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Don’t use TOTP for password resets

May 26, 2016

I saw a thread on the github issue tracker where someone was complaining that email verification and password reset tokens were too long in ASP.NET Identity 3. They are and I think this is a valid complaint because an end user can’t possibly type in the entire thing if necessary (for some reason line breaks are notorious in email readers and it always seems to happen to my sister in-law).

A suggestion was made on this thread to replace the normal data protection token generator with the TOTP (time based one-time password) generator so it would produce nice short 6 digit code. The problem with this is that an attacker can try to mount a brute force attack guessing all the possible codes within the validity window of the TOTP code (3 minutes in the ASP.NET Identity implementation). That means an attacker could try to guess all one million codes and they might get lucky in that much time. If they do, then they will be able to reset the password and pwn the account.

To properly mitigate against this, you need to do brute force protection on failed password reset requests. ASP.NET Identity does not do this for password reset requests (they do for login requests for both passwords and 2FA codes). I suppose they don’t perform this brute force check for password reset requests because the assumption is that they are using the default data protection mechanism which does mitigate this attack. So by default, ASP.NET Identity 3 is safe from this, but please don’t replace the plumbing without knowing what the consequences are.

Yea, security is hard.

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