CVE-2021-37634: LeafKit allows XSS with untrusted user input

Severity: medium (CVSS 6.1 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.7% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2021-08-09 · updated 2026-06-17
Weakness: CWE-79

Description

Leafkit is a templating language with Swift-inspired syntax. Versions prior to 1.3.0 are susceptible to Cross-site Scripting (XSS) attacks. This affects anyone passing unsanitised data to Leaf's variable tags. Before this fix, Leaf would not escape any strings passed to tags as variables. If an attacker managed to find a variable that was rendered with their unsanitised data, they could inject scripts into a generated Leaf page, which could enable XSS attacks if other mitigations such as a Content Security Policy were not enabled. This has been patched in 1.3.0. As a workaround sanitize any untrusted input before passing it to Leaf and enable a CSP to block inline script and CSS data.

Affected products

vapor Leafkit: before 1.3.0 — fixed in 1.3.0

How to fix

Upgrade to a fixed version: vapor Leafkit 1.3.0. Check the vendor advisory for mitigations.

References

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