# CVE-2026-29085: Hono: SSE Control Field Injection via CR/LF in writeSSE()

> Attackers can inject arbitrary SSE fields via CR/LF characters in streamSSE() fields, potentially altering client-side event processing.

- **Severity:** Medium 6.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 4 Mar 2026
- **Updated:** 5 Mar 2026
- **Also known as:** CLEANSTART-2026-DB28209, CLEANSTART-2026-FR80106, GHSA-p6xx-57qc-3wxr, MINI-5jx2-249r-qq79, MINI-64xr-h3mc-352j, MINI-6m73-74x5-h4h4, MINI-jchp-v8mj-v6v7, MINI-mgm6-m9wq-537c, ROOT-APP-NPM-CVE-2026-29085

## What is CVE-2026-29085?

CVE-2026-29085 is a medium-severity vulnerability in honojs hono, published on 4 Mar 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

Hono is a Web application framework that provides support for any JavaScript runtime. Prior to version 4.12.4, when using streamSSE() in Streaming Helper, the event, id, and retry fields were not validated for carriage return (\r) or newline (\n) characters. Because the SSE protocol uses line breaks as field delimiters, this could allow injection of additional SSE fields within the same event frame if untrusted input was passed into these fields. This issue has been patched in version 4.12.4.

Weakness: CWE-74 (Improper Neutralization of Special Elements in Output Used by a Downstream Component)

## Which versions are affected by CVE-2026-29085?

2 open source packages ship an affected version, including hono. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `hono` | npm | All versions | 4.12.4 |
| `hono` | npm | All versions | 4.12.4 |

Products: Hono Hono.

## How do I fix CVE-2026-29085?

Upgrade hono to 4.12.4 or later.

## Is CVE-2026-29085 being exploited?

There is no known exploitation of CVE-2026-29085 in the wild and no public exploit code.

## Other vulnerabilities in honojs hono

- [CVE-2026-27700](https://insights.sbomify.com/CVE-2026-27700/): Hono is Vulnerable to Authentication Bypass by IP Spoofing in AWS Lambda ALB conninfo (high 8.2)
- [CVE-2026-22817](https://insights.sbomify.com/CVE-2026-22817/): JWT Algorithm Confusion via Unsafe Default (HS256) in Hono JWT Middleware Allows Token Forgery and Auth Bypass (high 8.2)
- [CVE-2026-22818](https://insights.sbomify.com/CVE-2026-22818/): JWT algorithm confusion in Hono JWK Auth Middleware when JWK lacks "alg" (untrusted header.alg fallback) (high 8.2)
- [CVE-2025-62610](https://insights.sbomify.com/CVE-2025-62610/): Hono Improperly Authorizes JWT Audience Validation (high 8.1)
- [CVE-2026-29045](https://insights.sbomify.com/CVE-2026-29045/): Hono: Arbitrary file access via serveStatic vulnerability (high 7.5)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-29085 and fix it if it is affected.

What it is: CVE-2026-29085 is a medium-severity vulnerability in honojs hono, published on 4 Mar 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-29085 in the wild and no public exploit code.

Affected packages and fixed versions:
- hono (npm): affected all versions; fixed in 4.12.4
- hono (npm): affected all versions; fixed in 4.12.4

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-2026-29085/ (Markdown: https://insights.sbomify.com/CVE-2026-29085/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-29085
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-29085
- OSV: https://osv.dev/vulnerability/CVE-2026-29085

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-29085/
Summaries are generated and may contain mistakes; the official description above is the record.