Buffer OverFlow in ILASM and ILDASM

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See Below in this post

  • To MSRC: Buffer OverFlow in ILASM and ILDASM - The entire email conversation with MSRC (secure@microsoft.com) going from the inital response to the final answer where they will not threat this as a vulnerability and will not issue a security advisory for it (the solution will be included in the next Service Pack)
  • Buffer Overflow in ILASM - original email containing my first thoughts
  • ILDASM Exception Creator- little tool created by Kerem to create .Net assemblies that crash ILDASM
  • ILDAM vulnerability ShellCode development - more code snippets and comments (now releated to trying to inject a shellcode into the vulnerable process)

The bottom line is that this is a real issue in 1.1 and 1.0 (2.0 seems to mitigate them), Microsoft has acknowlege the problem but will not release a patch any time soon.

So be careful when you ILDASM something.

I also think that this issue needs further research since when we were testing the Overflows we were finding them in several places in ILASM and ILDASM (which means that there are probably many more variations still to be discovered/mapped)

To MSRC: Buffer OverFlow in ILASM and ILDASM

First email

[sent to secure@microsoft.com on 30th november 2003]

Dear MSRC,

A buffer overflow occurs in ILASM if a *.il file contains a call to a function of +/- 64k size. For example:



call instance void [System.Windows.Forms]System.Windows.Forms.Form::AAAAAAAAAAAAAAAAAAA .... (+/- 64k) AAAAA();

(see ILASM overflow 1.gif and ILASM overflow 2.gif)

To test this issue, on the command line, assemble the provided IilasmPoc.il file using ILASM and an exception will occur.

This issue has been confirm in the .Net framework 1.1. In 2.0 this seems to be fixed since no exception is thrown

Buffer Overflow In ILDASM

To test this issue,on the command line, disassemble the provided IldasmPoc.dll using ILDASM and an exception will occur. Which causes a heap overflow.

This issue has been confirmed in the .Net framework 1.1. In v2.0.an exception is thrown but it doesn't initially seem to be exploitable

The Exception on the ILDASM 1.1 is an 'Unhanded Exception which occurs at 0x7c92142e (ntdll.dll) in ildasm.exe: 0xC0000005: Access violation reading location 0x41414141 (see ILDASM2.PNG).

The debug result of the application shows that the exception is occurring in calloc.c source code of the ntdll.dll in windows xp service pack 2 debug symbols provided code block.

The Actual section of the exception is given below.

pvReturn = HeapAlloc(_crtheap, HEAP_ZERO_MEMORY, size); if (pvReturn == NULL) cmp dword ptr [pvReturn],ebx jne $L19640+1 (7C3423B6h) pvReturn = HeapAlloc(_crtheap, HEAP_ZERO_MEMORY, size); push esi push 8 push dword ptr [__crtheap (7C38C944h)] call dword ptr [__imp__HeapAlloc@12 (7C37A07Ch)] mov dword ptr [pvReturn],eax <-- The Exception Occurs Here

ILDASM Proof of Concept generator (see ILDASM_PoC folder)

To help with the debugging of this issue we created a simple PoC which generates a 74,3 kilobytes static method using Emit Namespaces inside the MyDynamicAsm.dll,

The CLR doesn't seem to be affected by this issue since the method can be called and executed using reflection. Note that it can't be called directly from c# (due to size restrictions on the vs.net compilers).

Ironically this method could be used as a protection for ildasm usage against compiled assemblies.


Original discovery: Dinis Cruz Further research, PoC and Documentation: Dinis Cruz and Kerem Kusmezer

Dinis Cruz Owasp .Net Project Leader www.owasp.net

final response: Its a vulnerability in 1.1, but no Security Patch will be released

From: "Microsoft Security Response Center" <secure@microsoft.com> Sent: 08 December 2005 11:32

Hi Dinis,

Great timing on your response, I have just received the feedback from the product team overnight.

The outcome from the investigation is as follows:

1. The ILDASM and ILASM tools are provided as part of the .Net Framework tools, SDK provided for developers and only for developers (no direct end user threat vectors - not installed by default etc).

2. As with any development tool being used by developers that accept input, the developer must ensure that what they are using to compile/ assemble/ open is considered trusted. This is equivalent to breaking Law 1 on http://www.microsoft.com/technet/archive/community/columns/security/essays/10imlaws.mspx

3. This issue has been fixed with Visual Studio 2005/ .Net Framework 2.0 - I have asked that this issue be fixed in any update shipped for the .Net 1.1, 1.0 SDK. (We would not issue a security bulletin for this).

Can you please let me know when you will post the message Owasp community, I would like to give a heads up to our Sec PR team to ensure the understand the underlying issue.

Thanks for not issuing the technical details, greatly appreciated, I think the message you post would be important to re-emphasize that developers who take un-trusted input for anything they build/ compile...whatever is extremely risky - this is one example of where that risk is realized.

Cheers Scott D.

Re: final response: Its a vulnerability in 1.1, but no Security Patch will be released

From: "Microsoft Security Response Center" <secure@microsoft.com> Sent: 08 December 2005 16:37

Hi Dinis,

I wanted to clarify something in my previous email, the assembler portion of this issue (ILASM) is included with the .Net framework, the disassembler is not, its part of the SDK.

This implies that we will need to fix the ILASM issue in the next .Net Framework 1.0/1.1 Service Pack.

Either way in all cases having developers ensure they are use trusted input to compiling is the best way to mitigate this issue.

Please let me know your thoughts on this issue, and I apologize for providing confusing information in the previous email.

Cheers Scott D.

From: Dinis Cruz [1] Sent: Tuesday, December 13, 2005 6:58 PM To: Microsoft Security Response Center Subject: Re: Buffer OverFlow in ILASM and ILDASM [6296sd]

Hi Scott

Do you have an idea of when this Service Pack will be released?

Dinis Dinis Cruz Owasp .Net Project Leader www.owasp.net

from: "Microsoft Security Response Center" <secure@microsoft.com> Sent: Wednesday, December 14, 2005 11:34 AM To: "Dinis Cruz" <dinis@ddplus.net> Subject: RE: Buffer OverFlow in ILASM and ILDASM [6296sd]

Hi Dinis,

I have some feedback from the product group that a SP for 1.1 of the framework will likely be next year sometime.

At a guess this may be later on in the year, I have seen no firm plans as of yet. I have also seen no discussion on an SP for 1.0. at this stage.

Fairly loose dates I know and I apologize, I wish I could provide you with more specific timelines but at this stage that is all we are aware off.

Cheers Scott D.

Buffer Overflow in ILASM

ILDASM Exception Creator

ILDAM vulnerability ShellCode development

  • Dinis: Kerem, can you upload to this thread a file with the exact size that triggers the Overflow (i.e ending in BCDE)
  • Kerem: Here you go File:Finalmagical zip.ppt

  • See File:ILDASM BO ShellCode.zip.ppt zip for a series of test projects and two files ILASM_BO_Tests.sxw and ILDASM_BO_Tests.sxw which contain some notes I was doing while I had a quick look at this issue.