T 0.15 Eavesdropping

Targeted attacks on communication connections, conversations, noise sources of all kinds or IT systems with the objective of collecting information are referred to as eavesdropping. It ranges from unnoticed, surreptitious eavesdropping on a conversation to highly specialised complex attacks in order to intercept signals transmitted over radio or transmission lines, e.g. with the help of antennae or sensors.

It is not only because of the low chance of discovery that wiretapping of lines or eavesdropping on wireless connections is a considerable threat for information security. In principle, there are no tap-proof cables. It is only the effort required by the eavesdropper that distinguishes the security of cables. Whether a line is actually wiretapped can only be ascertained through high metrological effort.

Particularly critical is the unprotected transmission of authentication data in plain-text protocols like HTTP, FTP or telnet, since they can easily be analysed automatically due to the clear structure of the data.

The decision to eavesdrop on information somewhere is in principle determined by the question of whether the information is worth the respective technical or financial effort and the risk of being discovered. The answer to this question essentially depends on the individual possibilities and interests of the attacker.

Examples:

• In the case of telephone calls, it is not only eavesdropping on conversations that can be of interest to an attacker. The information which is transmitted in signalling can be misused by an attacker as well e.g. due to an incorrect setting in the terminal resulting in the password being transmitted in plain text at the time of login.
• An attacker can easily eavesdrop on the entire communication if wireless transmission is unprotected or insufficiently protected (e.g. if a WLAN is protected only with WEP).
• Emails can be read throughout their entire journey through the network if they are not encrypted. Unencrypted emails should therefore not be compared with conventional letters but with postcards.