The sapi_header_op function in main/SAPI.c in PHP before 5.4.38, 5.5.x before 5.5.22, and 5.6.x before 5.6.6 supports deprecated line folding without considering browser compatibility, which allows remote attackers to conduct cross-site scripting (XSS) attacks against Internet Explorer by leveraging (1) %0A%20 or (2) %0D%0A%20 mishandling in the header function.