From The Collaborative International Dictionary of English v.0.48 [gcide]:
Joe \Joe\, n.
See Johannes.
[1913 Webster]
From The Free On-line Dictionary of Computing (19 Sep 2003) [foldoc]:
joe
<jargon, security> A computer account whose user name and
password are the same. Joes are considered harmful, as are
any passwords which are easy to guess.
(1995-06-21)