# CVE-2026-15631: @fastify/http-proxy vulnerable to prefix escape via WebSocket path traversal

> Attackers can bypass proxy restrictions to access internal upstream endpoints via WebSocket path traversal in @fastify/http-proxy.

- **Severity:** High 8.7 (`CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:H/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 18 Jul 2026
- **Updated:** 20 Jul 2026
- **Also known as:** GHSA-7hrw-592w-9wh2

## What is CVE-2026-15631?

CVE-2026-15631 is a high-severity path traversal vulnerability in @fastify/http-proxy, published on 18 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

Impact: @fastify/http-proxy versions from 9.4.0 up to and including 11.5.0 fail to validate the resolved WebSocket destination path against the configured rewrite prefix. The WebSocket routing path in WebSocketProxy.findUpstream resolves the destination via the WHATWG URL constructor, which collapses dot segments, so a crafted upgrade request with path traversal sequences can escape the rewrite prefix and reach upstream endpoints that were not meant to be exposed by the proxy. This is a variant of CVE-2021-21322 in a code path that never went through the HTTP fix in fastify/reply-from. Exploitation requires a non-normalizing WebSocket client, since browsers and the ws package normalize the request path before sending, but raw HTTP clients or downstream proxies that forward the request target unchanged make the attack reachable in production topologies. 

Patches: upgrade to @fastify/http-proxy 11.6.0. 

Workarounds: none.

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

## Which versions are affected by CVE-2026-15631?

It affects 1 product.


Products: Fastify Fastify\/Http-Proxy.

## How do I fix CVE-2026-15631?

No fix has been published for CVE-2026-15631 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-15631 being exploited?

There is no known exploitation of CVE-2026-15631 in the wild and no public exploit code.

## Other vulnerabilities in @fastify/http-proxy

- [CVE-2026-16117](https://insights.sbomify.com/CVE-2026-16117/): @fastify/http-proxy vulnerable to prefix escape via URL-encoded characters (critical 10.0)
- [CVE-2026-33805](https://insights.sbomify.com/CVE-2026-33805/): @fastify/reply-from vulnerable to connection header abuse enabling stripping of proxy-added headers (critical 9.0)
- [CVE-2026-85124](https://insights.sbomify.com/CVE-2026-85124/): @fastify/http-proxy vulnerable to prefix escape via backslash dot-segments (high 7.5)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-15631 and fix it if it is affected.

What it is: CVE-2026-15631 is a high-severity path traversal vulnerability in @fastify/http-proxy, published on 18 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-15631 in the wild and no public exploit code.

Affected product: @fastify/http-proxy. No package-level version data is available, so check the vendor advisory linked from the reference page.

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-2026-15631/ (Markdown: https://insights.sbomify.com/CVE-2026-15631/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-15631
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-15631
- OSV: https://osv.dev/vulnerability/CVE-2026-15631

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-15631/
Summaries are generated and may contain mistakes; the official description above is the record.