Is it ethical to ask about the professional profile of a candidate at another university before a job interview? I’m on the hiring committee for a Asst. Prof position. One of the applicants is working as a (non-TT) TA/postdoc at a dept in another university (but for only appx 6 mo) where I have a colleague (friend of a friend). 

Is it ethical for me to ask my colleague about this applicant? 

I could request my colleague keep my inquiry confidential, but in a competitive department, doing so may make public that this applicant is looking for another job. I don’t want to cause problems.
In most situations fitting the pattern you describe, you should not ask your colleague.  There are several possible cases:


If the candidate has requested their application be kept confidential, you absolutely must not reach out to them or to anyone else the candidate has not listed as a reference.
If the candidate is currently in a tenure-track or other permanent position, I would not ask without checking with the candidate first, because outing them could be very problematic.
If the candidate is currently in a soft-money or other contingent position, then I would similarly be very careful about asking, but the consequences are not as dire as in the previous case.
If the candidate is in a postdoc or similar position where they are expected to be looking for a permanent position, then asking is a violation of the implied confidentiality of the job application process, but it is a common violation.