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

Severity: medium (CVSS 6.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.3% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-03-04 · updated 2026-06-17
Weakness: CWE-74

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.

Affected products

honojs hono: before 4.12.4 — fixed in 4.12.4

How to fix

Upgrade to a fixed version: honojs hono 4.12.4. Check the vendor advisory for mitigations.

References

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