Brute Force Attacks

Brute Force Attacks

These attacks are aimed at gaining access to a system by repeated
attempts at authentication. Most services that require a username and
password, and have no facility for account lockout, are vulnerable to
this type of attack.
Brute force methods are commonly used to crack password files, as this
can be done reasonably quickly on a local system. Common tools used
in this case are:
• crack - A Unix based program.

• L0phtcrack - A Windows based program.
Attacking network based services can be more time consuming as the
response time will depend heavily on the network load. Tools exist to
crack the following services:
• telnet.
• ftp.
• http.
• CGI logins.
To improve the chances of a successful brute force attack, one part of a
two part authentication is needed. This can be obtained from other
network or system vulnerabilities, e.g. finger or null sessions, or by
“dumpster diving” and other social engineering methods.

Dictionary Attack

Once a username has been established, it is expedient to first try a
dictionary based attack which tries words from various dictionaries
until a match is found. The dictionaries available vary in size and scope
as well as subject. There are specific themes dictionaries available such
as Star Wars dictionaries that can be used in conjunction with other
information to produce a more targeted attack.
Failing a dictionary attack, a true brute force method can be followed,
which attempts every combination of characters from a known subset
until a match is found. This can be very time consuming if this subset is
large or if the minimum password length is relatively long.