The official ruby site reports:
A vulnerability was found that file creation routines can create unintended files by strategically inserting NUL(s) in file paths. This vulnerability has been reported as CVE-2012-4522. Ruby can handle arbitrary binary patterns as Strings, including NUL chars. On the other hand OSes and other libraries tend not. They usually treat a NUL as an End of String mark. So to interface them with Ruby, NUL chars should properly be avoided. However methods like IO#open did not check the filename passed to them, and just passed those strings to lower layer routines. This led to create unintentional files.