Is CVE-2026-86251 exploited in the wild?

No. Junglewise Threat Intelligence has no report of CVE-2026-86251 being exploited in the wild.

How severe is CVE-2026-86251?

Medium severity, with a CVSS score of 5.9, published 6 September 2026.

Which products and versions are affected?

h3 (npm) SEMVER:introduced=0,fixed=1.15.9.

Is there a fix for CVE-2026-86251?

Yes. h3 (npm): upgrade to 1.15.9 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

How is CVE-2026-86251 attacked?

It is attacked over the network.

Am I affected by CVE-2026-86251?

Check whether you run h3 (npm) 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.

CVE-2026-86251: h3 serveStatic path traversal via double-encoded dot segments

CVE-2026-86251 · Severity: medium · CVSS 5.9 · Published 2026-09-06

h3's serveStatic utility, used to serve static assets from web servers and CDNs, contains a path traversal vulnerability that allows attackers to bypass security checks and read arbitrary files. An attacker can craft requests with double-encoded paths (e.g., `%252e%252e`) that survive validation but are later decoded by backend systems (CDNs, S3, object storage), enabling unauthorized access to sensitive files outside the intended asset directory.

Technologies: h3 (npm). Vendors: npm.

Quick answers

Is CVE-2026-86251 exploited in the wild?: No. Junglewise Threat Intelligence has no report of CVE-2026-86251 being exploited in the wild.
How severe is CVE-2026-86251?: Medium severity, with a CVSS score of 5.9, published 6 September 2026.
Which products and versions are affected?: h3 (npm) SEMVER:introduced=0,fixed=1.15.9.
Is there a fix for CVE-2026-86251?: Yes. h3 (npm): upgrade to 1.15.9 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-86251 attacked?: It is attacked over the network.
Am I affected by CVE-2026-86251?: Check whether you run h3 (npm) 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.

How to fix

h3 (npm): upgrade to 1.15.9 or later
```
npm install h3@1.15.9
```

Technical details

The vulnerability stems from conflicting decode stages in h3's static file serving. When `serveStatic` processes a URL like `/%252e%252e/etc/passwd`, the H3Event constructor first decodes it to `/%2e%2e/etc/passwd` (preserving %25 sequences). A second redundant `decodeURI()` call in serveStatic does not further decode `%2e` (since encodeURI never encodes the dot character), leaving `%2e%2e` intact. The `resolveDotSegments()` function only checks for literal `.` characters and fast-returns without traversal protection. URL-based backends (CDNs, S3) then interpret `%2e%2e` as `..` per RFC 3986/URL Standard, enabling path traversal. Affected versions: h3 ≤ 1.15.8. Patched in 1.15.9. Only URL-based backends are vulnerable; filesystem-based backends are not affected.

Affected products

h3js h3 ≤ 1.15.8

Identifiers

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

Timeline

2026-03-20: disclosed
2026-03-20: patched: Version 1.15.9 released

References

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