The popen API function in TSRM/tsrm_win32.c in PHP before 5.2.11 and 5.3.x before 5.3.1, when running on certain Windows operating systems, allows context-dependent attackers to cause a denial of service (crash) via a crafted (1) "e" or (2) "er" string in the second argument (aka mode), possibly related to the _fdopen function in the Microsoft C runtime library. NOTE: this might not cross privilege boundaries except in rare cases in which the mode argument is accessible to an attacker outside of an application that uses the popen function.