CVE 2009-3557
The tempnam function in ext/standard/file.c in PHP before 5.2.12 and 5.3.x before 5.3.1 allows context-dependent attackers to bypass safe_mode restrictions, and create files in group-writable or world-writable directories, via the dir and prefix arguments.
See the 
CVE page on Mitre.org
for more details.
