H3: Unbounded Chunked Cookie Count in Session Cleanup Loop may Lead to Denial of Service

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

5.3 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

EPSS

• CVE-2026-86250: 0.49% estimated exploitation activity in the next 30 days; percentile 42.05 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary The setChunkedCookie() and deleteChunkedCookie() functions in h3 trust the chunk count parsed from a user-controlled cookie value (chunkedN) without any upper bound validation. An unauthenticated attacker can send a single request with a crafted cookie header (e.g., Cookie: h3=chunked999999) to any endpoint using sessions, causing the server to enter an O(n²) loop that hangs the process. Details The chunked cookie system stores large cookie values by splitting them into numbered chunks. The main cookie stores a sentinel value chunkedN indicating how many chunks exist. When setting a new chunked cookie, the code cleans up any previous chunks that are no longer needed. The vulnerability is in getChunkedCookieCount() at src/utils/cookie.ts:244-249: This value is consumed without validation in the cleanup loop of setChunkedCookie() at src/utils/cookie.ts:182-190: The same issue exists in deleteChunkedCookie() at src/utils/cookie.ts:227-232: The exploit chain through sessions: 1.…

Published by the source

23 Mar 2026, 21:44 UTC

Last modified by the source

1 Oct 2026, 00:00 UTC

Source last checked

26 Oct 2026, 13:06 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-q5pr-72pq-83v3

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-q5pr-72pq-83v3

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# Patch Next advisory brief: CVE-2026-86250

Advisory: H3: Unbounded Chunked Cookie Count in Session Cleanup Loop may Lead to Denial of Service
IDs: CVE-2026-86250, GHSA-q5pr-72pq-83v3
Record: https://patchnext.com/advisories/cve-2026-86250 (revision 2, source checked 26 Oct 2026, 13:06 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: h3 2.0.1-rc.18. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm h3: affected >= 2.0.0-beta.4, < 2.0.1-rc.18; fixed in 2.0.1-rc.18

## Suggested inspection (read-only)
- `npm ls h3 --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on h3.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-q5pr-72pq-83v3 (OSV record GHSA-q5pr-72pq-83v3 lists affected ranges: npm h3 >=2.0.0-beta.4 <2.0.1-rc.18 (fixed))
- Verified: https://github.com/advisories/GHSA-q5pr-72pq-83v3 (GitHub-reviewed advisory GHSA-q5pr-72pq-83v3 lists affected ranges: npm h3 >=2.0.0-beta.4 <2.0.1-rc.18)

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