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CVE-2026-86250 – Denial of Service (Unbounded Resource Consumption) – h3js h3 versions 2.0.0-beta.4

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Summary

CVE-2026-86250 is a Denial-of-Service (DoS) vulnerability in h3, the minimal HTTP framework maintained by the h3js project that underpins Nitro and the Nuxt ecosystem. The flaw resides in the chunked-cookie handling logic (setChunkedCookie() / deleteChunkedCookie()), which fails to validate the chunk count parsed from a user-supplied cookie value, allowing a single unauthenticated request to trigger an expensive O(n²) cleanup loop that can hang the Node.js event loop. The issue is rated HIGH severity (CVSS 8.7).

Technical details

  • Root cause: h3 supports "chunked cookies," where oversized cookie values are split across multiple cookies and tracked via a sentinel value (e.g., __chunked__N) indicating the total chunk count. The function that extracts this integer suffix does not enforce an upper bound on N.
  • Trigger conditions: An attacker sends a single HTTP request containing a crafted Cookie header (e.g., Cookie: h3=__chunked__999999) to any endpoint that uses h3’s session/chunked-cookie cleanup functionality.
  • Attack vector: Network-based, unauthenticated, and requires no user interaction — a single ~27-byte forged cookie payload is sufficient to trigger the condition.
  • Impact: When setChunkedCookie() or deleteChunkedCookie() perform cleanup, they iterate through the attacker-claimed chunk count without validation, resulting in an O(n²) loop that consumes excessive CPU and blocks the Node.js event loop, causing the server process to hang — a denial of service for the affected application.

Affected software

  • Product: h3 (npm package h3), maintained by h3js
  • Affected versions: 2.0.0-beta.4 up to (but not including) 2.0.1-rc.18
  • Fixed version: 2.0.1-rc.18 and later

Severity

  • CVSS Score: 8.7 (High)
  • CVSS v4.0 Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
  • Weakness classification: CWE-400 (Uncontrolled Resource Consumption)

Mitigation and recommended actions

  • Immediate: Upgrade the h3 package to version 2.0.1-rc.18 or later, which introduces validation and a bounded maximum on the chunked-cookie count before it is used in cleanup loops.
  • If immediate patching is not possible:
    • Deploy a reverse proxy or WAF rule that rejects or strips Cookie headers containing suspicious __chunked__<large-number> patterns.
    • Monitor for and rate-limit requests carrying abnormally structured cookie headers.
    • Restart/monitor affected Node.js processes with process supervisors configured to auto-recover from event-loop hangs as a stopgap until the patched version is deployed.

References

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