Heap-based buffer overflow in the BERDecBitString function in Microsoft ASN.1 library (MSASN1.DLL) allows remote attackers to execute arbitrary code via nested constructed bit strings, which leads to a realloc of a non-null pointer and causes the function to overwrite previously freed memory, as demonstrated using a SPNEGO token with a constructed bit string during HTTP authentication, and a different vulnerability than CVE-2003-0818.  NOTE: the researcher has claimed that MS:MS04-007 fixes this issue.