In the past, UNIX has had a reputation for being poor with regard to various aspects of system security. One reason for this is that in general the source code is available for study and experimentation, and this inevitably leads to the discovery of security vulnerabilities of various types. Far from being a problem, however, I would argue that this is a positive advantage because it makes the vulnerabilities known so that fixes can be found and distributed quickly and efficiently. I would further suggest that other operating systems must also suffer their share of similar problems, but that these other systems are not so thoroughly explored and, therefore, not so well debugged.
Most of the security problems that are encountered in modern UNIX and Linux installations are due to administrator error, where some available security feature has not been installed or has been configured incorrectly, or where security has been deliberately sacrificed in favor of some degree of user convenience.