[temuze]

I used to do a similar thing, then I realized it was a potential problem.

Let's say you have an account at AcmeCo. Let's say AcmeCo has a breach and I can see your password hash. Let's say the company uses a weak password hash (e.g. MD5), or no salt and it's easy to reference a rainbow table.

From this rainbow table, I can look up your hash and see that your password is "lulzSecret2$AcmeCo".

Now let's say you're in another leak from BetaCo. Similar situation -- I see that your password is "lulzSecret2$BetaCo2". Maybe the two is because you were forced to rotate your password once.

It doesn't take a genius to guess what your algorithm is.

But we can take it another level. Maybe I'll try all the major banks and guess passwords using your algorithm ("lulzSecret2$bofa", "lulzSecret2$chase"). Most banks require 2fa, but most of the time they keep it to text-based 2fa.

If I know your phone number from one of the breaches (happens all the time), maybe I can hijack your SIM card (this also happens all the time) and boom, I'm into your bank account.

[snotrockets]

Assume the function is a cryptographically appropriate hash function, you can reduce the risk of suggested attack to almost nil, considering the number of inputs you'd need for such attack