CVE-2026-33128: h3

Critical severity, CVSS 10.0. EPSS: 0.7% chance of exploitation in the next 30 days.

H3 is a minimal H(TTP) framework. In versions prior to 1.15.6 and between 2.0.0 through 2.0.1-rc.14, createEventStream is vulnerable to Server-Sent Events (SSE) injection due to missing newline sanitization in formatEventStreamMessage() and formatEventStreamComment(). An attacker who controls any part of an SSE message field (id, event, data, or comment) can inject arbitrary SSE events to connected clients. This issue is fixed in versions 1.15.6 and 2.0.1-rc.15.

Affected products

• h3 h3: before 1.15.6 (fixed in 1.15.6); version 2.0.0 only; version 2.0.1 only

Published 2026-03-20. Last modified 2026-06-17.