Recently a number of articles have discussed the security of browsers' password managers. There are many ways to build a secure password manager, and each browser uses a slightly different approach. In this edition of Security in Depth, we'll look at some of the key security decisions that went into designing the password manager for Google Chrome. As always, we welcome your feedback and suggestions.

Password managers improve security in two ways. First, they let users use more complex, harder-to-guess passwords because the password manager does the work of remembering them. Second, they help protect users from phishing pages (spoof pages that pretend to be from another site) by carefully scrutinizing the web page's URL before revealing the password. The key to the security of a password manager is the algorithm for deciding when to reveal passwords to the current web page. An algorithm that isn't strict enough can reveal users' passwords to compromised or malicious pages. On the other hand, an algorithm that's too strict won't function on some legitimate web sites. This may cause users to use more memorable (and less secure) passwords. Worse, users typically assume the browser is "broken," and become more willing to supply passwords to any page (including harmful ones), since they no longer trust the browser to make correct distinctions. The same side effects are possible if the password manager produces spurious warnings on legitimate sites; this simply trains users to ignore the warnings.

The password manager's algorithm is based on the browser's same-origin policy, which we've touched on before. The password manager supplies a password to a page only if the page is from the same origin (same scheme, host, and port) as the original page that saved the password. For example, this algorithm protects passwords from active network attackers by not revealing passwords saved on HTTPS pages to HTTP pages.

Because the same-origin policy does not distinguish between different paths, it's tempting to think that we could further improve security by requiring the paths to match as well; for example, passwords saved at https://example.com/login would not be sent to https://example.com/blog. However, this design works poorly with sites where users can log in from several places (like Facebook), as well as sites which store dynamically-generated state in the path. Furthermore, creating this "finer-grained" origin wouldn't actually improve security against compromised sites because other parts of the browser (like the JavaScript engine) still obey the same-origin policy. Imagine that example.com has a cross-site scripting vulnerability that lets an attacker inject malicious content into https://example.com/blog. An attacker would not need users to log in to this page; instead, the attacker could simply inject an <iframe> pointing to https://example.com/login and use JavaScript to read the password from that frame.

Besides checking the page hosting the password field, we can also check where password data is going to be sent when users submit their information. Consider a scenario that occurred a few years ago on a popular social networking site that let users (or in this case, attackers) customize their profile pages. At the time, an attacker could not include JavaScript on his profile page, but could still use malicious HTML — a password field set to send data back to the attacker's web server. When users viewed the attacker's profile, their password managers would automatically fill in their passwords because the profile page was part of the same origin as the site's login page. Lacking JavaScript, the attacker could not read these passwords immediately, but once the users clicked on the page, their data was sent to the attacker's server. Google Chrome defends against this subtle attack by checking the page to which the password data is submitted, once again using the same-origin policy. If this check fails, the password manager will not automatically fill in passwords when the page is loaded. The downside is that this can trip up legitimate web sites that dynamically generate their login URLs. To help users in both cases, the password manager waits for users to type their user names manually before filling in any passwords. At this point, if a page is really malicious, these users have most likely already fallen for the scam and would have proceeded to type in their passwords manually; continuing to refuse to fill in passwords would merely give the impression that the browser is "broken."

A number of other proposals to improve password manager security seem reasonable but don't actually make users more secure. For example, the password manager could refuse to supply passwords to invisible login fields, on the theory that legitimate sites have no need to do this and invisible fields are used only by attackers. Unfortunately, attackers trying to hide password fields from users can make the fields visible but only one pixel tall, or 99% transparent, hidden behind another part of the page, or simply scrolled to a position where users don't normally look. It is impossible for browsers to detect all the various ways password fields can be made difficult to notice, so blocking just one doesn't protect users. Plus, a legitimate site might hide the password field initially (similar to Washington Mutual), and if it does, the password manager wouldn't be able to fill in passwords for this site. 

We've put a lot of thought into the password manager's design and carefully considered how to defend against a number of threats including phishing, cross-site scripting, and HTTPS certificate errors. By using the password manager, you can choose stronger, more complex passwords that are more difficult to remember. When the password manager refuses to automatically fill in your password, you should pause and consider whether you're viewing a spoof web site. We're also keen to improve the compatibility of the password manager. If you're having trouble using the password manager with your favorite site, consider filing a bug.

Posted by Adam Barth and Tim Steele, Software Engineers