Software Engineering Institute

Researchers at the University of California at Berkeley have determined that by monitoring the delays between SSH packets transmitted across the network, it is possible to make educated guesses about the keystrokes typed by the user. This vulnerability relies primarily upon the fact that in interactive SSH sessions, each keystroke made by the user causes the SSH client to transmit one IP packet to the SSH server. Similarly, as the remote server echoes the typed characters back to the user, the SSH server sends individual IP packets back to the SSH client.

This behavior (which is common to many terminal programs) creates identifiable patterns in the packet data that yield information about the user's activities. For example, as described in the Berkeley paper, if a system administrator logs into a remote Unix system and types the su command to become the superuser, the process will look similar to this:

sysadmin@hostname % su
Password:
root@hostname #

Since this conversation takes place over an interactive terminal session, "sysadmin@hostname % " will be transmitted as group of characters (most likely in a single data packet), each character in "su" will result in two data packets (one keystroke and one echo), and "Password: " will be transmitted as a single data packet. Then, as the user types the root password, each character typed will result in a single data packet, but an echo will not be sent. This lack of an echo response is trivial to detect within a captured data stream and as a result, the attacker will know exactly which keystrokes (and how many) represent the system's root password. Then, by using the technique documented in the Berkeley paper, it is possible to analyze the delays between each keystroke of the password to simplify a brute-force attack against it.

This vulnerability has some noteworthy limitations that warrant mention

• The attacker needs accurate timing information for the victim's session, which means that this technique may prove to be fruitless when conducted over a high-latency network connection with many simultaneous sessions present.
• Successful exploitation of this vulnerability does not yield a usable password, it merely simplifies a brute force attack. Such attacks (even when simplified) will still take a significant amount of time and will be easily detectable by observant victims.
• At present, this technique has only been demonstrated with short sequences of keystrokes produced by touch typists with repeatable typing styles.

This vulnerability reduces the number of guesses needed to perform brute-force attacks against passwords typed in an existing SSH session.

The CERT/CC is currently unaware of a practical solution to this problem.

Mask echo behavior in terminal programs

This technique is dependent upon the previously mentioned echo behavior to identify password locations, so adding dummy echo characters to the password prompt (such as asterisks) would present a significant challenge to attackers.