TABLE OF CONTENTS A vulnerability is present in the OpenSSL software library in the SSL version 2 key exchange portion. Specifically the vulnerability is commonly referred to as the OpenSSL SSLv2 Malformed Client Key Remote Buffer Overflow Vulnerability (CAN-2002-0656.) This vulnerability affects many machines worldwide, so analysis and exploitation of this vulnerability is of high priority. The vulnerability arises from allowing a user to modify a size variable that is used in a memory copy function. The user has the ability to change this size value to whatever they please, causing more data to be copied. The buffer that overflows is found on the heap and is exploitable due to the data structure the buffer is found in.
OpenSSL's problem is caused by the following lines of code:
memcpy(s->session->key_arg, &(p[s->s2->tmp.clear + s->s2->tmp.enc]), (unsigned int) keya);
A user has the ability to craft a client master key packet, controlling the variable 'keya.' By changing 'keya' to a large number, more data will be written to s->session->key_arg then otherwise expected. The key_arg variable is actually an 8 byte array in the SSL_SESSION structure, located on the heap.
Since this vulnerability is in the heapspace there may or may not be an exploitation technique that works across multiple platforms. The technique presented in this Case Study will work across multiple platforms and does not rely on any OS specific memory allocation routines. We are overwriting all...
