Categories: controlnoticeauthenticationinformnotify

Handling unusual account activities with multiple factors


Use additional factors to notify users of unusual activities and authenticate when accounts may have been compromised.

For Internet services, prevent suspicious access to the account, and/or make the account owner aware of unusual activities.


Many Internet services are using password-based authentication which is really convenient (compared to strong authentication) but has apparent drawbacks:

  • The password itself does not change.

    Unless the account owner changes the password, all sign-ins use the same identity-password combination. If the password is stolen by a malicious party through some means, it can be reused until it is changed.

  • The password can be cracked.

    Many account owners tend to use weak passwords that can be brute-force attacked online. Sometimes the hacker can obtain the database dump which contains hashed passwords, which can be brute-forced offline.

  • The account owner has no way to ensure their exclusive possession of the password.

    The account owner has no evidence if the password is stolen by a malicious party.

  • The only way to control who can access is to reset the password.

    As a result, if the account owner want to ensure only they have the password, they can only reset the password. In some scenarios, resetting the password is not enough.

  • The password can be seen by people around.

    When the account owner enters the password on a computer keyboard or the screen of a mobile device (mobile phone / tablet), people around have the ability to snoop the password and record it.

Thus, a service can't tell if the user is legitimate even if the identity-password combination provided by the user is correct. Fortunately, a service can usually receive some meta information to help determine if an activity is unusual. In case of such unusual activities, a service needs to prevent suspicious access to an account, and/or inform the account owner of the unusual activities.


How can a service that uses username-password authentication and involves a lot of privacy identify unusual sign-ins, confirm the identify of the user, and inform the account owner of such unusual activities?

The pattern described here is a tradeoff between pure password authentication which is insecure, and pure multi-factor authentication which is inconvenient. It increases privacy at the cost of usability.

The fallback multi-factor authentication should be picked carefully with the understanding of the service.

  • In the provided example, Facebook makes use of its resource of friendship and photos. Their decision is based on the assumption that it is very unlikely for a hacker to recognize the friends. Actually the assumption may not hold true in some scenarios, because many of the photos are public and can be viewed under another account, or can be identified with the help from a large-scale tagged photo collection and machine learning.
  • Persuading the user into carrying a hardware token everywhere only for occasional multi-factor authentication may be difficult, but it might worth the effort for financial services.

The strategy to identify unusual activities should also be considered seriously. It should be also to identify most of the activities, with a false positive rate that is not too high. It should balance the cost of multi-factor authentication.


First, the service should be able to identify unusual sign-ins. Then the service may use multi-factor authentication to confirm the identity of the user.

The user should be informed of unusual activities, or have some ways to see recent events, and even do something.

  • Identify Unusual Activities

Today, a web service may appear as a website or an application on the user's devices (including mobile devices and the PCs). The service can use meta information to determine if a sign-in with the correct username-password combination is suspicious.

The strategies described here has both false positives and false negatives.

  • A Website

Typically, a sign-in to a website is in the form of an HTTP request, which contains many customized settings of the browser, including the type of the browser and operating system as well as the architecture (User-Agent header), the Cookie (Cookie header), language preferences (Accept-Languages header). Apart from these, the website can get the IP address of the user, which may be mapped to a certain country/area through GeoIP.

The above meta information can be used to tell if a browser is new to the website. The website can have its rules to determine if an access is suspicious, for example, an access from a new country / browser / operating system is considered suspicious.

  • An Application

By running native code, the application can collect some identifiers of the machine, including the operating system environment settings (e.g. the list of running processes), the hardware parameters (such as the ID of the CPU), and device UUIDs (provided by mobile operating systems like iOS). By completing a network request, the service also retrieves the IP address of the machine.

The above meta information can be used to tell if a machine is new to the service. The service can have its rules to determine if a sign-in is suspicious, for example, an access from a new country / machine / operating system is considered suspicious.

  • Require Multi-factor Authentication

In case of a suspicious sign-in, multi-factor authentication may be a way to let the legitimate user in. The service can request one more authentication except password, such as:

  • A software token

    Examples include Google Authenticator which runs on mobile phones and implements RFC6238 TOTP security tokens..

  • A hardware token (disconnected)

    Examples include a token issued by a bank which displays digits, which is similar to a software token.

  • A hardware token (connected)

    The token may exchange a longer secondary password than the previous one, which means it's safer.

  • Personal data like date of birth, SSN

    Obviously not a good choice here because it cannot be changed.

  • An one-time password (OTP) sent to the registered E-mail address / mobile phone

    Depending the type of the service, maybe the user uses the same password for the E-mail address, or maybe the mobile phone is stolen and the service runs on the mobile phone.

Using multi-factor authentication only in case of suspicious sign-ins is more convenient to using it all the time, but is less secure.

  • Notify Account Holders of Unusual Activities

When an suspicious sign-in is detected, it may be a sign that the password has already been leaked. Depending on the type of the service, it can notify the user about the suspicious sign-in through E-mail, telephone, or other means.

Here the immediate notification can also be used in the multi-factor authentication.

For services that can be logged on from multiple devices at the same time, the user should be able to check the existence of other sessions, and review recent sign-in events.



This pattern has some limitations. For example, it relies on accurate identification of suspicious sign-ins based on meta information, where the meta information including the IP address can be spoofed by an experienced attacker.

If the fallback multi-factor authentication only happens occasionally to the legitimate account owner, they may be unprepared to such authentication, leading to a decreased usability.


  1. Gmail

Gmail displays information about other sessions (if any) in the footer, linking to a page named "Activity on this account" which lists other sessions and recent activities to the Gmail account. The user has the option to sign out other sessions.

In case of annoying false positives, the user may choose to disable the alert for unusual activity. The disable takes about a week, "to make sure the bad guys aren't the ones who turned off your alerts."

  1. Facebook

When Facebook detects an unusual sign-in, it shows social authentication that displays a few pictures of the user's friends and asks the user to name the person in those photos.

  1. Dropbox

The Security tab of the Settings of the Dropbox website displays all web browser sessions logged in to the account, and enables the user to log out one or more of them. The name of the browser, operating system, and the IP address and corresponding country are displayed to help the user make a choice.

It also displays all devices that are linked to the account, and allows the user to unlink one or more of them.


  • Polakis, I., Lancini, M., Kontaxis, G., Maggi, F., Ioannidis, S., Keromytis, A. D., & Zanero, S. (2012, December). All Your Face are Belong to Us: Breaking Facebook's Social Authentication. In Proceedings of the 28th Annual Computer Security Applications Conference (pp. 399-408). ACM.

General Comments

  • Determining the Scope

I started with Gmail's display of account activities. It displays unusual activities regarding an account, which involves identifying unusual activities where the password entered is correct. For some other services, correct passwords can be rejected from a new device / location.

So, the scope of this pattern is to handle unusual activities (including sign-ins).

  • Relevant Information

This pattern includes multi-factor authentication and two-step authentication, which are well studied. But the general topic about informing the user of unusual activities seems to be lack of literature.