CVE-2026-86250

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.

  • Published Sep 6, 2026
  • CVSS 8.7 high
  • 0.5% chance of exploitation in the next 30 days (EPSS)
  • A fix is available

Affected software

CVE-2026-86250 at the National Vulnerability Database