limbcrypt versions prior to 2.5.5 contain a number of buffer overflow vulnerabilities that stem from imporper or lacking input validation. By passing a longer then expected input to a number of functions (multiple functions are affected) the user can successful make libmcrypt crash.
Another vulnerability is due to the way libmcrypt loads algorithms via libtool. When the algorithms are loaded dynamically the each time the algorithm is loaded a small (few kilobytes) of memory are leaked. In a persistant enviroment (web server) this could lead to a memory exhaustion attack that will exhaust all avaliable memory by launching repeated requests at an application utilizing the mcrypt library.
The solution to both of these problem is to upgrade to the latest release of libmcrypt, 2.5.5.