# H3 denial of service via unbounded chunked cookie count

Source: https://junglewise.ai/threats/duplicate-advisory-h3-unbounded-chunked-cookie-count-in-session-cleanup-11041bf9  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-09-06. Updated: 2026-10-01  
License: CC BY 4.0 with a link back

Severity: high · CVSS 7.5 · Published 2026-09-06

H3 is a Node.js HTTP framework that handles session management through a cookie chunking system. An unauthenticated attacker can send a crafted HTTP cookie header with an extremely large chunk counter value, causing the server's session cleanup logic to enter a quadratic-time loop that consumes CPU and hangs the process, resulting in denial of service for all users.

Technologies: [h3 (npm)](https://junglewise.ai/threats/technologies/h3). Vendors: [npm](https://junglewise.ai/threats/vendors/npm), [UnJS](https://junglewise.ai/threats/vendors/unjs).

## 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

## References

- [https://github.com/h3js/h3/security/advisories/GHSA-q5pr-72pq-83v3](https://github.com/h3js/h3/security/advisories/GHSA-q5pr-72pq-83v3)

## Related threats

- [h3 serveStatic path traversal via percent-encoded dot segments](https://junglewise.ai/threats/cve-2026-86253-h3-servestatic-path-traversal-via-percent-encoded-dot-segments)
- [h3 SSE event injection via unsanitized carriage return](https://junglewise.ai/threats/cve-2026-86252-h3-sse-event-injection-via-unsanitized-carriage-return)
- [h3 serveStatic path traversal via double-encoded dot segments](https://junglewise.ai/threats/cve-2026-86251-h3-servestatic-path-traversal-via-double-encoded-dot-segments)
- [H3: Unbounded Chunked Cookie Count in Session Cleanup Loop may Lead to Denial of Service](https://junglewise.ai/threats/cve-2026-86250-h3-unbounded-chunked-cookie-denial-of-service)
- [H3 open redirect via protocol-relative path in redirectBack()](https://junglewise.ai/threats/cve-2026-86205-h3-open-redirect-via-protocol-relative-path-in-redirectback)