Hono: Non-breaking space prefix bypass in cookie name handling in getCookie()

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

4.8 · CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N

EPSS

• CVE-2026-39410: 0.28% estimated exploitation activity in the next 30 days; percentile 20.97 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary 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. Details Browsers follow RFC 6265bis and only trim SP (0x20) and HTAB (0x09) from cookie names. Other characters, such as the non-breaking space (U+00A0), are preserved as part of the cookie name. For example, the browser treats the following cookies as distinct: However, parse() previously used JavaScript's trim(), which removes a broader set of characters including U+00A0. As a result, both names are normalized to: This mismatch allows attacker-controlled cookies with a U+00A0 prefix to shadow or override legitimate cookies when accessed via getCookie(). Impact An attacker who can set cookies (e.g., via a man-in-the-middle on a non-secure page or other injection vector) can bypass cookie prefix…

Published by the source

8 Apr 2026, 00:17 UTC

Last modified by the source

10 Sep 2026, 03:51 UTC

Source last checked

26 Oct 2026, 12:47 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-r5rp-j6wh-rvv4

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-r5rp-j6wh-rvv4

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# Patch Next advisory brief: CVE-2026-39410

Advisory: Hono: Non-breaking space prefix bypass in cookie name handling in getCookie()
IDs: CVE-2026-39410, GHSA-r5rp-j6wh-rvv4
Record: https://patchnext.com/advisories/cve-2026-39410 (revision 2, source checked 26 Oct 2026, 12:47 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 4.8)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: hono 4.12.12. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm hono: affected < 4.12.12; fixed in 4.12.12

## Suggested inspection (read-only)
- `npm ls hono --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on hono.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-r5rp-j6wh-rvv4 (OSV record GHSA-r5rp-j6wh-rvv4 lists affected ranges: npm hono >=0 <4.12.12 (fixed))
- Verified: https://github.com/advisories/GHSA-r5rp-j6wh-rvv4 (GitHub-reviewed advisory GHSA-r5rp-j6wh-rvv4 lists affected ranges: npm hono >=0 <4.12.12)

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