Unrestricted file upload vulnerability in the avatar upload functionality in Simple Machines Forum before 2.0.6 and 2.1 allows remote authenticated users to execute arbitrary code by uploading a file with an executable extension, then accessing it via a direct request to the file in an unspecified directory.
https://github.com/SimpleMachines/SMF2.1/issues/701
http://www.securityfocus.com/bid/63275
http://www.openwall.com/lists/oss-security/2013/10/25/3
http://www.openwall.com/lists/oss-security/2013/10/23/6
http://download.simplemachines.org/index.php?thanks%3Bfilename=smf_2-0-6_changelog.txt