Tuesday, February 17, 2009

Google warning: is your site abused through redirects

Google recently wrote in one of its official blogs that it is possible for spammers to take advantage of your website without ever setting a virtual foot in your server. Spammers can do this by abusing open redirects.

What are open redirects?

Many websites use links that redirect their website visitors to another page. Some redirects are left open to any arbitrary destination. These redirects can be abused by spammers to trick web surfers and search engines into following links that seem to be pointing to your website although they redirect to a spammy website.

That means that people who think that they visit your website will be redirected to highly questionable web pages that might contain adult content, viruses, malware or phishing attempts.

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Which redirects on your website could be abused?

Spammers are very inventive. According to Google, they have managed to use the redirect spam on a wide range of websites, including the websites of large well-known companies and the websites of small local government agencies.

For example, the following redirection types can be abused:

  1. Scripts that redirect users to a file on the server can be abused by spammers. The links on your website could look like this:

    http://www.example.com/download.php?url=http://www...
    http:///www.example.com/get/pdf/?http://www...

  2. Site search result pages with automatic redirect options. If the result pages of your internal site search feature contain an URL variable that sends your website visitors to other pages, spammers might be able to exploit them:

    http://www.example.com/search?q=keyword&page=1&url=...

  3. Affiliate tracking links. Affiliate tracking links often allow people to direct website visitors to other pages. Spammers might enter their own URLs in the tracking links. Example:

    http://www.example.com/track.php?affid=123&url=...

  4. Proxy pages. Proxy sites send people through to other websites and they can be abused by spammers:

    http://myproxy.example.com/?url...

  5. Interstitial pages. Some websites show an interstitial page when users leave a website to let users know that the information found on the link is not under their control. These URLs usually look like this:

    http://www.example.com/redirect/http://www...
    http://www.example.com/out?http://www...
    http://www.example.com/cgi-bin/redirect.cgi?http://www...

How to find out if your website is abused

Even if you find none of the URLs above on your website, your site still may have open redirects. Do the following to check if your website is abused by spammers:

  1. Make a site search on Google

    Go to Google.com and search for "site:yourdomain.com". Replace yourdomain.com with your own domain name. If you see web pages that have nothing to do with your website then it's likely that someone exploits a security hole on your website.

  2. Check your web server logs for URL parameters like "=http:" or "=//". If your redirection URLs get a lot of traffic, this could also be caused by spammers.

  3. If you get user complaints about content or malware that you know cannot be found on your website then your website users might have seen your URL before they were redirected to the malware site.

What you can do to protect your website

It's not easy to to make sure that your redirects aren't exploited. The reason for that is that an open redirect is not a bug or a security flaw. There are some things that you can do to protect your website:

  1. Check the referrer. Your redirect scripts should only work if they area accessed from another web page of your website. The redirect script should not work if the user accesses the script directly or from a search engine.

  2. If possible, make sure that the script can only redirect to web pages and files that are on your own websites. You could use a whitelist of allowed destination domains.

  3. Use the robots.txt file of your website to exclude search engines from the redirect scripts on your website. That will make your website less attractive for hackers.

  4. Add a signature or a checksum to your redirect links so that only you can use the script.

Open redirect abuse is a big issue for Google right now. If you secure your scripts, spammers will move over to other websites and leave your website alone.

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2 comments:

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