Vulnerability · Published Sep 6, 2026 · Assigned by VulnCheck

CVE-2026-86250h3 before 2.0.1-rc.18 Denial of Service via Unbounded Chunked Cookie

High 7.5

In plain English

A high-severity uncontrolled resource consumption in h3 from h3js. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.5% (higher than 42% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 3.1)7.5High

Exploit odds (30 days)0.5%top 57.8% of all CVEs

Exploited in the wildNo reportsCISA SSVC: none

Attack toolingNoneno public tooling known

Official description

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 packages & fixed versions

Ecosystem	Package	Fixed in	Advisory
npm	h3	2.0.1-rc.18	GHSA-q5pr-72pq-83v3

Affected products (as reported)

Vendor	Product	Versions
h3js	h3	2.0.0-beta.4 – before 2.0.1-rc.18

Weakness: CWE-400 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

References

GitHub Security Advisory (GHSA-q5pr-72pq-83v3) vendor-advisory
VulnCheck Advisory: h3 before 2.0.1-rc.18 Denial of Service via Unbounded Chunked Cookie third-party-advisory