C# ZipFile Password Cracker Project [Open Source]
After my on the 7-Zip bugs CVE-2017-17969 and CVE-2018-5996, I continued to spend time on analyzing antivirus software.
As it happens, I found a new bug that as the last two bugs turned out to affect 7-Zip as well.
Since the antivirus vendor has not yet published a patch, I will add the name of the affected product in an update to this post as soon as this happens.
UPDATE 2018-06-05 : The antivirus vendor I was talking about was.
As we have seen in some of my earlier blog posts, the UnRAR code is very fragile.
Therefore, it is hardly surprising that any changes to this code are likely to introduce new bugs.
Very abstractly, the bug can be described as follows: The initialization of some member data structures of the RAR decoder classes relies on the RAR handler to configure the decoder correctly before decoding something.
Unfortunately, the RAR handler fails to sanitize its input data and passes the incorrect configuration into the decoder, causing usage of uninitialized memory.
Now you may think that this sounds harmless and boring.
Admittedly, this is what I thought when I first discovered the bug.
Surprisingly, it is anything but harmless.
In the following, I will outline the bug in more detail.
Finally, we will see how the bug can be exploited for remote code execution.
The Bug This new bug arises in the context of handling solid compression.
The idea of solid compression is simple: Given a set of files e.
This can yield a higher compression rate, in particular if there are many files 7 zip source codes are somewhat similar.
In the RAR format before version 5solid compression can be used in a very flexible way: Each item representing a file of the archive can be marked as solid, independently from all other items.
The idea is that if an item is decoded that has this solid bit set, the decoder would not reinitialize its state, essentially continuing from the state of the previous item.
Obviously, one needs to make sure that york city new york zip code manhattan decoder object initializes its state at the beginning for the first item it is decoding.
Let us have a look at how this is implemented in 7-Zip.
The RAR handler has a method NArchive::NRar::CHandler::Extract that contains a loop which iterates with a variable index over all items.
That seems to be correct, right?
Well, the problem is that RAR supports three different encoding methods excluding version 5and each item can be encoded with a different method.
In particular, for each of these three encoding methods there is a different decoder object.
Interestingly, the constructors of these decoder objects leave a large part of their state uninitialized.
The first item will cause the solidStart flag to be set to false.
At first sight, this may not look too bad.
These variables may now hold a size that is larger than the actual buffer, allowing a heap-based buffer overflow.
Obviously, the list is not complete.
The Fix In essence, the bug is that the decoder classes do not guarantee that their state is correctly initialized before they are used for the first time.
As we have seen, this does not turn out very well.
UnRAR seems to implement the first option.
Igor Pavlov, however, chose to go with a variant of the second option for 7-Zip.
In case you want to patch a fork of 7-Zip or you are just interested in the details of the fix, you might want to have a look atwhich summarizes the changes.
On Exploitation Mitigation In the on the 7-Zip bugs CVE-2017-17969 and CVE-2018-5996, I mentioned the lack of DEP and ASLR in 7-Zip before version 18.
Shortly after the 7 zip source codes of that blog post, Igor Pavlov released 7-Zip 18.
Moreover, all dynamic libraries 7z.
Hence, most of the running code is subject to ASLR.
However, all main executables 7zFM.
This means that not only are they not subject to ASLR, but you cannot even enforce ASLR with a tool like EMET or its successor, the 7 zip source codes Defender Exploit Guard.
Obviously, ASLR can only be effective if all modules are properly randomized.
I discussed this with Igor and convinced him to ship the main executables of the new 7-Zip 18.
The 64-bit version still runs with the standard non-high entropy ASLR presumably because the image base is smaller than 4GBbut this is a minor issue that can be addressed in a future release.
On an additional note, I would like to point out that 7-Zip never allocates or maps additional executable memory, making it a great candidate for Arbitrary Code Guard ACG.
In case you are using Windows sonora zip code nogales, you can enable it for 7-Zip by adding the main executables 7z.
Writing a Code Execution Exploit Normally, I would not spend much time thinking about actual weaponized exploits.
However, it can sometimes be instructive to write an exploit, if only to learn how much it actually takes to succeed in the given case.
The platform we target is a fully updated Windows 10 Redstone 4 RS4, Build 17134.
Each of these three methods will invoke a different executable 7zFM.
Since we want to exploit the lack of ASLR in these modules, we need to fix the extraction method.
Hence, we go with the second method.
Exploitation Strategy Using the bug from above, we can create a Rar decoder that operates on mostly uninitialized state.
So let us see for which Rar decoder this may allow us to corrupt the heap in an attacker-controlled manner.
One possibility is to use the Rar1 decoder.
Moreover, bytePlace is determined by the input stream, so it is attacker-controlled as well but cannot be larger than 0xff.
