CVE-2026-85124 @fastify/http-proxy vulnerable to prefix escape via backslash dot-segments
An unauthenticated attacker can bypass proxy prefix restrictions using backslash dot-segments to access hidden internal upstream endpoints.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- None listed
What is CVE-2026-85124?
CVE-2026-85124 is a high-severity path traversal vulnerability in @fastify/http-proxy, published on 3 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
@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.
Weakness: CWE-22
Which versions are affected by CVE-2026-85124?
It affects 1 product.
Products include Fastify Fastify\/Http-Proxy.
How do I fix CVE-2026-85124?
No fix has been published for CVE-2026-85124 yet. Limit exposure of the affected software until one is available.
Upgrade, and resolve every user-supplied path against an allowed base folder before opening it. More about Path traversal.
Is CVE-2026-85124 being exploited?
There is no known exploitation of CVE-2026-85124 in the wild and no public exploit code.
Other vulnerabilities in @fastify/http-proxy
Sources
Also known as GHSA-qv33-689p-2xq5.