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

> Attackers can inject malicious scripts into web pages via unsanitized user input in LeafKit templates, potentially compromising user sessions.

- **Severity:** High 7.4 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:N/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** A user has to click or open something
- **Fix available:** Yes
- **Published:** 9 Aug 2021
- **Updated:** 4 Aug 2024
- **Also known as:** GHSA-rv3x-xq3r-8j9h

## 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 (Improper Neutralization of Input During Web Page Generation)

## 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.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `github.com/vapor/leaf-kit` | SwiftURL | All versions | 1.3.0 |
| `github.com/vapor/leaf-kit` | SwiftURL | All versions | 1.3.0 |

Products: Vapor Leafkit.

## How do I fix CVE-2021-37634?

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

## 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](https://insights.sbomify.com/CVE-2026-28499/): LeafKit's HTML escaping may be skipped for Collection values, enabling XSS (medium 6.9)
- [CVE-2026-27120](https://insights.sbomify.com/CVE-2026-27120/): Leaf-kit html escaping does not work on characters that are part of extended grapheme cluster (medium 6.1)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-37634 and fix it if it is affected.

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

Affected packages and fixed versions:
- github.com/vapor/leaf-kit (SwiftURL): affected all versions; fixed in 1.3.0
- github.com/vapor/leaf-kit (SwiftURL): affected all versions; fixed in 1.3.0

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2021-37634/ (Markdown: https://insights.sbomify.com/CVE-2021-37634/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-37634
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-37634
- OSV: https://osv.dev/vulnerability/CVE-2021-37634

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-37634/
Summaries are generated and may contain mistakes; the official description above is the record.