Thread:Project:Support desk/Strange user accounts created

Hi, Lately, many new strange/akward accounts are created on my wiki (version 1.15.1). For example the users with account/user names:

Nealshcame Latashamrf Tommiecden Jermainesq Juliorqter Samanthaer Reynahbane Glennaazul Loboutinlobouti Cheapnhl Tiffanydiscount Northwomens

these users do not (yet) contribute. Is this a security risk? How to avoid this? Please reply! Gert