This post is aimed towards reverse engineering beginners that have already heard about API hashing. If you don’t know, what Ghidra is or how to use it, you will need to brush over some parts of this post.
What is API Hashing
In case, you don’t follow this blag closely, I’ll quickly summarize, what I mean with API hashing: if a malware author doesn’t want to include API function names in the malware - neither in the import address table nor as strings somehow passed to GetProcAddress for example - they can use API hashing. This involves calculating some sort of hash for each combination of DLL file name and API function name (often only the latter) and inclusion of those hashes in the malware instead.
Given an API hash, the malware can enumerate all loaded DLLs and their exported functions to calculate hashes with the same custom algorithm and compare the result to the given hash, ultimately enabeling resolution of the corresponding API function. The topic is covered more thoroughly in the post about API hashing in the REvil ransomware.
Identifying the API Resolution Function
We will be looking at the Zloader sample with SHA256 hash
4029f9fcba1c53d86f2c59f07d5657930bd5ee64cca4c5929cbd3142484e815a
In another blag post about string obfuscation, we stumbled upon the API hashing function of Zloader: The function FUN_030a3170 is called in $190$ places and each time, it receives some small integral number and a larger value fitting into a DWORD. This alone slightly smells like API hashing but a dead give-away is the fact that the returned value of the function is always CALLed shortly after:
pcVar1 = (code *)FUN_030a3170(0,0x6aa0e84);
iVar2 = (*pcVar1)(2,0);
So let us rename FUN_030a3170 to ev_ResolveApi and take note of a few argument combinations:
| First Argument | Second Argument | Call |
|---|---|---|
0 |
0x6aa0e84 |
f(2,0) |
1 |
0xf3c7b77 |
f(0,puVar5,puVar4,0xcf0000,0x80000000,0x80000000,0x80000000,0x80000000,0,0,uVar3,0); |
9 |
0xabc78f7 |
f(puVar1,1,&local_14,0) |
A quick look at the decompiled code of the function should instantly make you loose interested in reverse engineering it top to bottom: It looks very convoluted and long. But let us not give up but leap our way to the goal.
The First Argument
So let’s skip over everything and only realize that the first argument is used to index the array named PTR_DAT_030bc2ec. The data PTR_DAT_030bc2ec[param_1] is then passed to the string deobfuscation function, analyzed in a previous blag post. Double clicking the array will show the following in the assembly listing view:
PTR_DAT_030bc2ec XREF[1]: FUN_030a3170:030a3224(R)
030bc2ec e8 c3 0b 03 addr DAT_030bc3e8 = 32h 2
030bc2f0 f5 c3 0b 03 addr DAT_030bc3f5 = 2Ch ,
030bc2f4 00 c4 0b 03 addr DAT_030bc400 = 37h 7
...
030bc348 e8 c3 0b 03 addr DAT_030bc3e8 = 32h 2
030bc34c db c4 0b 03 addr DAT_030bc4db = 3Bh ;
Ghidra identified each of the array entries as a pointer. So let’s interpret each entry of this array as an obfuscated string and decrypt it:
| Index | DLL Name |
|---|---|
| 0 | kernel32.dll |
| 1 | user32.dll |
| 2 | ntdll.dll |
| 3 | shlwapi.dll |
| 4 | iphlpapi.dll |
| 5 | urlmon.dll |
| 6 | ws2_32.dll |
| 7 | crypt32.dll |
| 8 | shell32.dll |
| 9 | advapi32.dll |
| 10 | gdiplus.dll |
| 11 | gdi32.dll |
| 12 | ole32.dll |
| 13 | psapi.dll |
| 14 | cabinet.dll |
| 15 | imagehlp.dll |
| 16 | netapi32.dll |
| 17 | wtsapi32.dll |
| 18 | mpr.dll |
| 19 | wininet.dll |
| 20 | userenv.dll |
| 21 | bcrypt.dll |
Hence the first argument to the function is an index into the above listed array of DLL names and hence almost certainly used to specify the DLL to use when resolving an API function.
