3128 - Proxy
Overview
The TCP port 3128 is commonly used by web proxy servers, such as Squid. A proxy server is simply a component that acts as an intermediary relay for clients accessing network resources. Instead of connecting directly to the resource, the client makes requests to the proxy server that fulfil, or not, the requests and transmit the result back to the client.
Web proxies, also known as HTTP proxies, forward HTTP requests or TCP sessions. The later are tunnelled using the CONNECT HTTP verb. As stated in the Squid documentation for CONNECT tunnel: "the proxy establishes a TCP connection to the specified server, responds with an HTTP 200 (Connection Established) response, and then shovels packets back and forth between the client and the server, without understanding or interpreting the tunneled traffic".
Open proxies
An open proxy is a proxy server that will forward unauthenticated client's requests, which may be leveraged to access services exposed on the proxy server's loopback interface or network resources otherwise inaccessible.
Network enumeration
nmap can be used to scan the network for exposed Proxy services:
Open proxies detection
Multiple techniques may be used to detect open web proxies, each one having its own advantages and disadvantages. The usage of some of the tools associated with each techniques is detailed below.
Attempt to access a well known website through the proxy.
Only requires a single request.
Requires the proxy server to have Internet access. Forwarding of internal traffic may be authorized while sending of Internet going traffic restricted.
nmap's NSE script http-open-proxy.
By default the script will attempt to access www.google.com.
masscan + Masscan-Proxies-Tester.
By default the script will attempt to access http://perdu.com.
Detailed in the "Access attempt to a well known website" section below.
Attempt to access localhost services (of the proxy server) through the proxy.
Requires a maximum of 65535 requests to exhaustively attempt to access every possible localhost TCP services.
Forwarding to remote hosts may be authorized while access to the loopback interface restricted.
proxychains + nmap full TCP connect scan.
Metasploit.
Detailed in the "HTTP(S) proxy usage" section below.
Attempt to access well known ports / services (or all ports) of remote internal hosts.
The more ports / hosts are scanned, the more thorough the approach will be.
May requires a tremendous amount of time depending on the number of ports / hosts scanned.
proxychains + nmap full TCP connect scan.
Detailed in the "HTTP(S) proxy usage" section below.
Access attempt to a well known website
The nmap's NSE script http-open-proxy or masscan followed by Masscan-Proxies-Tester Python script can be used to detect open proxy servers that can reach the Internet. Masscan-Proxies-Tester takes as input a masscan scan output in the List format.
HTTP(S) proxy usage
Tools may natively support the specification of a proxy to channel HTTP / HTTPS requests or TCP sessions through a web proxy server. For tools that do not offer such mechanism natively (such as nmap), proxychains can be used to force the TCP connections made by the given application to pass through the specified proxy.
Note that a number of restrictions apply when conducting ports scan through a proxy (usage of full TCP connections, no forwarding of ICMP requests, etc.). Refer to the [General] Ports scan note for more information on how to conduct a ports scan through a Proxy server.
In additions to HTTP(S) proxies, proxychains also supports SOCKS4 / SOCKS5 and TOR proxies. For more information on SOCKS proxies, refer to the [General] Pivoting note.
The Metasploit's auxiliary/scanner/http/squid_pivot_scanning module can also be used to directly conduct network scan through an exposed Squid proxy:
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