Quick answers
- Is This vulnerability exploited in the wild?
- No. Junglewise Threat Intelligence has no report of This vulnerability being exploited in the wild.
- How severe is This vulnerability?
- High severity, with a CVSS score of 7.5, published 6 September 2026.
- Which products and versions are affected?
- h3 (npm) >= 2.0.0-beta.4, < 2.0.1-rc.18; UnJS H3 >= 2.0.0-beta.4, < 2.0.1-rc.18.
- Is there a fix for This vulnerability?
- A patch was published on 2026-03-21: Fixed in version 2.0.1-rc.18 Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
- How is This vulnerability attacked?
- It is attacked over the network, in a software library.
- Am I affected?
- Check whether you run h3 (npm) or UnJS H3 at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.
Technical details
The vulnerability is an uncontrolled resource consumption flaw in the setChunkedCookie() and deleteChunkedCookie() functions. These functions parse a chunk count from user-supplied cookie values (e.g., __chunked__999999) without validating an upper bound, then execute nested cleanup loops that scale O(n²) with the attacker-controlled count. The attack requires only network access and no authentication or user interaction; any request to a session-using endpoint triggers the vulnerable code path.
Affected products
- Unjs H3 >= 2.0.0-beta.4, < 2.0.1-rc.18
Timeline
- 2026-03-21: disclosed
- 2026-03-21: patched: Fixed in version 2.0.1-rc.18