MS ASN library is fraught not only with integer overflow, but also with stack overflow.

MS ASN library is fraught not only with integer overflow, but also with stack overflow.


After eEye published the vulnerability with ASN library, many people discussed it, and

focused on whether we can exploit it and gain privilege.

Theoretically speaking, we can gain privilege, but in fact, it's very difficult, because

it needs a very LARGE value to cause an integer overflow. This happened when copying data into
heap buffer, and will cause an error with writting buffer firstly, so it's difficult to be
exploited. If an example can deal with above 512M data when bit string heap corruption, it's
possible to exploit it.

To some special ASN library functions, they exist stack overflow. If this kind of ASN
function is used by some programs or services, we can exploit it. But it's regrettable,
because we don't find this kind of programs or services. If these programs exist, it's easy to
exploit(only stack overflow).

This is ASN1BERDecDouble function in ASN1 library(not Win2K+SP4):

call ASN1BERDecTag
test eax, eax
jz error
lea eax, [ebp+arg_4]
push edi
push eax
push ebx
call ASN1BERDecLength 〈-----When the value is bigger than 0X10C, trigger a stack
overflow
test eax, eax
jz error
mov edx, [ebp+arg_4]
cmp edx, edi
jnz short l1

l1:
mov eax, [ebx+20h]
lea ecx, [edx+eax]
lea esi, [eax+1]
mov [ebx+20h], ecx
movzx ecx, byte ptr [eax] <-------Pay attention to EAX. We can control it to fit to
condition, not the 0X84.

test cl, 80h
mov [ebp+var_8], ecx
jz l2

l2:
test cl, 40h
jz short l3

l3:
lea ecx, [edx-1]
lea edi, [ebp+var_10C] 〈–This is stack not heap. If ECX bigger than 0X10C, it
causes to overwrite stack.
mov eax, ecx
push 2Ch
shr ecx, 2
repe movsd
####################################################

But this vulnerability is fixed in Win2K+SP4. We found another similar function:

ASN1PERDecDouble. It can be exploited in W2K+SP4, but the new hotfix has fixed it.

Although we don't find system program that calls ASN1PERDecDouble or ASN1BERDecDouble,

but if these programs call THIS two functions and not be fixed, we can exploit to gain
privilege without doubt. Especially to ASN1PERDecDouble, it's dangerous in WIN2K+SP4.

The next thing is only to find these applies or services.

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