MIME::Words versions before 5.521 for Perl contain a vulnerability in the decode_mimewords function that allows memory exhaustion. This occurs when repeated "=?" sequences that do not form complete encoded words cause the function to append large error messages repeatedly, leading to significant memory consumption. From version 5.518, MIME::Parser decodes multipart boundaries using this function, enabling a remote sender to trigger this exhaustion via a crafted Content-Type header.