How to protect your browser from Unicode domain phishing attacks

Graham Cluley
Graham Cluley
@[email protected]

How to protect your browser from Unicode domain  phishing attacks

Do you trust а

Of course you do! So, do you feel okay about visiting the website at https://www.а

The URL I’ve linked to isn’t the real Apple technology company that makes shiny iPhones, Homepods, and iMacs. Instead, it’s a Unicode domain which – rather than using the conventional ASCII characters that make up the vast majority of websites you’re likely to visit – contains foreign characters.

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So the “а” of а is actually a Cyrillic “а” (U+0430) rather than the ASCII character “a” (U+0061).

What’s that? You couldn’t tell the difference? No, neither can I. And, as we’ve described before, that’s a problem that phishers and online crooks are only too happy to take advantage of in their pursuit of your passwords and other sensitive information.

You see, it’s not just “а” and “a” that can be mixed up. There are countless ways in which bad guys can take advantage of the many Unicode characters that look remarkably similar to common ASCII characters. Which means that you and I are at risk of visiting a site believing it to be legitimate, when in fact it’s designed to scam us in what is known as an IDN Homograph attack.

Browsers are beginning to get better at warning users when they visit a site with an internationalized domain name (IDN), with some now displaying the URL in the browser bar in its Punycode form. That means you might spot you’re visiting xn– rather than the real

But human nature means that we will more-often-than-not fail to check the browser bar, and not notice that we’re not on the website we intended.

For that reason, I strongly recommend that you get some help.

There are a range of browser extensions and plugins that can warn you when you visit a website with an internationalized domain name. Having tried a few solutions, my preference is for a browser add-on called IDN Safe.

IDN Safe not only warns you that you are visiting a URL with an internationalized domain name, but it also *blocks* the webpage (which is far more likely to grab your attention!).

Website blocked

Of course, if you *did* want to visit that URL it would be a nuisance if you were now being blocked from reaching it. So, IDN Safe includes an allow-list feature to allow you to visit specific sites that you decide are legitimate.

IDN Safe isn’t for everyone. In particular, if you are – say – Chinese and in the habit of visiting websites that take advantage of internationalized domain names you may find it a ruddy nuisance. But, for most of us, I think it’s a sensible addition to our security toolbox – and may stop you from being phished or scammed one day.

Furthermore, Firefox users may benefit from making a change to their browser settings which will force the Punycode version of the URL to be displayed in their browser bar.

I talk more about IDN Safe in the latest edition of the “Smashing Security” podcast. You can listen via the player below, or check out the “Smashing Security” website for show notes.

Smashing Security #066: 'Passwords, pirates, and postcards'

Listen on Apple Podcasts | Spotify | Pocket Casts | Other... | RSS
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Graham Cluley is an award-winning keynote speaker who has given presentations around the world about cybersecurity, hackers, and online privacy. A veteran of the computer security industry since the early 1990s, he wrote the first ever version of Dr Solomon's Anti-Virus Toolkit for Windows, makes regular media appearances, and is the co-host of the popular "Smashing Security" podcast. Follow him on Twitter, Mastodon, Threads, Bluesky, or drop him an email.

10 comments on “How to protect your browser from Unicode domain phishing attacks”

  1. R. Dale Barrow

    It looks like Firefox 58.0.2 (64 bit) on Windows 10 shows punycodes by default. This may be recent. Yay! I did not need to "about:config" to see what was going on.

  2. Andrew King

    Great post Graham! I was aware of the problem, but it is really very helpful to know such add-ins exist. I will re-share on LinkedIn.

  3. Alfonso

    I was sure that Firefox 58.0.2 (64bit) on Win 7 Prof. was secure. This happened to me a couple of days ago: Mozilla FF browser froze while checking a web site that had Kaspersky's "green seal". I couldn't take screenshots at all. None of the usual hacks worked. I shut down the computer, started 15 minutes later and there it was>>> The frozen browser. So I ended up taken pictures with my camera.
    Mozilla has not responded. Kaspersky helped me by fixing the problem. I have sent Kaspersky the 14 pictures.
    With all these problems shouldn't we be concerned of having Mozilla FF, Kaspersky and others holding our passwords to everything? BHH are getting ahead of the game.

    Thank you Graham! Keep up the good work

  4. Alfonso

    Hello again Graham. Copy and paste your head title>> "????????’???? ???????????????? ???????? ???????? ???????????????????????????? ???????? ???? ???????????????????????????? ????????????".<<
    and try to change the font to any of the other most used fonts. >>It stays the same<< "CAMBRIA MATH" WHY?

    1. Graham CluleyGraham Cluley · in reply to Alfonso

      That's because it's not a font thing. It's Unicode characters. Crazy isn't it?

  5. David


    The vulnerability could not be reproduced in recent versions of Apple's Safari web browser on macOS, Microsoft's Edge, or Internet Explorer 11 in iTnews testing.

    Recent beta versions of Chrome are also not vulnerable, and Apple's mobile Safari browser for iOS displays the domain name correctly.

  6. Andrew Udvare

    Could just not have IDNs and the problem is solved. The benefits do not outweigh the security issues. Arguably an en-* locale system should warn on IDN domains at minimum.

  7. Steve Borsch

    Would be interested in your thoughts on browser extensions overall. Why? Because when I read "Read and change all the data on the websites you visit." it gives me pause…

    …especially as I stay on top of cyber security and read posts like this one:

    Rogue Chrome, Firefox Extensions Hijack Browsers; Prevent Easy Removal

    As such there are *very* few extensions in any of my browsers. Your thoughts?

    1. Vytautas Buozys · in reply to Steve Borsch

      When google just released chrome and its store for apps. In the beginning I was excited and curious. But later I noticed that these extensions need to much my personal info. It might be that some of them are time savers, but unrelated permissions giving a big stop of using them.

  8. Soryte

    Another issue are browsers on mobile devices,,, I haven't tested lately but last time I checked my iPad there was no real URL on the status bar.
    And seemingly most folks now use mobile devices to browse :o

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