Is there a way for a subscriber to edit admin's name??

Hello there guys!

I have a 3.4.2 wp installed and logged in today to my admin panel. the first thing i noticed was that my name at the “Hello ‘name'” notice was changed some how!

after researching a bit, i found out that this name belongs to a girl that tried to subscribe. i don’t realy know if she got access to all admin panel features, just know that she was able to change the admin name (my name) to her name and that there is no new listing for her in my users list.

I use WP for 5 years on several websites and was shocked by this incident.

Do any of you might suggest how can such thing happen? what should i do in order to secure my site?

Thanks in advanced,
Koby.