An IE7 Bug That Returns MSIE 6.0 User Agent String

IE7, don’t you just love it? What they gave us with one had in terms of CSS actually sort of working they took away with some of the stupidest niggly little bugs ever. Here’s another potentially large one i’m stumbled right into recently.

If your user agent string is more than 260 characters in length then your user agent string suddenly and almost magically becomes “Mozilla/4.0 (compatible; MSIE 6.0)”

Not what you’d expect to happen. Now, I know what your thinking. Why on god’s earth would your user agent string BE that long. The answer is simple. Toolbars and Spyware.

Toolbars and Spyware

The cool thing about Windows is that toolbars, spyware and crapware in general can and do add pointless shit to your Internet Explorer user agent string via the registry. Sounds good doesn’t it. Now consider the following:

1. Most store bough Windows PCs come pre-installed with tons of stupid toolbars and crapware
2. IE 7 isn’t much more secure than IE6 was in terms of spyware
3. Home users do not know or care about viruses and spyware

With me?

Try it yourself

You can easily trigger this bug yourself by editing the registry. Fire up regedit and navigate to HKLM/Software/Microsoft/Windows/CurrentVersion/Internet Settings/5./0User Agent/Post Platform. Create a new string key and fill up full of crap (enough to pass the 260 character threshold)

Next, close all instances of IE. Fire UP IE again and type the following into the address bar:

javascript:alert(navigator.userAgent)

Hit enter and IE7 should now tell you that it’s IE6. Awesome!

I firmly believe that this is one of the reasons behind problems such as the thickbox positioning bug that many people experience with IE7.

The workaround?

This is of course just another argument against relying on the user agent string to determine browser version. It really shouldn’t be done this way, yet it’s the method built into many JavaScript libraries including jQuery with it’s unreliable $.browser.version property.

If you really need to know if a browser is IE6 or IE7 then use something more solid like object detection. Simply testing for something like the following would be a better option than extracting unreliable data from the user agent string

A possible patch below redefines the browser version as “7.0″ only if the browser has already been identified as IE6 and the the presence of the XMLHttpRequest object suggests otherwise.

if(jQuery.browser.msie && parseInt(jQuery.browser.version) == 6 && this.XMLHttpRequest) {
jQuery.browser.version = "7.0"
}

I’m thinking though that maybe this might not be bulletproof. If IE8 presents the same or similar “Hi i’m IE6!” behaviour then the above patch could incorrectly identify IE8 as IE7. This might not be as serious as IE7 vs. IE6 but it’s still an issue.

I just wish that Microsoft would put a bit more effort into writing good software. It’s not like they don’t have the resources.