Passwords: Threats and Counter-Measures

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There’s little doubt that passwords are an inconvenience. Unfortunately they remain the most practical way for most of us to keep our on-line identities to ourselves. Without them, or if you don’t keep them secret, it would be far easier for someone else to masquerade as you, to read and modify any of your information and to take any action in your name.

Given the power that knowing someone’s password gives it’s not surprising that a lot of people, from so-called “friends” to serious criminals, try to find them out. This factsheet explains the most common ways that is done and some of the ways that systems and their users can either prevent it or, at least, reduce the damage when it happens. Precautions against password loss are most effective when systems and users cooperate – systems designers should beware of making their controls so inconvenient that users look for easier ways around them.

Users’ tolerance of inconvenience will vary based, among other things, on the organisational culture and how they perceive the importance of the services and information that is being protected. The choice of measures must therefore take those factors into account. Users in universities and colleges are likely to fall somewhere between users in regulated businesses and users of social networks, though their expectations may be set by the latter and they may need reminding that they are in a different environment.

Threats to Passwords

The following are currently the most common ways for passwords to become known to people other than their rightful owners.


The simplest way to discover someone’s password is to have them tell you it.  This can be done by persuading them to type it into a website you control (commonly known as phishing), by installing a keylogger (either hardware or software) on a computer, or by reading traffic on an unencrypted wireless or wired network. For intruders these methods have the great benefit that it does not matter how long or complex a password the user has chosen: the intruder can simply read it.

Cracking of hashes/brute force

If the intruder cannot obtain the password then he can simply use a program to generate billions of possible passwords (often using the same techniques as are suggested for choosing passwords) and try each of them against the account. The crudest way to do this is to simply attempt to log in using each generated password: the resulting flood of password failures should be easy for a system administrator to spot, but since attackers continue to use this approach it seems it is still reasonably successful. Attempts may be made against obscure authenticated services, such as SSH and LDAP, to reduce the chances of detection.

Offline cracking

Brute force attacks are much less obvious if the intruder can obtain a copy of an encrypted password, for example if a system’s password file can be downloaded, if a hash has been included in a public file, or if an unknown machine can join an authentication group. Once the intruder has one or more encrypted passwords he can do the brute force guessing on his own machine (using modern hardware and algorithms this may take only a few minutes for short passwords), or even use a cloud service, and then return to login to the target once the correct password has been discovered.

Password recovery/reset systems

An intruder may not need to get the password from the user if he can persuade the authentication system to either mail it to him or change it to something of his choice. Systems to allow the legitimate user to recover or change a password they have forgotten can also let other people do the same. Helpdesk operators need to be particularly careful to check the identity of anyone asking for a password reset. On-line systems that rely on “secret questions” such as “name of first school” or “birthday” are trivial to defeat if that information can be found on a social network. Systems that send reminders to a backup e-mail address or phone number can fail if the user changes address or number allowing the abandoned backup to be registered by someone else.

Educated guesswork

It should be obvious that the same techniques used to guess the answers to secret questions can also be used to guess passwords. Anything based on something your friends will know, or that is available from a website, is a very poor choice as a password.

Reuse of Passwords

Most people now have many different accounts on different systems in both their private and work lives. Although best practice is to have a different password for every account, unfortunately it’s much more common to reuse the same password on different services. That means that an organisation doesn’t just have to worry about the above attacks against its own systems, it has to worry about the same attacks on all other systems where the same password has been used. This probably means that an organisation can no longer completely control whether its passwords are secure: it should also develop plans and systems to detect and respond when a password has been compromised.

Default passwords

Equipment and software often has standard pre-configured passwords which, of course, are well known to intruders. Such passwords should always be changed, though it can still be hard to find out where they may have been used. A related problem is where a password is set for the user by a local administrator. Unless the user is required to change the password to one that the administrator does not know, doubt can always be raised which of the two people who knew the password was actually logged in and responsible for the account’s activity. If there are reasons that users cannot be forced to change their passwords on first use then procedures need to be carefully designed and followed to ensure that suspicion does not fall on the wrong person.

Password embedded in code

Passwords are also sometimes disclosed by being included in scripts or programs. While this may appear an easy way to automate access to an interactive system it carries high risks of disclosure and alternatives should be used wherever possible. If there is no other alternative then the script or program must be very carefully protected against deliberate or accidental access. The worst possible outcome is for a script containing a plaintext password to end up on a public website. 

Measures to Protect Passwords

Having identified the most likely threats to passwords, organisations and their users should implement appropriate behaviours and technical measures to protect against those risks. Measures are likely to involve both preventing passwords being lost and minimising the damage when they are. Different systems and information may be subject to different risks, so may require different measures. For example two-factor authentication may be appropriate for researchers and administrators dealing with sensitive information and operators managing key networks and systems even if it is considered too costly for general use.


