My colleague would like to use my CERN account because I have special access. I have my doubts about his/her request, but am not sure how to tactfully respond.

The password is yours.  Consider it like your toothbrush:  never share it and change it regularly.  There is no legitimate reason for you ever to be asked to disclose your password -- not by the CERN Computer Security Team, the CERN Service Desk, or your colleague.  In fact, sharing your password is a direct violation of the CERN Computing Rules, as set out in Operational Circular No. 5.  Your colleague may not be aware of this, but you should bring it to his/her attention as you politely decline.

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