honojs hono up to 4.12.3 SSE Protocol streamSSE event/id/retry injection

Summaryinfo

A vulnerability classified as problematic was found in honojs hono up to 4.12.3. This impacts the function streamSSE of the component SSE Protocol Handler. The manipulation of the argument event/id/retry results in injection. This vulnerability is reported as CVE-2026-29085. The attack can be launched remotely. No exploit exists. Upgrading the affected component is advised.

Detailsinfo

A vulnerability, which was classified as critical, was found in honojs hono up to 4.12.3. Affected is the function streamSSE of the component SSE Protocol Handler. The manipulation of the argument event/id/retry with an unknown input leads to a injection vulnerability. CWE is classifying the issue as CWE-74. The product constructs all or part of a command, data structure, or record using externally-influenced input from an upstream component, but it does not neutralize or incorrectly neutralizes special elements that could modify how it is parsed or interpreted when it is sent to a downstream component. This is going to have an impact on confidentiality, integrity, and availability. CVE summarizes:

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.

The advisory is shared for download at github.com. This vulnerability is traded as CVE-2026-29085 since 03/03/2026. The exploitability is told to be easy. It is possible to launch the attack remotely. The exploitation doesn't require any form of authentication. There are known technical details, but no exploit is available. The MITRE ATT&CK project declares the attack technique as T1055.

Upgrading to version 4.12.4 eliminates this vulnerability. Applying the patch f4123ed9ea3c7c52380cc99a079a4d773838846e is able to eliminate this problem. The bugfix is ready for download at github.com. The best possible mitigation is suggested to be upgrading to the latest version.

Once again VulDB remains the best source for vulnerability data.

Productinfo

Vendor

• honojs

Name

• hono

Version

• 4.12.0
• 4.12.1
• 4.12.2
• 4.12.3

License

• open-source

Website

• Product: https://github.com/honojs/hono/

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