The man-in-the middle attack intercepts a communication between two systems. For example, in an http transaction the target is the TCP connection between client and server. Using different techniques, the attacker splits the original TCP connection into 2 new connections, one between the client and the attacker and the other between the attacker and the server, as shown in figure 1. Once the TCP connection is intercepted, the attacker acts as a proxy, being able to read, insert and modify the data in the intercepted communication.
Figure 1. Illustration of man-in-the-middle attack
The MITM attack is very effective because of the nature of the http protocol and data transfer which are all ASCII based. In this way, it’s possible to view and interview within the http protocol and also in the data transferred. So, for example, it’s possible to capture a session cookie reading the http header, but it’s also possible to change an amount of money transaction inside the application context, as shown in figure 2.
Figure 2. Illustration of a HTTP Packet intercepted with Paros Proxy.
The MITM attack could also be done over an https connection by using the same technique; the only difference consists in the establishment of two independent SSL sessions, one over each TCP connection. The browser sets a SSL connection with the attacker, and the attacker establishes another SSL connection with the web server. In general the browser warns the user that the digital certificate used is not valid, but the user may ignore the warning because he doesn’t understand the threat. In some specific contexts it’s possible that the warning doesn’t appear, as for example, when the Server certificate is compromised by the attacker or when the attacker certificate is signed by a trusted CA and the CN is the same of the original web site.
MITM is not only an attack technique, but is also usually used during the development step of a web application or is still used for Web Vulnerability assessments.
MITM Attack tools
There are several tools to realize a MITM attack. These tools are particularly efficient in LAN network environments, because they implement extra functionalities, like the arp spoof capabilities that permit the interception of communication between hosts.
- Cain e Abel
MITM Proxy only tools
Proxy tools only permit interactiion with the parts of the HTTP protocol, like the header and the body of a transaction, but do not have the capability to intercept the TCP connection between client and server. To intercept the communication, it’s necessary to use other network attack tools or configure the browser.
- OWASP WebScarab
- Paros Proxy
- Burp Proxy
- Odysseus Proxy
- Fiddler (by Microsoft)