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active man-in-the-middle attack (MitM)

This definition is part of our Essential Guide: Antimalware tools and techniques security pros need right now

What is an active man-in-the-middle attack?

Active man-in-the-middle (MitM) is an attack method that allows an intruder to access sensitive information by intercepting and altering communications between the user of a public network and any requested website.

Avoiding logging in to sensitive sites from public locations can protect the user from conventional man-in-the-middle attacks. However, in an active MitM attack, the perpetrator manipulates communications in such a way that they can steal information for sites accessed at other times. The attacker can then use that information for identity theft or other types of fraud.

An active MitM may be conducted in a number of ways. Here's one method:

  1. The attacker listens to communications transmitted over a public network.
  2. The victim accesses the Internet over the network and browses to an innocuous website, such as a mainstream news site.
  3. The website server processes the request and responds to it.
  4. The attacker intercepts the response sent from the server and interjects an IFrame object targeting their chosen site.
  5. When the user's browser receives the compromised response, it invisibly requests that website along with the cookie storing user credentials for the site.
  6. This response allows the attacker to log in to the site and interact in any way that the valid user can.

The attacker may also use cache poisoning to prolong the attack.

Roi Saltzman and Adi Sharabani reported on active man-in-the-middle attacks in February 2009. Although the researchers stress that you can't protect yourself from active MitM attacks completely, they offer a number of suggestions for safer browsing. Saltzman and Sharabani recommend that before connecting to a public network, you delete all cookies and cache files. That should mean there is no data for an attacker to steal. When you disconnect from the public network, you should repeat the process so that any suspect data generated during the session will be removed. The researchers also recommend dedicating one browser to public browsing and never using that browser to access sites with sensitive data.

This was last updated in December 2009

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