Note the choice of words here: I did not say that I am sure that the argument is used to specify the DLL which is used to resolve a function; but only, that I am almost certain. When reverse engineering like this, you should keep the amount of certainty for every statement in the back of your head. So if something doesn’t make sense anymore, you can track back and more easily assess, where to dig deeper. In this case, I don’t see a lot of other possibilities, what this DLL name will be used for otherwise.
The second argument
Let us do a similar trick with the second argument: don’t reverse engineer the whole function but just look at the four places, where the second argument appears while keeping in mind that we believe it to specify an API hash of the function to be resolved:
uVar1 = param_2 % uVar1;
...
uVar2 = FUN_030b9a70(uVar2,param_2,0,0);
...
pcVar5 = FUN_030a3620(iVar4,param_2);
...
*local_14 = param_2;
The alleged API hash is passed into the two functions FUN_030b9a70 and FUN_030a3620. We will now take a look at the two, keeping an eye out for code that calculates an API hash to then compare it to the passed argument.
On first glance, the first of the two functions looks promising: it contains some arithmetic operations and calls a few other functions. But looking at the return value, one can instantly see that it either returns 0xa1 or 0. So these are probably not the droids we are looking for. The second function - FUN_030a3620 - looks at least as promising as the first one: it contains the two constants 0x60 and 0x18 at the very top and also uses a few (nested) loops.
So if someone would point a gun to my head and ask me for an opinion, which of the two you should investigate further, I’d definitely choose the second. And you should always imagine that someone is pointing a gun to your head while reverse engineering. We don’t have no time for anything else.
I nearly forgot to repeat a life hack from the very same blag post already referenced a few times, which should clear up, why I got so excited about the two constants 0x60 and 0x18: 0x18 is the offset of the Optional Header within the PE header and 0x60 is the offset of the Data Directories within that Optional Header. We don’t need to understand everything here but can simply assume that there is some sort of PE parsing going on (that is parsing of the Windows Portable Executable file format). And you need PE parsing to list exports from loaded DLLs, hence you need PE parsing to calculate API hashes of loaded functions.
Lucky for us, the API hash passed in as an argument is only used in one single line:
if (uVar3 == param_2) {
And since it does not make much sense to compare an API hash with anything else but another API hash, it is reasonable to assume that uVar3 also contains an API hash. It is also plausible that it contains the API hash calculated by the malware based on loaded DLL names and their exported functions. Since the value of uVar3 comes out of FUN_030a3140 let’s rename that function to pr_ApiHash. It receives local_90 and -1 as arguments. So let’s just assume for now that local_90 is somehow derived from DLL and function names and dive into pr_ApiHash.
The API Hashing Function
Lazy time is over now. We finally need to understand some code and what exactly, pr_ApiHash does with its arguments to arrive at an API hash. Since we already assumed that the first argument contains some data derived from DLL and function names, let us focus on the second argument for now: It is first compared with -1 - which makes sense because we already observed this value as an argument - and another function, FUN_030a2fe0, is called with the alleged DLL and function names as arguments. Let’s look into FUN_030a2fe0 and retype its argument to BYTE *:
int __cdecl FUN_030a2fe0(BYTE *param_1) {
int iVar1;
int iVar2;
if (param_1 != (BYTE *)0x0) {
iVar2 = -1;
do {
iVar1 = iVar2 + 1;
iVar2 = iVar2 + 1;
} while (param_1[iVar1] != '\0');
return iVar2;
}
return 0;
}
If the passed data is the NULL pointer, the function will return 0. Otherwise, it will initialize the variable iVar2 with -1 and increase value passed into the function until it is the NULL terminator. During each iteration, the return variable iVar2 is incremented by one. Since this is a do-while loop, this incrementation happens at least one time. Staring at this code a bit more, you can see that this function will interpret the passed argument as a string and return its length. This is huge because we can now guess the type of the argument and also the type of the variable passed into this function: it probably is just char * as opposed to some complex data structure derived from DLL and function names.
