The iconv_substr function in PHP 5.2.4 and earlier allows context-dependent attackers to cause (1) a denial of service (application crash) via a long string in the charset parameter, probably also requiring a long string in the str parameter; or (2) a denial of service (temporary application hang) via a long string in the str parameter.  NOTE: this might not be a vulnerability in most web server environments that support multiple threads, unless these issues can be demonstrated for code execution.