# CVE-2026-86251: h3 before 1.15.9 Path Traversal via Double Decoding

> Attackers can read arbitrary files from backend storage via double-encoded path traversal in the h3 serveStatic utility.

- **Severity:** High 8.2 (`CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 6 Sep 2026
- **Updated:** 9 Sep 2026
- **Also known as:** GHSA-676v-2c25-vqpj, GHSA-72gr-qfp7-vwhw

## What is CVE-2026-86251?

CVE-2026-86251 is a high-severity path traversal vulnerability in h3js h3, published on 6 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

h3 versions before 1.15.9 contain a path traversal vulnerability in the serveStatic utility. A double-decoding flaw allows a request path containing double-encoded dot sequences (e.g. %252e%252e) to be decoded to %2e%2e, which survives resolveDotSegments() because that function only checks for literal '.' characters. When the resulting asset ID is resolved by URL-based backends (CDN, S3, object storage), %2e%2e is interpreted as '..' per RFC 3986, enabling path traversal to read arbitrary files from the backend.

Weakness: CWE-22 (Improper Limitation of a Pathname to a Restricted Directory)

## Which versions are affected by CVE-2026-86251?

2 open source packages ship an affected version, including h3.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `h3` | npm | All versions | 1.15.9 |
| `h3` | npm | All versions | 1.15.9 |

## How do I fix CVE-2026-86251?

Upgrade h3 to 1.15.9 or later.

## Is CVE-2026-86251 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-86251 exists.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-86251 and fix it if it is affected.

What it is: CVE-2026-86251 is a high-severity path traversal vulnerability in h3js h3, published on 6 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-86251 exists.

Affected packages and fixed versions:
- h3 (npm): affected all versions; fixed in 1.15.9
- h3 (npm): affected all versions; fixed in 1.15.9

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-86251/ (Markdown: https://insights.sbomify.com/CVE-2026-86251/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-86251
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-86251
- OSV: https://osv.dev/vulnerability/CVE-2026-86251

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-86251/
Summaries are generated and may contain mistakes; the official description above is the record.