Is CVE-2026-86253 exploited in the wild?

No. Junglewise Threat Intelligence has no report of CVE-2026-86253 being exploited in the wild.

How severe is CVE-2026-86253?

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

Which products and versions are affected?

h3 (npm) SEMVER:introduced=2.0.0,fixed=2.0.1-rc.15.

Is there a fix for CVE-2026-86253?

Yes. h3 (npm): upgrade to 2.0.1-rc.15 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

How is CVE-2026-86253 attacked?

It is attacked over the network, in a software library.

Am I affected by CVE-2026-86253?

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-2026-86253: h3 serveStatic path traversal via percent-encoded dot segments

CVE-2026-86253 · Severity: medium · CVSS 5.9 · Published 2026-09-06

h3 is a popular Node.js HTTP framework that includes a static file serving utility. An unauthenticated attacker can exploit improper URL decoding to read arbitrary files on the server, including sensitive configuration files, environment variables, source code, and system files. The vulnerability bypasses security boundaries that are supposed to restrict file access to a designated directory.

Technologies: h3 (npm). Vendors: npm.

Quick answers

Is CVE-2026-86253 exploited in the wild?: No. Junglewise Threat Intelligence has no report of CVE-2026-86253 being exploited in the wild.
How severe is CVE-2026-86253?: Medium severity, with a CVSS score of 5.9, published 6 September 2026.
Which products and versions are affected?: h3 (npm) SEMVER:introduced=2.0.0,fixed=2.0.1-rc.15.
Is there a fix for CVE-2026-86253?: Yes. h3 (npm): upgrade to 2.0.1-rc.15 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-86253 attacked?: It is attacked over the network, in a software library.
Am I affected by CVE-2026-86253?: 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 2.0.1-rc.15 or later
```
npm install h3@2.0.1-rc.15
```

Technical details

The vulnerability is a path traversal (CWE-22) in the serveStatic() function's handling of URL paths. The root cause is in src/utils/static.ts line 86, where h3 uses FastURL (from srvx) to parse request URLs. FastURL extracts the pathname via raw string slicing without normalizing dot segments or decoding percent-encoded characters. When a request arrives with %2e%2e (percent-encoded dot-dot), FastURL leaves it verbatim in event.url.pathname. The code then calls decodeURI() on this raw path, converting %2e to a literal dot, producing traversal sequences like /../. These unsanitized paths are passed directly to user callbacks (getMeta, getContents) without validation, allowing them to escape the static root directory when performing filesystem operations. The vulnerability affects h3 v2.x and earlier on Node.js; it does not affect runtimes with pre-parsed URL objects. Patches are available in versions ≥1.15.6 and ≥2.0.1-rc.15.

Affected products

h3 h3 <1.15.6, 2.0.0 to 2.0.1-rc.14

Identifiers

CVE-2026-86253
GHSA-wr4h-v87w-p3r7

Timeline

2026-03-18: disclosed
2026-03-18: patched: Patches released in v1.15.6 and v2.0.1-rc.15

References

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