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

> Attackers can bypass HTML escaping in Leaf-kit using extended grapheme clusters to inject scripts, leading to XSS in web applications using this library.

- **Severity:** Medium 6.1 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/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:** 20 Feb 2026
- **Updated:** 24 Feb 2026
- **Also known as:** GHSA-4hfh-fch3-5q7p

## What is CVE-2026-27120?

CVE-2026-27120 is a medium-severity cross-site scripting vulnerability in vapor leaf-kit, published on 20 Feb 2026. 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. 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.

Weakness: CWE-87 (Improper Neutralization of Alternate XSS Syntax), CWE-79 (Improper Neutralization of Input During Web Page Generation), CWE-75 (Failure to Sanitize Special Elements into a Different Plane)

## Which versions are affected by CVE-2026-27120?

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 | 1.4.1 |
| `github.com/vapor/leaf-kit` | SwiftURL | All versions | 1.4.1 |

Products: Vapor Leafkit.

## How do I fix CVE-2026-27120?

Upgrade github.com/vapor/leaf-kit to 1.4.1 or later.

## Is CVE-2026-27120 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-27120 exists.

## Other vulnerabilities in vapor leaf-kit

- [CVE-2021-37634](https://insights.sbomify.com/CVE-2021-37634/): LeafKit allows XSS with untrusted user input (high 7.4)
- [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)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-27120 and fix it if it is affected.

What it is: CVE-2026-27120 is a medium-severity cross-site scripting vulnerability in vapor leaf-kit, published on 20 Feb 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-27120 exists.

Affected packages and fixed versions:
- github.com/vapor/leaf-kit (SwiftURL): affected all versions; fixed in 1.4.1
- github.com/vapor/leaf-kit (SwiftURL): affected all versions; fixed in 1.4.1

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-2026-27120/ (Markdown: https://insights.sbomify.com/CVE-2026-27120/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-27120
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-27120
- OSV: https://osv.dev/vulnerability/CVE-2026-27120

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-27120/
Summaries are generated and may contain mistakes; the official description above is the record.