So let us rename FUN_030a2fe0 to strlen and retype the two arguments to pr_ApiHash according to what we just learned. While we are at it, realize that uVar4 is the value returned from pr_ApiHash and rename that variable to ApiHash.
uint __cdecl pr_ApiHash(char *SomeString,int StrLen) {
byte bVar1;
uint uVar2;
uint uVar3;
uint ApiHash;
if (StrLen == -1) {
StrLen = strlen(SomeString);
}
ApiHash = 0;
if ( (SomeString != (char *)0x0) && (0 < StrLen) ) {
ApiHash = 0;
do {
bVar1 = FUN_030a5260();
ApiHash = (uint)(byte)*SomeString + (ApiHash << (bVar1 & 0x1f));
if ( (ApiHash & 0xf0000000) != 0 ) {
uVar3 = (ApiHash & 0xf0000000) >> 0x18;
uVar2 = FUN_030a9b90(0xfffffff,0xffffffff,0);
ApiHash = FUN_030aeef0(~(uVar2 | ~ApiHash | uVar3),(uVar2 | ~ApiHash) & uVar3,(HINSTANCE)0x0);
}
SomeString = (char *)( (byte *)SomeString + 1);
StrLen = StrLen + -1;
} while (StrLen != 0);
}
return ApiHash;
}
Code-Level Obfuscation
Now we need to get really un-lazy. There are three functions used during calculation of the API hash with names FUN_030a5260, FUN_030a9b90 and, FUN_030aeef0. Each of these functions needs special attention.
- Even though the return value of
FUN_030a5260is used, Ghidra did not correctly guess the function signature and somehow determine that it is avoidfunction. Change the signature (HotkeyF), check “Use Custom Storage” and change the returned data type tointand the storage location toEAX. ChoosingEAXis often correct and I suggest to just try it and justify later if the resulting decompiled code makes sense. Again, purely for time-efficiency reasons. The result will be a convoluted function that ends withreturn _DAT_030be374 ^ 0xa2df808b. Follow_DAT_030be374and change the type toddw(HotkeyDthree times). This reveals that this global variable contains the value0xA2DF808F. Xor-ing with0xa2df808bresults in4. Hence, we can renameFUN_030a5260toReturn4. - Similarly,
FUN_030a9b90is identified to be avoidfunction. Performing the same procedure as above (adapt the signature to return anintinEAX) will lead to a very simple decompiled function that only Xors the first two arguments. Hence you can rename it toXor(and also remove the last parameter if you feel tidy). - Finally,
FUN_030aeef0only seems to calculate the binary or of the two parameters, hence rename it toBor(and, again, remove the third parameter if you like).
The above three functions are probably caused by anti-analysis techniques employed by the malware author. The technique used in the first function is called “constant unfolding” because it is the opposite of the compiler optimization technique called constant folding. Constant folding evaluates constant expressions during compile time to avoid unnecessary calculations during run time. Constant unfolding does the reverse: it identifies constants - 4 in this case - and replaces them with some sort of calculation - _DAT_030be374 ^ 0xa2df808b in this case - during compile/build time.
Similarly, the other two function employ the opposite of the compiler optimization technique called inlining: Instead of performing the arithmetic operation in-line (here, a simple Xor / Binary Or), a function is called that performs this operation. In addition to that, unnecessary instructions where inserted into this un-in-lined function that make the code harder to read. Specifically the condition of a branch like
if ( ( ( (param_2 == 0xb4c6d61) && (fuLoad != param_1)) &&
(in_stack_0000000c != (HINSTANCE)0xb4c6d61)) &&
( ( (int)in_stack_0000000c << 7 | (uint)in_stack_0000000c) == 0)) {
that is never taken is called opaque predicate. In addition to that, both function contain some jung instructions without any side effects. A lot of them have been removed by the Ghidra decompiler, but since identifying those is hard - even heuristically - some still remain.
Re-Implementing the Hashing Function
Offentimes you want to emulate API hashing in a different language because it enables you to annotate API resolution calls during static analysis. Re-implementing an algorithm will often also get rid of any implementational details that may have even been introduces by a compiler or obfuscator during built. This in turns eases identification of overlaps in the hashing method between different malware families, which in turn may indicate a link between the families.
