How is your password attacked?

We protect most of our systems and information with authentication credentials consisting of a username and a password. This is single-factor authentication using something we know (the password).

The passwords we use are open to attack, either by guessing the password and using it to log in, or as a result of a breach where user credentials have been stolen and the lists are subsequently attacked.

Below are some common attack methods used against passwords, along with potential countermeasures.

Social engineering

Attackers will attempt to gain your authentication credentials simply by asking. This can also be combined with other attacks to make them more effective. Most passwords are based on something personal; by discovering details about you, the attacker can build a profile of likely words. Think of the film Wargames, in which Matthew Broderick discovers that the creator of WOPR has left a publicly accessible backdoor with his dead son’s name as the password.

Here, the countermeasures are to educate the user about the danger of social engineering and how attackers use social media as a profiling tool.


There are various forms of password sniffing or logging that can be used by an attacker. Typically, sniffing is where credentials sent over the network – in particular over wireless networks – can be intercepted (sniffed) by an attacker recording the transmitted packets. An additional method relies on infecting computers with malware that captures key strokes being typed (key logging). This can be combined with screen capture to record the use of virtual keyboards and drop-down boxes (such as the selecting of letters of your password), typically used by banking Trojans. Finally, there are physical key loggers that can be attached or built into a keyboard to capture key strokes. The latest versions of these have wireless interfaces built-in. Physical key loggers were mentioned in some of the reports about the Sumitomo Mitsui Banking Corporation in 2004. Wireless accessible KVM (Keyboard, Video and Mouse) over IP were installed in attacks on Santander and Barclays branches in 2013.

Encryption of traffic over the network, up to date anti-malware on devices and awareness of attempts to install hardware are important countermeasures. The PCI DSS mandates looking for rogue wireless access points, so physical inspection can be combined with checks for malicious hardware.

Password brute forcing

There are various brute force attacks, including attacks on the login screen or against the stored credentials.

Single account

Login screens can be attacked by repeatedly guessing the password and submitting the guess until it is accepted. Lockout mechanisms, such as only allowing four guesses before freezing an account permanently or for a defined period of time, can prevent or slow down these attacks. Captcha can also be used to prevent most automated attacks. Log analysis of failed login attempts should indicate that a potential attack is underway.

Stolen password lists

Stolen passwords lists are often protected by a cryptographic function called a ’hash’; popular forms of hash algorithms are MD5 and SHA1. A hash converts the input into a fixed length message digest; the same input generates the same message digest. An attack will take a guess at a password, which is then hashed using the appropriate algorithm and the resulting message digest is then compared to those in the list of stolen password hashes: a match indicates a correct guess. This is a time consuming process which can be sped up using various techniques including Rainbow Tables, which are pre-computed message digests that can be compared to the stolen password list. If a match is detected, then the plain text version of the password can be found.

To prevent the use of pre-computed hash tables, passwords are often concatenated with a random value (‘salt’) unique to a system before being hashed. Other techniques to protect against brute forcing include using a hash algorithm multiple times: the attacker must know how many iterations were used. The Linux shadow password file contains a line per account; the password field consists of a number of elements that include the hash algorithm, the salt, and the message digest. Linux also applies the selected hash thousands of times.

Password brute forcing can make use of parallel and distributed processing. Some attack methods make use of multiple GPUs in a machine, and each GPU can have thousands of cores. A 25 GPU cluster can process 95^8 combinations in just 5.5 hours, enough to brute force every possible eight-character password containing upper- and lower-case letters, digits, and symbols.

Stolen password lists are often posted on bulletin boards for other hackers to crack, and some hackers offer password cracking services.


Hackers can also use botnets, which can consist of tens of thousands to millions of machines, to attack passwords. They can be deployed to brute force passwords lists, or to brute force account credentials with each machine sending a few guesses to the login page to stay within the account lockout rules – of course, when millions of machines are used, accounts can be guessed and accessed.


Given sufficient resources it is always possible to brute force a password, but a high work factor (defined as the amount of effort – usually measured in units of time – needed to break a cryptosystem) will make it impractical to complete a brute force attack.

Strong passwords are able to resist attempts to crack a user’s credentials. The strength is measured in its effectiveness in resisting guessing and brute-force attacks; this is a function of length, complexity, and unpredictability of the password.


The longer the password, the larger the combination space will be. If we assume just lower case letters then the following applies; as the length of the password increases, the number of potential combinations increases exponentially.

Number of letters Sample Combination space
1 a 26
2 aa 676
3 aaa 17576
8 aaaaaaaa 208827064576
10 aaaaaaaaaa 141167095653376


For more complex passwords (by adding upper case and numbers) the combination space increases further.

Number of letters Sample Combination space
1 A 62
2 A9 3844
3 A9a 238328
8 A9aA9aA9 218340105584896
10 A9aA9aA9aA 839299365868340224


Complex does not mean strong

A complex password is not necessarily a strong password, if we look at a typical complex password rule, such as:

  • Minimum eight characters
  • Must use upper and lower case
  • Must use numeric characters
  • Must use symbols

This can result in a password such as:


This is not a strong password, even though it meets the complexity rules. The complexity of a password depends on the combination of the symbols used within the password not being used in a predictable way. The number of available symbols is dependent on the characters accessible through the keyboard and accepted by the application.


Part of preventing a password being broken is its unpredictability. A predictable password would be one found in a dictionary, for example. There is a class of password attacks known as a dictionary attack, in which word lists – often from a dictionary – are used as the source of guesses in the attack. Word lists are not just dictionary lists, but could be lists of football teams or players; the potential source of lists is vast with the Internet having lists of just about every topic from girl’s names to the top million used passwords. This means that, for those Manchester United fans who use a player’s name as their password, there is a list of every player that has ever been in their squad. The tools that perform these attacks automatically switch numbers and symbols for letters based on well accepted rules and will automatically append sequences of numbers to the end of the word. If you used the player’s name Ryan Giggs as the basis of your password – i.e. Ry4nGi66s1973 – this can be guessed by most tools that will accept a list of Man Utd players.

Don’t forget that social engineering or looking at your Facebook page could reveal information that may help an attacker select a word list to use in an attack; you could be making your password more predictable by what you say about yourself online.


In order to create a strong password that is resistant to attack a user must select a password that is long, complex and not based on dictionary words or using ‘leet speak’ to convert letters to numbers or symbols. The longer and more complex it is the more resistant the password will be to attack. Combining passwords (something you know) with a second factor, such as a token (something you have, like your mobile phone), will create a strong authentication system.