101 lines
3.0 KiB
Markdown
101 lines
3.0 KiB
Markdown
# Antimalware Signature Evasion
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## Check Binary Against AV
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* Split the binary to create signature
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```sh
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head --bytes <number of bytes> > sig.exe
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```
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* To test running antivirus, split it in half multiple times and check if it would be detected by the AV. Check for bad bytes
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### Automation
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* [Powersploit's Find-AVSignature](https://github.com/PowerShellMafia/PowerSploit/blob/master/AntivirusBypass/Find-AVSignature.ps1) finds it automatically
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* [rasta-mouse's Threat Check](https://github.com/rasta-mouse/ThreatCheck)
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## Static Code
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* Obfuscation of classes
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* __Class Hierarchy Flattening__, create proxies via interfaces
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* __Class Splitting & Coalescing__, shift variables to other classes
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* __Dropping Modifiers__, drop private use public only
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* Obfuscation of methods
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* __Method Proxy__, proxy method or object replacement
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* __Method Scattering & Aggregation__, coalesce or split methods
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* __Method Clone__, replication of methods
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## Property Based
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* Properties may be hash checksum or entropy of a binary
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* While having the source code at hand, simple white spaces can change the checksum or adding strings to the text section may lower the entropy
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### Bit Flipping
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* When only the binary is accessible add some `NOP`s or do bit flipping via
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```python
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import sys
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with open (sys.argc[1], "rb") as _f:
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orig = list(_f.read)
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for i,c in enumerate(orig):
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current=list(orig)
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current[i] = chr(ord(c) ^ 0xde)
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path = f"{i}.exe"
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with open (path, "wb") as _f:
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_f.write(''.join(str(x) for x in current)
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print("[+] Success")
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return 0
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```
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* Verify if the portable executables generated from the script above are still well formed
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```sh
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FOR /L %%A IN (1,1,10000) DO (
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signtool verify /v /a flipped\\%%A.exe
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)
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```
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## Behavior Based
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* __Import Adress Table (IAT)__ is the PLT of windows. It contains addresses of the (standard) lib functions provided by DLLs.
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* IAT can be found in a PE's header under `IMAGE_OPTIONAL_HEADER`. The references are provided by an API call or the __Thunk Table__
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* __Dynamic Loading__ can be used to call the address of a library function instead of IAT
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### Dynamic Loading
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* The structure of a call has to be defined, an example is [GetComputerNameA](https://docs.microsoft.com/en-us/windows/win32/api/winbase/nf-winbase-getcomputernamea)
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* Get the handle of the module where the address that will be called resides in
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* Get the process address of the loaded function address
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```c
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#include <windows.h>
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#include <stdio.h>
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#include <lm.h>
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// Compilation via i686-w64-mingw32-g++ -fPIC nosig.c -o nosig.exe
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typedef BOOL (WINAPI* myNotGetComputerNameA)(
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LPSTR lpBuffer,
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LPDWORD nSize
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);
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int main ( void ) {
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HMODULE hkernel32 = LoadLibraryA("kernel32.dll");
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myNotGetComputerNameA notGetComputerNameA = (myNotGetComputerNameA) GetProcAddress(hkernel32, "GetComputerNameA");
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printf("GetComputerNameA: 0x%p\\n", notGetComputerNameA);
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CHAR hostName[260];
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DWORD hostNameLength = 260;
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if (notGetComputerNameA(hostName, &hostNameLength)) {
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printf("hostname: %s\\n", hostName);
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}
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}
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```
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