CVE-2026-86250: h3js h3

High severity, CVSS 7.5. EPSS: 0.5% chance of exploitation in the next 30 days.

h3 versions before 2.0.1-rc.18 fail to validate the chunk count parsed from user-controlled cookie values in setChunkedCookie() and deleteChunkedCookie() functions. Attackers can send a crafted cookie header with an extremely large chunk count to trigger an O(n²) cleanup loop that hangs the server process.

Affected products

• h3js h3: from 2.0.0-beta.4, before 2.0.1-rc.18 (fixed in 2.0.1-rc.18)

Published 2026-09-06. Last modified 2026-09-18.