CVE-2026-86252: h3js h3

Medium severity, CVSS 5.3. EPSS: 0.4% chance of exploitation in the next 30 days.

h3 versions before 1.15.9 fail to sanitize carriage return characters in EventStream data and comment fields, allowing attackers to inject arbitrary SSE events by including unsanitized carriage returns. Attackers can inject event type directives, split single push calls into multiple browser-parsed events, or escape comment fields to inject data, bypassing the prior CVE fix that only addressed newline injection.

Affected products

• h3js h3: before 1.15.9 (fixed in 1.15.9); from 2.0.0-beta.0, before 2.0.1-rc.17 (fixed in 2.0.1-rc.17)

Published 2026-09-06. Last modified 2026-09-08.