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Stop URL spoofing attacks in their tracks

In the conclusion of this series, learn how to secure Windows systems from URL spoofing attacks using anti-spoofing browser features, e-mail policy settings and word of mouth.

In this two-part series, Serdar Yegulalp explains how URL spoofing targets Windows users and how to protect systems from attacks. Part one detailed how URL spoofing works and how to educate users on its warning signs. Part two below covers anti-spoofing browser features, domain spoofing, weaknesses in international domain names and e-mail vulnerabilities.

In my previous tip, I offered some tricks to help you and your users identify URL spoofing scams -- but user education can only go so far. Today I'll discuss steps you can take to help lock down Windows systems.

Use browser-based features when available
As spoofing becomes more common, newer Web browsers are being programmed to identify such scams. For example, Mozilla's Firefox 1.0.1 can detect when certain tactics are being employed (i.e. site redirection that falsely claims to be SSL-protected). It then warns the user accordingly. Consider this another reason to dump Internet Explorer. Also be mindful of third-party plug-ins like CoreStreet's SpoofStick, which can also help protect you from spoofing scams.

Set up a spoof@ e-mail address where potential spoof messages can be sent and analyzed
An overwhelming number of spoof e-mails forced both eBay and PayPal to set up spoof@ addresses where people can forward the scams as attachments. Each company's security team analyzes the URLs and routing information in each e-mail to quickly identify and shut down offenders. If you create such an e-mail account, you should assign someone to monitor it continually to keep up with your volume of spoofed traffic.

Enforce reverse DNS authorization if possible
Reverse DNS authorization insures that a given piece of e-mail is indeed coming from the professed sender's domain. Unfortunately, not all ISPs consistently support reverse DNS authorization, which means that a perfectly legitimate e-mail may bounce.

Accept and send only plaintext e-mails
This fairly radical maneuver is a great way to expose spoof URLs. All hyperlinks are displayed in plaintext-only format. A bogus link will be obvious. How to enforce such a policy on inboound e-mail depends on your mail setup. For Exchange, you can use a third-party product called Aloaha.

If you have to send automated e-mails from your domain, you may also be wise to send plaintext-only e-mails and educate recipients about your decision. Make it clear that if anyone receives non-plaintext e-mail from your domain, URLs in that e-mail may be spoofed. If there's no pressing need to send HTML e-mails from your domain, it's better not to do so.

Beware of URL spoofs that take advantage of International Domain Name (IDN) system weaknesses
This is a new and dangerous variety of URL spoofing that relies on IDN system weaknesses to render bogus URLs that appear to be legitimate, even when using SSL. It creates URLs using international characters that look like conventional Roman or Latin characters. To demonstrate this problem, Secunia's Eric Johanson conducted a proof-of-concept exploit where the URL was invisibly redirected to This is called a homograph attack, in which an attacker or phisher spoofs the domain and URLs of businesses. There is no easy way to detect or work around such attacks at this time.

Homograph attacks will only work in browsers configured to support internationalized domain names. Internet Explorer does not support such domains by default, but Mozilla and Firefox do. To disable this feature in Mozilla-based browsers, go to about:config and set network.enableIDN to "false." However, until the IDN system can be hardened against spoofing, your best defense is to spread word about spoofs as quickly as possible to avoid being taken by them.

Serdar Yegulalp is the editor of the Windows 2000 Power Users Newsletter.

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