The Authentication Problem

Authentication is a tricky problem. The goal of authentication is to verify the identify of the person, device, machine, etc. that is attempting to gain access to the protected system. There are many factors to consider when designing an authentication system. Here is a brief sampling:

  • How much security is necessary?
  • Do we require username?
  • How strong should the password be?
  • Do we need multi-factor authentication?

The need for authentication typically means that the data being accessed is sensitive in some way. This can be something as simple as a todo list or a user’s email, or as important as banking or top secret information. It can also mean that the data being accessed is valuable in some way such as a site that requires a subscription. So, the security necessary is dependent on the data being protected.

Usually, authentication systems require a username and some form of a password. For more secure systems, multi-factor authentication is used. Multi-factor authentication means that multiple pieces of information are used to authenticate the user. These vary depending on the security required. In the United States, federal regulators recognize the following factors:

  • Something the user knows (e.g., password, PIN)
  • Something the user has (e.g., ATM card, smart card)
  • Something the user is (e.g., biometric characteristic such as a fingerprint)

A username and a password is an example of a single-factor authentication mechanism. When you use an ATM machine, you supply it with an ATM card and then use a PIN. This is an example of two-factor authentication.

The U.S. Federal Financial Institutions Examination Council (FFIEC) recommends the use of multi-factor authentication for financial institutions. Unfortunately, most of the authentication systems currently in place are still single-factor authentication systems, despite asking for several pieces of information. For example, if you log into your bank system you use a username and password. Once the username and password pass, you are often asked for additional information such as answers to challenge questions. These are all examples of things the user knows, thus only a single factor.

Some institutions have begun using additional factors to identify the user such as a one-time password sent to an email address or cell phone. This can be cumbersome, however, as it can often take additional time to receive this information. To combat this, browser cookies are used after the first successful authentication. After the user logs in for the first time, they are offered a chance to have the system place a “secure token” on their system. Subsequent logins use this secure token in addition to the username and password to authenticate the user. This is arguably a second factor as it’s something the user has, as opposed to something they know. On the other hand, it is extremely easy to duplicate or steal cookies.

There are other ways that two-factor authentication can be circumvented as well. Since most institutions only use a single communication mechanism, hijacking that communication medium can result in a security breach.

Man-in-the-middle attacks use fake websites to lure users in and steal the authentication information the user uses to authenticate. This can happen transparently to the user by forwarding the information to the actual institution and letting the user continue to access the system. More sophisticated attacks have the user “fail” authentication the first time and let them in on subsequent tries. The attacker can then use the first authentication attempt to gain access themselves.

Another method is the use of Trojans. If a user can be tricked into installing malicious software into their system, an attacker can ride on the user’s session, injecting their own transactions into the communications channel.

Defending against these attacks is not easy and may be impossible in many situations. For instance, requiring a second method of communication for authentication may help to authenticate the user, but if an attacker can hijack the main communication path, they can still obtain access to the user’s current session. Use of encryption and proper training of users can help mitigate these types of attacks, but ultimately, any system using a public communication mechanism is susceptible to hijacking.

Session Security

Once authentication is complete, session security comes into play. Why go through all the trouble of authenticating the user if you’re not protecting the data they’re accessing? Assuming that the data itself is protected, we need to focus on protecting the data being transferred to and from the user. Additionally, we need to protect the user’s session itself.

Session hijacking is the term used to identify the stealing of a user’s session information to gain access to the information the user is accessing. There are four primary method of session hijacking.

  • Session Fixation
  • Session Sidejacking
  • Physical Access
  • Cross-site Scripting

Physical access is pretty straightforward. This involves an attacker directly accessing the user’s computer terminal and copying the session data. Session data can be something as simple as an alphanumeric token displayed right in the URL of the site being accessed. Or, it can be a piece of data on the machine such as a browser cookie.

Session fixation refers to a method by which an attacker can trick a user into using a pre-determined session ID. Once the user authenticates, the attacker gains access by using the same session ID. The system recognized the session ID as an authenticated session and lets the user in without verification.

Session Sidejacking involves an attacker intercepting the traffic between a user and the system. If a session is not encrypted, the attacker can obtain the session ID or cookie used to identify the user’s session. Once this information is obtained, the attacker can use the same information to gain access to the user’s session.

Finally, cross-side scripting is when an attacker tricks the user’s computer into sending session information to the attacker. This can happen when a user accesses a website that contains malicious code. For instance, an attacker can create a website with a special link to a well-known site such as a bank. The link contains additional code that, when run, sends the user’s authentication or session information to the attacker.

Encryption of the communications channel can mitigate some of these attack scenarios, but not all of them. Programmers should ensure that additional information is used to verify a user’s session. For instance, something as simple as verifying the user’s source IP address in addition to a session cookie is often enough to mitigate both physical access and session sidejacking. Not allowing a pre-defined session ID can prevent session fixation. And finally, proper coding can prevent cross-side scripting.

Additionally, any session information stored on the remote system being accessed should be properly secured as well. Merely securing the data accessed isn’t enough if an attacker can access the remote system and steal session information.


Finally, how and when should a user be unauthenticated? Unauthentication is often overlooked when designing a secure system. If the user fails to log out, then attacks such as session hijacking become easier. Unauthentication can be tricky, however. There a number of factors to consider such as:

  • How and when should a user’s session be closed?
  • Should a user’s session time out?
  • How long should the timer be?

Most unauthentication currently consists of a user’s session timing out. After a pre-determined period of inactivity, the system will log a user out, deleting their session. Depending on the situation, this can be incredibly disruptive. For example, if a user’s email system has a short time out, they run the risk of losing a long email they’ve been working on. Some systems can mitigate this by recording the user’s data prior to logging them out, making it available again upon login so the user doesn’t lose it. Regardless, the longer the time out, the less secure a session can be.

Other unauthentication mechanisms have been discussed as well. When a physical token such as a USB key is used, the user can be unauthenticated if the key is removed from the system. Or, a device with some sort of radio in it, such as bluetooth, can unauthenticate the user if it is removed from the proximity of the system. Unfortunately, user’s will likely end up leaving these devices behind, significantly reducing their effectiveness.

As with authentication, unauthentication methods can depend on the sensitivity of the data being protected. Ultimately, though, every system should have some form of automatic unauthentication.

Data security in general can be a difficult nut to crack. System designers are typically either very lax in their security design, often overlooking session security and unauthentication, or they can be very draconian, opting to make the system very secure at the expense of the user. Designing a user-friendly, but secure, system is difficult, at best.


2 thoughts on “The Authentication Problem”

  1. Nice article! Lots of details and good explanations. You gave me more to think about when writing code, for sure.


  2. I’m glad you find it useful. If you’re interested in security in general, I find Bruce Schneier’s blog ( to be quite useful. He’s a security researcher, so he’s quite knowledgeable on the subject.

    If you’re a web developer, then I’d like to direct your attention to the OWASP project ( OWASP publishes a top ten list of most common security problems for websites. Good stuff to keep in mind.

    And finally, if you’re a PHP developer, then I would recommend checking out Chris Shiflett’s book, Essential PHP Security. Excellent information for keeping PHP applications secure. For the server side of things, I’d recommend checking out the Suhosin extension for PHP ( and Mod Security ( Suhosin is generally a drop-in and won’t bother you unless you’re doing something either very unusual, or wrong. Mod Security can be a bit of a bear and takes a bit more time to dig into. Both are excellent if used properly.

    And keep your eyes here, I’m sure I’ll be posting more soon.. ;)

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