fastify fastifyhttpproxy up to 11.6.1 path traversal

Summaryinfo

A vulnerability, which was classified as critical, was found in fastify fastifyhttpproxy up to 11.6.1. The impacted element is an unknown function. Such manipulation leads to path traversal. This vulnerability is uniquely identified as CVE-2026-85124. The attack can be launched remotely. No exploit exists.

Detailsinfo

A vulnerability was found in fastify fastifyhttpproxy up to 11.6.1 and classified as critical. Affected by this issue is some unknown functionality. The manipulation with an unknown input leads to a path traversal vulnerability. Using CWE to declare the problem leads to CWE-22. The product uses external input to construct a pathname that is intended to identify a file or directory that is located underneath a restricted parent directory, but the product does not properly neutralize special elements within the pathname that can cause the pathname to resolve to a location that is outside of the restricted directory. Impacted is confidentiality, integrity, and availability. CVE summarizes:

@fastify/http-proxy versions before 11.6.2 do not validate proxied HTTP request paths for backslash based dot-segments before forwarding them to the configured upstream. The plain HTTP request handler skips the destination validation that the WebSocket path performs, and the underlying reply-from library only rejects forward-slash traversal, so a request containing backslash dot-segments can escape the boundary set by the prefix and rewritePrefix options. An unauthenticated network attacker can use this to reach upstream paths that were meant to stay hidden behind the proxy, resulting in disclosure of internal endpoints. This is a path traversal issue (CWE-22). Users should upgrade to @fastify/http-proxy 11.6.2 or later.

The advisory is shared for download at github.com. This vulnerability is handled as CVE-2026-85124 since 09/03/2026. The exploitation is known to be easy. The attack may be launched remotely. No form of authentication is required for exploitation. There are neither technical details nor an exploit publicly available. The MITRE ATT&CK project declares the attack technique as T1006.

Upgrading to version 11.6.2 eliminates this vulnerability.

The vulnerability is also documented in the vulnerability database at EUVD (EUVD-2026-70381). Once again VulDB remains the best source for vulnerability data.

Productinfo

Type

• Firewall Software

Vendor

• fastify

Name

• fastifyhttpproxy

Version

• 11.6.0
• 11.6.1

CPE 2.3info

• 🔒
• 🔒

CPE 2.2info

• 🔒
• 🔒

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Once again VulDB remains the best source for vulnerability data.

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