Two-factor Authentication

Most of the attacks described above can be made much harder if the password is not the only thing required to login. A variety of two-factor systems are available which require in addition either a biometric measurement (e.g. a fingerprint) or possession of a particular device (which may range from a dedicated token to a smartphone). Two-factor systems may be somewhat less convenient to use than simple passwords or be limited to particular hardware, so are most appropriate for accounts that have access to high-value services or information. For this level of security they may well be easier to use than a very long and complex static password.

Protecting password files

To check that the user has typed in the correct password, systems must have a reference to check against. An attacker who can obtain a copy of this reference file can run cracking programs against it and will almost inevitably succeed in discovering the passwords for several user accounts. Password files should therefore be among the best protected information the organisation holds, held on well-secured machines with limited access and, unless this is impossible, holding only salted hashes rather than the actual passwords. The choice of hashing algorithm can significantly affect the time to crack a password file - try to use the strongest (i.e. slowest) one available.

Federated authentication

Implementing federated or single sign on, using a central authentication server, has several security benefits. It reduces the number of systems on which passwords need to be stored, and should also ensure that secure protocols are used to transfer them over networks. Reducing the number of passwords users need to remember should help them use more complex and secure passphrases. However because the same password/phrase can now give access to multiple systems, it is even more important to secure the central authentication server, and for users to be careful against phishing or key logging attacks.

Password Complexity

Making passwords more complex increases the difficulty of attacks that rely on brute force or educated guessing. However it has no effect on attacks that reset the password or record it as the user types it in. The invention of rainbow tables as an alternative to brute-force attacks has made even complex passwords vulnerable in a few minutes if they are too short: most authorities now recommend the use of passphrases or sequences of random words to ensure sufficient length.

Password Lock-out

A common approach to reduce the risk of brute-force attempts to log in to an account is to either lock the account or increase the delay between login attempts when there have been repeated failures. This can be effective in slowing down attacks and giving responders time to react to an alarm. However it can cause problems when a user forgets to update a password stored in a browser or device if the automatic retries trigger the lock-out alarm.

Self-test for Problems

A number of password cracking programs are available, so it makes sense for authorised staff to run them against the organisation’s own password files. This must be carefully planned to minimise the security and legal risks to the organisation, its staff and information: testers should only need to know that a particular account was cracked, not what its password was. The exercise must be designed to help users select and remember better passwords, otherwise it risks reducing security rather than enhancing it.


When a password has been compromised, the unauthorised user will normally behave differently from the authorised one. Logs of when accounts are used, and where from, may reveal early indications when this happens. It may also be possible to directly identify unauthorised use of the account.

Patterns of Use

Many accounts will show fairly obvious patterns both in when they are used (what times and what days of the week), and where users log in from. Indeed these may sometimes be a matter of policy: access to sensitive information may only permitted at designated locations and times. Changes to these patterns may indicate that there is a problem with the account. Unfortunately they may also be the result of legitimate events, such as the account owner being on holiday or having a deadline to meet. Even near simultaneous logins from different parts of the planet may only indicate that the user is having problems with their VPN set up. Login attempts from IP addresses that appear on address blocklists are almost always a bad sign. Some online services use signals such as these as a trigger for enhanced authorisation measures: if the site has suspicions that the user is not who they claim to be then they are asked to provide extra proof before being granted access.

Suspicious Activity

Most attacks on university accounts seem to be aimed at using the university’s e-mail facilities either to phish more accounts (a deceptive e-mail from within the university is likely to be more convincing) , or else to send bulk e-mail. Monitoring for spam or phishing mails being sent from university accounts can often provide an early indication of problems; limiting the rate at which accounts can send mail may limit the damage caused, though dealing with the problem is still urgent.

Some types of attacker publish passwords, or more commonly password files, that they have been able to obtain. The aim seems either to be to embarrass the organisation whose security has been breached, or to seek help in cracking hashed passwords. Monitoring the sites used for publication can therefore be an effective way to discover problems. Unfortunately most of the use of these sites is legitimate and harmless, but Janet CSIRT and other incident response teams have developed monitoring tools that increase the likelihood that an alert will actually indicate a problem.

Password Timeouts

A method sometimes proposed to limit the impact of password compromises is to require users to change them regularly. Time-limits for password age used to be set based on the time taken to discover them using brute-force methods, however since the invention of rainbow tables this would imply lifetimes of minutes or hours. Protecting hashed passwords against discover is now a better measure against this threat. Limited lifetimes may still help by disabling unused accounts in case account management procedures fail to do so, and by ensuring that changes to password policy or technology can be completed when all old passwords have expired, but these may well imply different expiry time-limits from those used previously. And, as the UK National CyberSecurity Centre's advice points out, any requirement to change passwords runs a significant risk of encouraging users to adopt sequences of passwords (e.g. by changing a digit) that increase the likelihood of a successful password guessing attack.

For More Information

See the National CyberSecurity Centre's advice on effective use of passwords.