The rb_get_path_check function in file.c in Ruby 1.9.3 before patchlevel 286 and Ruby 2.0.0 before r37163 allows context-dependent attackers to create files in unexpected locations or with unexpected names via a NUL byte in a file path.
www.ruby-lang.org/en/news/2012/10/12/poisoned-NUL-byte-vulnerability