CVE-2026-86250 h3 before 2.0.1-rc.18 Denial of Service via Unbounded Chunked Cookie
Attackers can cause a denial of service by sending crafted cookies with large chunk counts, hanging the server process in the h3 library.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-86250?
CVE-2026-86250 is a high-severity denial of service vulnerability in h3js h3, published on 6 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
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.
Weakness: CWE-400
Which versions are affected by CVE-2026-86250?
2 open source packages ship an affected version, including h3.
| Package | Affected | Fixed in |
|---|---|---|
h3 npm | From 2.0.0-beta.4 | 2.0.1-rc.18 |
h3 npm | From 2.0.0-beta.4 | 2.0.1-rc.18 |
How do I fix CVE-2026-86250?
Upgrade h3 to 2.0.1-rc.18 or later.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-86250 being exploited?
There is no known exploitation of CVE-2026-86250 in the wild and no public exploit code.
Sources
Also known as GHSA-q5pr-72pq-83v3.