What’s Vulnerable › Vulnerabilities

CVE-2026-86252: h3 before 1.15.9 SSE Event Injection via Carriage Return

• Severity: medium (CVSS 5.3 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.4% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-09-06 · updated 2026-09-08
• Weakness: CWE-74

Description

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; from 2.0.0-beta.0 before 2.0.1-rc.17 — fixed in 1.15.9, 2.0.1-rc.17

How to fix

Upgrade to a fixed version: h3js h3 1.15.9, 2.0.1-rc.17. Check the vendor advisory for mitigations.

References

• 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
• https://github.com/h3js/h3/security/advisories/GHSA-4hxc-9384-m385

Get alerted when CVE-2026-86252 or the next vulnerability affects your stack