So this would give us a pretty good though not perfect read-write primitive.
Therefore, we will target the vtable pointer of an object that is placed after the Rar1 7 zip source codes on the heap.
The idea is to use a Rar3 decoder object for this purpose, which we will use at the same time to hold our payload.
Roughly speaking, we will do this by making an allocation of the size of the Rar1 decoder object, writing the desired data to it, and then freeing it at some point before the actual Rar1 decoder is allocated.
A straightforward way to achieve this is to activate Low Fragmentation Heap LFH on the corresponding allocation size, then spray the LFH with multiple of those replacement objects.
This actually works, but because allocations on the LFH are randomized since Windows 8, this method will never be able to place the Rar1 decoder object in constant distance to any other object.
Therefore, we try to avoid the LFH and place our object on the regular heap.
This will activate LFH for these allocation sizes and prevent such small allocations from destroying our clean heap structure.
Note that it is unavoidable to allocate some decoder before allocating that Rar1 decoder, because only this way the solidStart this web page will be set to false and the next decoder will not be initialized correctly see above.
If everything works as planned, the Rar1 decoder reuses our replacement object, and the Rar3 decoder object is placed with some constant offset after the Rar1 decoder object.
Allocating and Freeing on the Heap Obviously, the above allocation strategy requires us to be able to make this web page allocations in a reasonably controlled manner.
Going through the whole code of the RAR handler, I could not find many good ways to make dynamic allocations on the default process heap that have attacker-controlled size and store attacker-controlled content.
Let us see how this works.
Add item ; } The class CItem has a member variable Name of type AString, which stores the ASCII name of the corresponding item in a heap-allocated buffer.
In particular, it seems that 7 zip source codes cannot write null bytes.
This is bad, because the replacement object we want to put on the heap requires a few zero bytes.
So what can we do?
Moreover, we see that AString keeps the same underlying buffer, unless it is too small to hold the desired string.
This gives rise to the following idea: Assume we want to https://money-promocode-jackpot.website/zip-code/code-in-miami-zip.html the hex-bytes DEAD00BEEF00BAAD00 to some heap-allocated buffer.
Note that we have replaced all null bytes in our data with code fife zip arbitrary non-zero byte 0x55 in this exampleensuring that the full string is written to the buffer.
This works reasonably well, 7 zip source codes we can 7 zip source codes this to write arbitrary sequences of bytes, with two small limitations.
First, we have to end our sequence with a null byte.
Second, we cannot have too many null bytes in our byte sequence, because this will cause a quadratic blow-up of the archive size.
Luckily, we can easily work with those restrictions in our specific case.
These allocations have the property that they will be freed eventually, and they can using the above technique be filled with almost arbitrary sequences of bytes.
Since these allocations are all made via the same stack-allocated object item and hence use the same AString object, the allocation sizes of this type need to be strictly increasing in their size.
We will use this allocation type mainly to put the replacement object on the heap.
This is useful in particular to cause many pending allocations of certain sizes in order to activate LFH.
Note that the copied string cannot contain any null bytes, which is fine for our purposes.
Combining the outlined methods carefully, we can construct an archive that implements the heap allocation strategy from the previous section.
ROP We leverage the lack of ASLR on the main executable 7zG.
This is not really a problem, but it makes the ROP chain somewhat ugly.
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On Reliability After some fine-tuning of the auxiliary heap allocation sizes, the exploit seems to work very reliably.
In order to obtain more information on reliability, I wrote a small script that repeatedly calls the binary 7zG.
Moreover, the script checks that calc.
Note that across all operating systems, the very same crafted archive is used.
This works well, presumably because most changes between the Windows 7 and Windows 10 heap implementation affect the Low Fragmentation Heap, whereas the rest has not changed too much.
Moreover, the LFH is still triggered for the same number of pending allocations.
Admittedly, it is not really possible to determine the reliability of an exploit empirically.
Conclusion In my opinion, this bug is a consequence of the design partially inherited from UnRAR.
If a class depends on its clients to use it correctly in order to prevent usage of uninitialized class members, you are doomed for failure.
We have seen how this at first glance innocent looking bug can be turned into a reliable weaponized code execution exploit.
Due to the lack of ASLR on the main executables, the only difficult part of the exploit was to carry out the heap massaging within the restricted context of RAR extraction.
Fortunately, the new 7-Zip 18.
Do you have any comments, feedback, doubts, or complaints?
I would love to hear them.
You can find my e-mail address on the page.
Alternatively, you are invited to join the discussion on or on.
How To Unzip A File In Windows 10 Using 7-Zip
Here's a super quick and easy Windows 10 tip to show you how to extract files from .zip .rar .7z and .tar files for free!. (Free, Safe, Open Source) More Windows 10 Tips. How to Install 7.
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