The following example of a reverse-connect proxy is from just one sample among many that we are seeing in the wild. Most of the data we have collected suggests they are based upon existing SOCKS protocol implementations. This bot sample was additionally designed to evade network port filtering. The proxy bot will iterate through a list of ports until a connection to the controller succeeds. For instance, if port 80 was unreachable it would then attempt to connect to the following ports (in-order): 8080, 3128, 21, 22, 53, 110, 5190, 143, 119, 137, 138, 443, 530, 873, 989, 990. One can see from the list of ports the miscreants have chosen that they are taking advantage of the common practice of allowing outbound connections to popular services by port and protocol without additional inspection. However many networks and most home consumer devices don't implement egress filtering at all and the first port (80/TCP) usually works fine.
Reverse Tunnel Proxy Malware Sample
File type(s): MS-DOS executable (EXE), OS/2 or MS Windows
Size: 14848 Bytes
In the following malware sample, we examine just the first two TCP sessions of the many that were extracted using the Chaosreader packet capture session reassembly tool (http://chaosreader.sf.net/). The packet capture was acquired during the execution of the referenced sample in an instrumented malware analysis environment (sandbox). The sessions below depict the reverse tunnel proxy announcement/registration phase which is followed immediately by controller-initiated spam relay attempts. See Figure 1 for a visual example.