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Medium, CVSS 5.9amended

h3: Double Decoding in serveStatic Bypasses resolveDotSegments Path Traversal Protection via %252e%252e

CVE-2026-86251GHSA-72gr-qfp7-vwhw

What to do

Vendor fix available. Upgrade each affected package to the fixed version on its release line (h3 1.15.9), redeploy, then verify what is actually installed.

This page has no inventory attached, so it cannot say whether you are affected. A vendor fix is not proof that your deployment is fixed.

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Affected packages

  • npm h3

    Affected
    <= 1.15.8
    Fixed in
    1.15.9

Sources

Severity and exploitation
Exploitation
No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.
CVSS
5.9 · CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N
EPSS
  • CVE-2026-86251: 0.43% estimated exploitation activity in the next 30 days; percentile 37.07 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.
Source description

As published by the source; long descriptions are shortened.

Summary The serveStatic utility in h3 applies a redundant decodeURI() call to the request pathname after H3Event has already performed percent-decoding with %25 preservation. This double decoding converts %252e%252e into %2e%2e, which bypasses resolveDotSegments() (since it checks for literal . characters, not percent-encoded equivalents). When the resulting asset ID is resolved by URL-based backends (CDN, S3, object storage), %2e%2e is interpreted as .. per the URL Standard, enabling path traversal to read arbitrary files from the backend. Details The vulnerability is a conflict between two decoding stages: Stage 1 — H3Event constructor (src/event.ts:65-69): This correctly preserves %25 sequences by escaping them before decoding. A request for /%252e%252e/etc/passwd produces event.url.pathname = /%2e%2e/etc/passwd — the %25 was preserved so %252e became %2e (not .). Stage 2 — serveStatic (src/utils/static.ts:86-88): This applies a second decodeURI(), which decodes %2e → ., producing…

Dates and revisions
Published by the source
Last modified by the source
Source last checked
  1. Revision 2 · amended ·

    Amended from ghsa record GHSA-72gr-qfp7-vwhw

  2. Revision 1 · created ·

    Published from osv record GHSA-72gr-qfp7-vwhw

Agent brief

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# Patch Next advisory brief: CVE-2026-86251

Advisory: h3: Double Decoding in `serveStatic` Bypasses `resolveDotSegments` Path Traversal Protection via `%252e%252e`
IDs: CVE-2026-86251, GHSA-72gr-qfp7-vwhw
Record: https://patchnext.com/advisories/cve-2026-86251 (revision 2, source checked 26 Oct 2026, 12:46 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.9)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: h3 1.15.9. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm h3: affected <= 1.15.8; fixed in 1.15.9

## 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-72gr-qfp7-vwhw (OSV record GHSA-72gr-qfp7-vwhw lists affected ranges: npm h3 >=0 <1.15.9 (fixed))
- Verified: https://github.com/advisories/GHSA-72gr-qfp7-vwhw (GitHub-reviewed advisory GHSA-72gr-qfp7-vwhw lists affected ranges: npm h3 >=0 <=1.15.8)

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