Date post: | 18-May-2015 |
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Reversing Microsoft Patches to reveal Vulnerable code
Harsimran Walia
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Introduction
Need
Process
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Finding a 0day vulnerability
Vulnerability reaches the
vendorVendor finds a
fix
Releases a patch to fix the
vulnerability
Introduction
Birth of a security patch
Microsoft patches
Reverse engineer the
patch
Locate the vulnerability
patched
Highlight the difficulties
Discussion in the presentation
For reversing and obtaining binary difference in my demos I would be using DarunGrim2
How DarunGrim works?• The schema of DarunGrim is shown in
the figure• To generate diffing results
– Binaries are disassembled in IDA Pro in thebackground and darungrim IDA plugin is runwhich creates the sqlite database
– Diffing Engine, the heart of DarunGrim2.The sqlite db from IDA and the binaries from GUIare fed into this engine as inputs
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Introduction
Algorithm ?
• Main algorithm of DarunGrim is Basic block fingerprint hash map• Each basic block is 1 entity whose fingerprint is generated from the
instruction sequence• Fingerprint hash generated by IDA Pro• Two fingerprint hash tables one each for unpatched and patched binary• For finding the binary difference, each unique fingerprint from original
binary is searched against the fingerprints of patched binary for a match• All fingerprints in the original binary hash tables are either matched or
unmatched
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Introduction
Algorithm ? Contd..
• For a function to be called matching, all the basic blocks in the function should be matching
• For unmatched functions DarunGrim calculates percentage match• Match rate based on fingerprint string match
– Similar to GNU Diff algorithm which is finding longest common subsequence
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Introduction
Vulnerability Vs Exploit based signatures
Exploit signatures • Created by using byte string patterns or regular expressions • These are exploit specific • They are used widely mainly because of the ease of their creation• Cater to only one type of input satisfying that vulnerability condition• Fail: different attacks can exploit the same vulnerability, so exploit based
signatures will fail • For eg. Exploit based signature
– ESig = “docx?AAAAAAAAAAA...”– It will fail if some exploit uses a long string of B’s instead of A’s
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Introduction
Vulnerability Vs Exploit based signatures
Vulnerability signatures • Based on the properties of the vulnerability and not on
the properties of the exploit• It is a superset of all the inputs satisfying a particular
vulnerability condition• For eg. Vulnerability based signature for previous case
– VSig = MATCH_STR (Buffer,"docx?(.*)$",limit)– Matches string in buffer with the regex– It is effective against any alphabet unlike exploit signature
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Introduction
VulnerabilitySignature
Exploit Signature
Vulnerability Vs Exploit based signatures
Vulnerability signatures contd..• For a good vulnerability signature
– It should strictly not allow any false negatives as even one exploit can pwn the system and create a gateway for the attacker into the network.
– It should allow very few false positives, as too many false positives may lead to a DoS attack for the system.
– The signature matching time should not create a considerable delay for the software and services.
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Introduction
• The first step of creating an undisclosed exploit is to find the vulnerability to exploit it.
• To verify if the patch released by Microsoft is working as per it is designed.
• To create vulnerability based signatures.
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Need
Finding patches
Extraction of files
Binary Differencing
Differencing Analysis
Debugging
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Process
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Process
• Pick a vulnerability and download its patch• Pick a vulnerability just before this one that patched the same
program or dll– If unavailable, use the same dll from your system
Finding patches
Problem
• GDR or QFE/LDR ??• File Versioning
Quick-fixUse open source
ms-patch-tools to easily get the file
versions to compare
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Process
Finding patches
DEMO C:\Users\harry\Documents\Camtasia Studio\
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Process
• The traditional way of extracting file from patch – <patchfilename>.exe /x– Works only till Windows XP and earlier versions of Windows
Problem
• Above method cannot be used on Win7 and Vista patches delivered as msu
Extraction of filesFinding patches
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Process
• Use expand command– expand -F:* <Saved_MSU_File_Name>.msu C:\<Folder_to_extract_in> – expand -F:* <Saved_MSU_File_Name>.cab C:\<Folder_to_extract_in>
Extraction of files
Solution
Finding patches
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Process
Extraction of filesFinding patches
DEMO
Finding patches
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Process
• DarunGrim v2 used for binary difference– Feed in the two binaries to be compared– Generates a list of functions with the %age match between the two files
Extraction of files
Binary Differencing
Problem
• Not every function %age < 100 is changed• Includes false positives which requires manual analysis
Finding patches
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Process
Extraction of files
Binary Differencing
DEMO
Finding patches
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Process
• Manual inspection of functions with less than 100% match– Remove false positives generated by problems like
• Instruction reorderingLot of reordering happening over different releases marks even the same blocks as unmatched
• Split blocksBlock in the graph which has only parent and the parent has only one child leads to a split block.causing a problem in the matching processCan be improved by merging the two blocks and treating as a single block.
Extraction of filesBinary Differencing
Differencing Analysis
• Hot patchingInstructions like mov eax, eax at the start of functions are a sign of hot patching leading to a
mismatch in the blockBy just ignoring the instruction we can get a match
• Compiler optimizations Different compilers and even different versions of the same compiler perform different
optimizations which also creates problems in getting proper difference
– Eventually reach a function which is indeed modified and might be the fix to the vulnerability being patched
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Finding patches
Process
Extraction of filesBinary Differencing
Differencing Analysis
Finding patches
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Process
Extraction of filesBinary Differencing
Differencing Analysis
DEMO unpatched.png
patched.png
Finding patches
• push [ebp-2Ch] ; unsigned intcall ??2@YAPAXI@Z ; operator new(uint)mov ebx, eaxpop ecxmov [ebp-18h], ebxmov [ebp-3Ch], ebxmov byte ptr [ebp-4], 1push dword ptr [ebp-2Ch]mov ecx, esipush ebxpush [ebp-30h]call sub_118000C func(const *,void *,long)mov edi, eaxtest edi, edijge short
• push [ebp-2Ch] ; unsigned intcall ??2@YAPAXI@Z ; operator new(uint)pop ecxmov [ebp-14h], eax ; ebp-14h = pBuffermov [ebp-40h], eaxmov byte ptr [ebp-4], 2push [ebp-2Ch]mov ecx, esipush ebxpush edicall sub_118000C func(const *,void *,long)mov esi, eaxtest esi, esijge short loc_118158A
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Process
Extraction of filesBinary Differencing
Differencing Analysis
Finding patches
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Process
Extraction of filesBinary Differencing
Differencing Analysis
Debugging
• To validate our finding of analysis by debugging– Getting a crash of the application– Creating a malformed file to get the crash
• Would be using Immunity Debugger
Finding patches
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Process
Extraction of filesBinary Differencing
Differencing Analysis
Debugging
DEMO
Conclusion
• Presented an overview of how the 1-day exploits and Vulnerability signatures can be created
• Attempt was made to understand the process involved in reversing and the problems faced during the execution of the process
• Only talked about Microsoft patches but concept not limited to this.
• Concepts presented can be perfected by interested audience
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Thanks
Questions??
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