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

What is CVE-2021-37634?

CVE-2021-37634 is a high-severity cross-site scripting vulnerability in vapor leaf-kit, published on 9 Aug 2021. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.

Official 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.

Weakness: CWE-79

Which versions are affected by CVE-2021-37634?

2 open source packages ship an affected version, including github.com/vapor/leaf-kit. It affects 1 product.

Products include Vapor Leafkit.

How do I fix CVE-2021-37634?

Upgrade github.com/vapor/leaf-kit to 1.3.0 or later.

Escape output for the context it lands in and set a strict Content Security Policy. Upgrading the affected component is the real fix. More about Cross-site scripting.

Is CVE-2021-37634 being exploited?

There is no known exploitation of CVE-2021-37634 in the wild and no public exploit code.

Other vulnerabilities in vapor leaf-kit

• CVE-2026-28499 LeafKit's HTML escaping may be skipped for Collection values, enabling XSS Medium 6.9
• CVE-2026-27120 Leaf-kit html escaping does not work on characters that are part of extended grapheme cluster Medium 6.1

Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• How this page is made

Also known as GHSA-rv3x-xq3r-8j9h.