user8637: All ASIC for Bitcoin do a double SHA256 hash on an arbitrary string. But maybe they might be used for pen testing :-)
Lets assume we have single SHA256 hash we want to crack. We also have an ASIC that does double SHA256. Well if we compare results computed by double SHA256 to passwords that generated SHA256 hash we will get nothing. But what if we do single SHA256 on SHA256 hash that we got on start using CPU, and then try to find SHA(SHA256(x))? If we find goodX it would be our password.
Would modifying firmware, or just mining proxy that feeds miner with work do the job?

user22: Most mining ASICs are hard-coded to perform sha256(sha256(x)) which may as well be a random number with regard to sha256(x). The only way in which an ASIC would be useful for password cracking is if someone happened to be using sha256(sha256(password)) as their scheme, which to my knowledge is not in popular use anywhere.

user88384: So, these answers are all great.  They are all missing one giant point.  ASICs are not ONLY for bitcoin miners, though the OP i believe was referring to mining ASICs.  But an ASIC (or Application Specific Integrated Circuit) can be designed for any number of calculations, such as password cracking.  I am certain at least nation state actors already have these.  So the answer is YES, ASICs can be used to crack passwords, but they would have to be specifically designed to crack passwords using a specific algorithm.

user34491: It will not help even if passwords were hashed with DSHA256. This link explains why. https://rya.nc/asic-cracking.html
Basically the ASIC loads the header into its memory and increments the nonce until the hash of the header is lower than the the specified target.
Therefore you cannot use this to brute force a password seeing as the logic is incompatible. 

8637: <p>All ASIC for Bitcoin do a double SHA256 hash on an arbitrary string. But maybe they might be used for pen testing :-)</p>

<p>Lets assume we have single SHA256 hash we want to crack. We also have an ASIC that does double SHA256. Well if we compare results computed by double SHA256 to passwords that generated SHA256 hash we will get nothing. But what if we do single SHA256 on SHA256 hash that we got on start using CPU, and then try to find SHA(SHA256(x))? If we find goodX it would be our password.</p>

<p>Would modifying firmware, or just mining proxy that feeds miner with work do the job?</p>
 22: <p>Most mining ASICs are hard-coded to perform sha256(sha256(x)) which may as well be a random number with regard to sha256(x). The only way in which an ASIC would be useful for password cracking is if someone happened to be using sha256(sha256(password)) as their scheme, which to my knowledge is not in popular use anywhere.</p>
 88384: <p>So, these answers are all great.  They are all missing one giant point.  ASICs are not ONLY for bitcoin miners, though the OP i believe was referring to mining ASICs.  But an ASIC (or Application Specific Integrated Circuit) can be designed for any number of calculations, such as password cracking.  I am certain at least nation state actors already have these.  So the answer is YES, ASICs can be used to crack passwords, but they would have to be specifically designed to crack passwords using a specific algorithm.</p>
 34491: <p>It will not help even if passwords were hashed with DSHA256. This link explains why. <a href="https://rya.nc/asic-cracking.html" rel="nofollow">https://rya.nc/asic-cracking.html</a></p>

<p>Basically the ASIC loads the header into its memory and increments the nonce until the hash of the header is lower than the the specified target.</p>

<p>Therefore you cannot use this to brute force a password seeing as the logic is incompatible. </p>
