# CVE-2026-126359: Hono: `css` helper does not escape a closing tag split between the template and an interpolated value, allowing HTML injection

> Apps using Hono's css helper with untrusted strings after a closing tag can render attacker-controlled HTML or script to viewers.

- **Severity:** Low 3.4 (`CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:L/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
- **Harmful if only installed:** No
- **Needs a specific call:** Yes (`css`)
- **Fix available:** Yes
- **Published:** 12 Nov 2026
- **Updated:** 12 Nov 2026
- **Also known as:** GHSA-3gg2-335m-rxc6

## What is CVE-2026-126359?

CVE-2026-126359 is a low-severity cross-site scripting vulnerability in honojs hono, published on 12 Nov 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

Hono is a Web application framework that provides support for any JavaScript runtime. From 3.12.0 until 4.13.16, the css helper in hono/css escapes interpolated string values separately and therefore fails to neutralize a closing style tag when < ends the template text or a preceding value and / begins the next attacker-controlled value. During server-side rendering, the combined output can close the <Style /> element early and cause the remainder of the interpolated value to be interpreted as HTML. Applications that place an untrusted string immediately after < in a css template, including adjacent untrusted interpolations, can expose viewers to attacker-controlled HTML or script and information accessible to that page. Other uses of the css helper are not affected. This issue is fixed in version 4.13.16.

Weakness: CWE-79 (Improper Neutralization of Input During Web Page Generation)

## Which versions are affected by CVE-2026-126359?

1 open source package ship an affected version, including hono.

An affected version is only exploitable if your software calls css. Many projects that include it are not exposed.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `hono` | npm | From 3.12.0 | 4.13.16 |

## How do I fix CVE-2026-126359?

Upgrade hono to 4.13.16 or later.

## Is CVE-2026-126359 being exploited?

There is no known exploitation of CVE-2026-126359 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-126359 and fix it if it is affected.

What it is: CVE-2026-126359 is a low-severity cross-site scripting vulnerability in honojs hono, published on 12 Nov 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: There is no known exploitation of CVE-2026-126359 in the wild and no public exploit code.
Exposure: An affected version is only exploitable if your software calls css. Many projects that include it are not exposed.

Affected packages and fixed versions:
- hono (npm): affected from 3.12.0; fixed in 4.13.16

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.
   If an affected version is present, also search the code for calls to css and report whether any exist.
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-2026-126359/ (Markdown: https://insights.sbomify.com/CVE-2026-126359/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-126359
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-126359
- OSV: https://osv.dev/vulnerability/CVE-2026-126359

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-126359/
Summaries are generated and may contain mistakes; the official description above is the record.