Friday, June 25, 2010

Open redirectors: some sanity

Open redirectors are a contentious issue. Old-school hackers think anyone who thinks they are serious is on drugs. New-school hackers are more evenly divided. I haven't yet seen a public, balanced list of reasons why you should be worrying about other problems. Here it is. For now, I'll concentrate on the central idea that open redirectors permit domain obfuscation and therefore facilitate phishing etc.
  • OMG! Open redirectors can send a user to whilst appearing to go to

    1. Not an issue: The only security indicator for URLs supported by browsers is the URL bar. The status bubble can be faked. This is to say that you can only securely do an URL check on the final landing page of a click. Check out the Browser Security Handbook.

    2. Not an issue: An easier way to fake an URL is to simply use mismatched anchor text vs. the actual href. End users make decisions based solely on the the text they read, not the underlying URL.

    3. Not an issue: We cannot seriously expect end users to make safe / dodgy distinctions based on any component of an URL. If we as a security community try and offload decisions like this on to end users, we're exhibiting basic misunderstandings. A case in point -- I just keynoted OWASP Stockholm with my colleague Ian Fette and he released an eye-opening statistic: 50% of users click through the phishing / malware interstitial in Google Chrome. Just to be clear, this is a dialog with a red background and a huge no-entry sign, with text such as "This website may harm your computer". Ouch, 50%, and that's a simple decision. It's time to stop suggesting users make complicated decisions based on URLs. The issue is becoming pretty moot with URL shorteners anyway.

    4. Not an issue: It's very easy for attackers to register a domain name that sounds offical but is not. Time and time again, even relatively technical users fall for phishing scams simply because a bad domain looks vaguely official. This backs up the previous point about users understanding URLs nicely.

The fact is, it's really easy to get a user's browser to come into contact with untrusted bits. Malware ads would be one example; there are plenty of others.
If you want to be a productive member of the security community, please do the following things:
  • Desist from seizing upon minor issues and declaring them "critical" in order to get attention. You may get quoted by some clueless reporter, but you'll still be a third-rate security researcher.

  • Get involved in hardening web app frameworks, browsers and plug-ins such that they are robust in the face of malicious data. Users are going to be exposed to bad stuff. Help tackle the problem at the roots.