# CVE-2026-59897: Hono incorrect header de-duplication in AWS API Gateway adapter

Source: https://junglewise.ai/threats/cve-2026-59897-hono-incorrect-header-de-duplication-in-aws-api-gateway-adapter  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-07-08. Updated: 2026-07-21  
License: CC BY 4.0 with a link back

CVE-2026-59897 · Severity: medium · CVSS 4.8 · Published 2026-07-08

Hono, a web framework, contains a flaw in its AWS API Gateway and VPC Lattice adapters that can cause certain security information to be ignored. When a request contains multiple headers with similar values, the system may accidentally delete one of them, such as an IP address in a security log. This could allow an attacker to bypass security rules like rate limiting or IP restrictions by tricking the system into seeing an incomplete list of data.

Technologies: [hono (npm)](https://junglewise.ai/threats/technologies/hono). Vendors: [npm](https://junglewise.ai/threats/vendors/npm).

## Quick answers

**Is CVE-2026-59897 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-59897 being exploited in the wild.

**How severe is CVE-2026-59897?** Medium severity, with a CVSS score of 4.8, published 8 July 2026.

**Which products and versions are affected?** hono (npm) >= 4.3.3, < 4.12.27, SEMVER:introduced=4.3.3,fixed=4.12.27.

**Is there a fix for CVE-2026-59897?** Yes. hono (npm): upgrade to 4.12.27 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-59897 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-59897?** Check whether you run hono (npm) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- hono (npm): upgrade to 4.12.27 or later

  ```sh
  npm install hono@4.12.27
  ```


## Technical details

A vulnerability exists in Hono's AWS API Gateway v1 and VPC Lattice adapters where repeated request headers are de-duplicated using substring comparison instead of exact matching. For example, if a header contains both '203.0.113.1' and '203.0.113.10', the former is dropped because it is a substring of the latter. This flaw affects middleware or application logic relying on the full header chain, such as X-Forwarded-For for IP restriction, rate limiting, or audit logging. An attacker can craft specific header values to ensure critical values are omitted before the application processes the request. The issue is fixed in version 4.12.27.

## Affected products

- honojs hono >= 4.3.3, < 4.12.27

## Identifiers

- CVE-2026-59897
- GHSA-xgm2-5f3f-mvvc

## Timeline

- 2026-06-23: disclosed: Initial disclosure by yusukebe
- 2026-07-08: advisory: NVD publication
- 2026-07-21: advisory: GitHub Advisory reviewed

## References

- [https://api.github.com/users/DavidCarliez](https://api.github.com/users/DavidCarliez)
- [https://github.com/DavidCarliez](https://github.com/DavidCarliez)
- [https://api.github.com/users/DavidCarliez/gists%7B/gist_id%7D](https://api.github.com/users/DavidCarliez/gists%7B/gist_id%7D)
- [https://api.github.com/users/DavidCarliez/repos](https://api.github.com/users/DavidCarliez/repos)
- [https://avatars.githubusercontent.com/u/271374756?v=4](https://avatars.githubusercontent.com/u/271374756?v=4)
- [https://api.github.com/users/DavidCarliez/events%7B/privacy%7D](https://api.github.com/users/DavidCarliez/events%7B/privacy%7D)

## Related threats

- [Hono: `css` helper does not escape a closing tag split between the template and an interpolated value, allowing HTML injection](https://junglewise.ai/threats/cve-2026-126359-hono-css-helper-does-not-escape-a-closing-tag-split-between-the)
- [Hono RegExpRouter middleware bypass via wildcard matching](https://junglewise.ai/threats/cve-2026-126358-hono-regexprouter-skips-wildcard-middleware-for-some-route-sets)
- [Hono mount() path traversal bypasses middleware scoping](https://junglewise.ai/threats/cve-2026-126357-hono-mount-and-app-mount-can-forward-a-path-that-bypasses)
- [Hono validator form multipart/form-data parameter bypass](https://junglewise.ai/threats/cve-2026-126356-hono-validator-form-skips-validation-for-multipart-form-data)
- [Hono csrf() middleware bypass via hyperlink auditing](https://junglewise.ai/threats/cve-2026-126355-hono-csrf-does-not-check-requests-sent-by-hyperlink-auditing)