CVE-2026-86252

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.

• https://github.com/h3js/h3/security/advisories/GHSA-4hxc-9384-m385
• https://www.vulncheck.com/advisories/h3-before-1.15.9-sse-event-injection-via-carriage-return