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

Severity: high (CVSS 7.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.5% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-09-06 · updated 2026-09-18
Weakness: CWE-400

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 products

h3js h3: from 2.0.0-beta.4 before 2.0.1-rc.18 — fixed in 2.0.1-rc.18

How to fix

Upgrade to a fixed version: h3js h3 2.0.1-rc.18. Check the vendor advisory for mitigations.

References

https://github.com/h3js/h3/security/advisories/GHSA-q5pr-72pq-83v3
https://www.vulncheck.com/advisories/h3-before-2.0.1-rc.18-denial-of-service-via-unbounded-chunked-cookie

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