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CVE-2026-15631

8.7 · HIGH

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.

Published Jul 18, 2026CWE-22EPSS 0.50%

Overview

CVE-2026-15631 is a high-severity vulnerability. It was published on July 18, 2026 and has a CVSS 3.1 base score of 8.7 (HIGH).

This vulnerability has a CVSS 3.1 base score of 8.7, rated HIGH. It can be exploited remotely over the network. No authentication or special privileges are required for exploitation.

Technical 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.

Remediation

Check the references section for vendor advisories, patches, and mitigation guidance. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.

References

Refcna.openjsf.org/security-advisories.htmlGitHubgithub.com/fastify/fastify-http-proxy/security/advisories/GHSA-7hrw-592w-9wh2

How CyberStrike helps

Frequently asked questions

What is CVE-2026-15631?

CVE-2026-15631 is a high-severity vulnerability. It was published on July 18, 2026 and has a CVSS 3.1 base score of 8.7 (HIGH).

How severe is CVE-2026-15631?

This vulnerability has a CVSS 3.1 base score of 8.7, rated HIGH. It can be exploited remotely over the network. No authentication or special privileges are required for exploitation.

How do I fix or remediate CVE-2026-15631?

Check the references section for vendor advisories, patches, and mitigation guidance. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.

How can CyberStrike help with CVE-2026-15631?

CyberStrike is an open-source, AI-augmented pentesting engine. Rather than flag a version number, it tests whether an issue like CVE-2026-15631 is actually reachable in your own applications and APIs, then verifies each finding before reporting it. That tells you if the weakness is a real, exploitable risk in your environment or a false positive you can safely deprioritize.

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