After performing the above-described steps and some minor adjustments to variable names in the pr_ApiHash function, we end up with the following:
uint __cdecl pr_ApiHash(char *SomeString,int StrLen) {
byte Four;
uint Mask;
uint HighNibble;
uint ApiHash;
if (StrLen == -1) {
StrLen = strlen(SomeString);
}
ApiHash = 0;
if ( ( SomeString != (char *)0x0 ) && (0 < StrLen) ) {
ApiHash = 0;
do {
_Four = Return4();
ApiHash = (uint)(byte)*SomeString + (ApiHash << ( (byte)_Four & 0x1f ));
if ( (ApiHash & 0xf0000000) != 0 ) {
HighNibble = (ApiHash & 0xf0000000) >> 0x18;
Mask = Xor(0xfffffff,0xffffffff);
ApiHash = Bor(~(Mask | ~ApiHash | HighNibble),(Mask | ~ApiHash) & HighNibble);
}
SomeString = (char *)( (byte *)SomeString + 1 );
StrLen = StrLen + -1;
} while (StrLen != 0);
}
return ApiHash;
}
You can just copy this into a text editor and change the syntax a bit until it is valid code of your language of choice. If your language natively supports bigints (like Python), better make sure to sprinkle it with enough & 0xffffffff. I decided to use Python for now and since I very much enjoy totally unnecessary optimizations, I ended up with the following:
def calc_hash(function_name):
mask = 0xf0000000
ret = 0
for c in function_name:
ret = ord(c) + (ret << 0x4)
if ret & mask:
ret = (~ret | mask) ^ (~ret | ~mask) >> 0x18
return ret & 0xffffffff
API Hash Lookup
We already know that the API hash 0x6aa0e84 from the kernel32.dll should resolve to a function that accepts two arguments like so f(2,0). So let us plug all exports from the kernel32.dll into the hashing function and check the result:
import pefile
pe = pefile.PE(data=open('C:\\Windows\\SysWOW64\\kernel32.dll', 'rb').read())
export = pe.DIRECTORY_ENTRY_EXPORT
dll_name = pe.get_string_at_rva(export.struct.Name)
for pe_export in export.symbols:
export_name = pe_export.name.decode('utf-8')
if calc_hash(export_name) == 0x6aa0e84:
print(export_name)
Which … fails by giving no result. Since I was pretty sure about everything but the data actually passed into pr_ApiHash, I decided to do some reversing around that next: Ghidra determined the type of variable local_90 to be undefined2 local_90 [50]. This is Ghidra’s way of telling you that it thinks it is an array with $50$ entries where each entry has a length of 2 bytes. Since we already established that the array is actual a string, I decided to retype it to char[100]:
...
FUN_0309ea50(local_90, uVar3);
cVar1 = *(char *)(iVar5 + param_1);
if (cVar1 != '\0') {
i = 0;
do {
local_90[i] = FUN_0309a690(cVar1);;
cVar1 = *(char *)(iVar5 + param_1 + 1 + i);
i = i + 1;
} while (cVar1 != '\0');
}
uVar4 = pr_ApiHash(local_90,-1);
...
So the values of that array come out of the function FUN_0309a690. Let’s take a close look at it: the function receives a value and either returns it or adds 0x20 and returns the result. Because the condition looks complicated and was hit pretty hard by the obfuscator the author probably uses, I was just lucky to know what adding the number 0x20 in the context of strings may mean: converting upper-case characters to lower-case characters. So my leap of faith was to assume that FUN_0309a690 actually is pr_toLower. And heureca! Running the above Python code with lower-cased export_name results in a single hit, namely CreateToolhelp32Snapshot which accepts two DWORD arguments according to the documentation. This is in-line with our observation from the table at the start of this post.
Summary
What can you take away from this post? Maybe it is that I’m just as lazy as a sloth and don’t even reverse engineer. Maybe, that adding or subtracting 0x20 means converting between upper and lower case strings. Maybe, that offsets 0x18 and 0x60 indicate PE parsing. Or maybe, that it is sometimes possible to understand a lot of about a malware without going into every single line of code and understanding everything.
Article Link: https://blag.nullteilerfrei.de/2020/06/11/api-hashing-in-the-zloader-malware/