Open HTTP redirectors

There has been much discussion about open e-mail relays, but very little about open HTTP redirectors. An open redirector is hosted by, but will unintentionally send you to This can have interesting effects on PageRank or can trick users into clicking on something that isn’t what it seems.

After many months of abuse by spammers, the redirect server is now closed.

Yahoo! has used a redirect server for a long time for tracking clicks from one Yahoo! website to another.

Last year, spammers started using in email messages to trick unsuspecting users into thinking that they were clicking on a Yahoo! website. They started sending out emails with links that looked like this:

Users saw the domain name and figured it must be some official Yahoo! site, not realizing that the server would redirect to another IP address. So we started blocking those types of URLs (easy to do since we’d never use a dotted-quad for anything legit). So the spammers switched to something a little more clever:

The trick here was a misuse of the clear-text “username:password@server” authentication feature. It made it look like you were accessing a URL, but in fact were going somewhere else. These were particularly insidious, since they didn’t even go through our redirect servers, so there was nothing we could do to block them. Microsoft got rid of the problem for us with an update to Internet Explorer 5 and 6 that simply disabled the feature altogether. Mozilla followed suit by displaying a warning dialog box when this type of URL is used:

You are about to log into the site “” with the username “,” but the website does not require authentication. This may be an attempt to trick you.

Is “” the site you want to visit?

So the spammers went back to abusing Yahoo!, but this time with actual hostnames:

This not only tricks email users, but when used on the web can (in theory) also influence PageRank-type algorithms.

We had no choice but to either maintain a whitelist (lots of server-side state to manage) or implement a digital signature algorithm. We went with the digital signature approach. So now you can safely click through to partner sites:

But if you try to recycle the same signature with a different URL, you’ll get a 403 Forbidden:

Finally, does what it’s supposed to do and nothing else. Frustrated spammers out there have probably already started to abuse someone else.


14 thoughts on “Open HTTP redirectors”

  1. For the record, it wasn’t “last year”. It was nearly four years ago that I tried to convince the Paranoid Yahoo Who Sat Near You and I that “fixing was something that needed to be done”, to which I was scoffed at, etc.

    I think it had more to do with people simply starting to block mail that contained “” which probably had a detrimental impact on yahoo-originated official spam.

  2. I know you yahoos like to pick on google, but don’t forget about the redirector as well…

  3. Note that links going through the Google redirector do not affect the target site’s Google ranking in any way.

    I think it would make more sense for the big search engine sites to document their redirectors publicly and ignore links going through any of them altogether, much like Google do for their own redirector.

  4. Another solution is to use URI blacklists such our SURBL lists, which list spammer web sites and are commonly used to block spam using message body aware programs. Redirection and shorting services such as are using SURBLs to deny redirection services to spammers.

  5. Wait, but then in your example, if the spammer’s website is in fact indexed by yahoo’s search engine, then they can trivially find the appropriate digital signature to include in their URLs so that the redirect will work…

    Is the premise supposed to be that URLs which appear in spam are probably not indexed? That seems unlikely to be a safe assumption.

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