The sapi_header_op function in main/SAPI.c in PHP 5.4.0RC2 through 5.4.0 does not properly determine a pointer during checks for %0D sequences (aka carriage return characters), which allows remote attackers to bypass an HTTP response-splitting protection mechanism via a crafted URL, related to improper interaction between the PHP header function and certain browsers, as demonstrated by Internet Explorer and Google Chrome. NOTE: this vulnerability exists because of an incorrect fix for CVE-2011-1398.