH 7.4

Sources

This product uses the NVD API but is not endorsed or certified by the NVD. Sources and licences