Today we have released MS11-044 to address CVE-2011-1271, a remote code execution vulnerability in the .NET framework. Here we would like to provide more technical information about this vulnerability and why we believe this issue to be unlikely to be exploited.
This root cause of CVE-2011-1271 is that there was a bug in the JIT compiler which would cause it to mistakenly determine that a given object is always null (or non-null) and would omit certain checks.
if ((value == null || value == new string) == false)
00000027 test esi,esi ; value == null?
00000029 je 00000075
0000002b xor edx,edx ; new string
0000002d mov ecx,6D913BD2h
00000032 call FFD20BC8
00000037 cmp eax,esi ; value == new string?
00000039 je 00000075
Console.WriteLine("Post-check Value is: " + value);
0000003b mov ecx,dword ptr ds:[03532090h] ; "Post-check value is: "
00000041 xor edx,edx ; Wrong here.
00000043 call 6D70B7E8 ; String.Concat()
00000048 mov esi,eax ;
0000004a call 6D72BE08 ; get Console.Out
0000004f mov ecx,eax
00000051 mov edx,esi
00000053 mov eax,dword ptr [ecx]
00000055 call dword ptr [eax+000000D8h] ; Console.WriteLine()
At offset 0x41, the optimizer has incorrectly concluded that value will always be null so it directly passes a null to String.Concat().
For CVE-2011-1271, the JIT compiler can introduce a logic flaw when running C# or IL code sequences very similar to those describe above. Depending on the .NET application’s business logic, if the NULL check (or non-NULL check) is used to make a security decision, for example the check of certain credentials, and if the attacker controlled data may leverage directly or indirectly this missing logic and gain advantages based on this, then there is a possibility of remote code execution. However, we do not believe this to be a common case for the majority of deployed .NET applications.
Special thanks to Reid Borsuk in .NET team.
Fermin Serna and Chengyun Chu, MSRC Engineering