Simple Machines Forum (SMF) 2.0.6, 1.1.19, and earlier allows remote attackers to impersonate arbitrary users via a Unicode homoglyph character in a username.
http://www.openwall.com/lists/oss-security/2013/12/30/3
http://www.openwall.com/lists/oss-security/2013/12/30/1
http://www.jakoblell.com/blog/2013/12/13/multiple-vulnerabilities-in-smf-forum-software/