I still use version 1.7.5 and was wondering if it is less secure than the more up to date 1.8.6
Generally speaking: yes. It's practically always advisable to keep software up to date, because new versions fix known (potentially exploitable) bugs in older versions.
For more information you can review the version history. From a quick glance notable security improvements appear to be:
You should also keep PHP, your webserver and your database updated, since those are part of the system's attack surface as well.
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