Wednesday, March 6, 2013

Can't live with them, can't live without them

Passwords have been around for approximately forever, and despised for nearly that long. However, while great strides have been made in improving password-based authentication, these improvements are not a panacea, often come with maintenance costs of their own, and sometimes even serve as additional attack vectors. While we should keep striving to improve authentication, it is also important to recognize that passwords are not going away any time soon, to understand the drawbacks of existing password solutions, and to try to improve them.

Many of the best practices for passwords (prohibiting reuse, requiring unguessable passwords, being able to remember passwords) seem impossible without a password manager. Firefox has implemented a password manager since inception. The built-in password manager detects the presence of login form and prompts the user to store the password via a notification.
We use data from the same Test Pilot study as in the last post, this time focusing on password statistics. Approximately 5.5% of users have disabled the password manager, which is enabled by default. However, are the remaining 94.5% of users actually using the password manager with intent?

To answer this question, let's first examine the number of users who have stored at least one password in the password manager (as obtained by querying nsILoginManager for all logins). The graph below shows the distribution of the the number of passwords stored in password manager for users who have no more than 30 passwords. This graph represents 96% of the nearly 12K users in the study.
The graph above shows 73.4% of users store at least one password in the password manager, but it's not clear at all that this is not accidental use: after all, 13.9% of users store only a single password, and it is doubtful that a password manager is necessary or beneficial in the single password case. We can also take a look at the distribution of the number of sites stored in the password manager, for users who have no more than 30 sites stored. 

Interestingly, the site distribution has a slightly longer tail than the password distribution, so this graph represents only 89% of users. The shape of the graph is very similar, however, and lends credence to the hypothesis that much of the information stored in the password manager represents accidental use, if we believe that the password manager is not beneficial in the case of a single site.

Because this study did not collect how frequently the password manager triggered on login forms, we can't definitively conclude that users storing only one password represents accidental use. Alternative explanations, ranked in order of increasing possibility according to my personal prejudice:
  1. I only use this browser for work and I don't care about my work password.
  2. I have a secure, memorizable password scheme but can't remember the requirements for this one site.
  3. I only have one main password but it doesn't meet this one site's requirements.
Does this data hint at anything interesting about password reuse? Let's examine mean number of passwords stored versus the number of sites.
This graph represents 97.5% of users and omits 43 outliers who have more than 100 passwords stored. The vertical error bars represent the standard deviation from the mean. This graph falls far south of x=y, the ideal case of storing one password per site. So we can conclude that even while using a password manager, people still reuse passwords across sites.

This level of reuse is not necessarily due to user choice. For example, subdomains on the same intranet frequently require the same password, due to LDAP linkage. This in itself is not a security problem if the security guarantees are identical across subdomains. However, it is a problem when those intranets outsource services to outside vendors through LDAP, requiring password reuse at external parties. Note to future study authors: please include counts for effective TLDs in addition to domains in order to account for this case.

In summary, it seems that even though 94.5% percent of people have the password manager enabled, far fewer users gain any benefit from the password manager. Over the years I have heard the following arguments against using password managers:
  • I only use one password so I don't need one.
  • They don't work across all my devices.
  • They don't work across all my browsers.
  • I don't trust local password managers against local attacks.
  • I don't trust cloud password managers because I don't trust third parties.
The first argument is especially egregious, considering the combined forces of account hijacking, phishing, and password database hacks. The second two arguments can't be solved with a local password manager, or even a browser-specific password manager. The fourth argument can be solved somewhat with master password, but only 1 out of 12K users had master password enabled (security.ask_for_password in about:config), so either that feature is undiscoverable, unusable, or regarded as too insecure to be effective. It is clear from the data that not enough people take advantage of password managers. I look forward to further progress from the identity team to solve some of these issues.

Many thanks to Paul Sawaya and Tanvi Vyas for advice on this post, and to Paul for writing the code to capture password manager statistics.