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3 years ago
# Fully Weaponized CVE-2021-40444
Malicious docx generator to exploit CVE-2021-40444 (Microsoft Office Word Remote Code Execution), works with arbitrary DLL files.
# Background
Although many PoC are already around the internet, I guessed to give myself a run to weaponizing this vulnerability,
as what I found available lacked valuable information that it's worth sharing, also considering Microsoft already
released a patch for this vulnerability.
So far, the only valuable resources I've seen to create a fully working generator are:
* [Blog by Ret2Pwn](
* [Twit by j00sean](
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The above resources outline a lot of the requirements needed to create a full chain. To avoid repeating too much
unnecessary information, I'll just summarize the relevant details.
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### Exploit Chain
1. Docx opened
2. Relationship stored in document.xml.rels points to malicious html
3. IE preview is launched to open the HTML link
4. JScript within the HTML contains an object pointing to a CAB file, and an iframe pointing to an INF file,
prefixed with the ".cpl:" directive
5. The cab file is opened, the INF file stored in the %TEMP%\Low directory
6. Due to a Path traversal (ZipSlip) vulnerability in the CAB, it's possible to store the INF in %TEMP%
7. Then, the INF file is opened with the ".cpl:" directive, causing the side-loading of the INF file via rundll32
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(if this is a DLL)
### Overlooked Requirements
There are quite a bit of overlooked requirements for this exploit to work, which caused even good PoCs, like
[the one by lockedbyte](, to fail working properly.
Maybe nobody explicitly "released" them to avoid the vulnerability to be exploited more. But now it's patched,
so it should not cause a lot of troubles to release the details.
#### HTML File
As for this [tweet]( by [Will Dormann](,
the HTML should be at least 4096 bytes in size in order to trigger the "Preview" within MS Word.
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#### CAB File
The CAB file needs to be byte-patched to avoid extraction errors and to achieve the ZipSlip:
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* `filename.inf` should become `../filename.inf`
* `filename.inf` should be exactly `<12-char>.inf`
* `CFFOLDER.typeCompress` should be 0 (not compressed)
* `CFFOLDER.coffCabStart` should be increased by 3
* `CFFOLDER.cCfData` should be 2
* `CFFILE.cbFile` should be greater than the whole `CFHEADER.cbCabinet`
* `CFDATA.csum` should be recalculated (or zeroed out)
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The reason for these constraints are many, and I didn't spend enough time to deeply understand all of them, but
let's see the most important:
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* **TypeCompress**: If the CAB is compressed, the trick to open it within an object file to trigger the INF write will fail
* **CoffCabStart**: CoffCabStart gives the absolute position of the first CFDATA structure, as we added a '../',
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we would need to increase this by 3 to point to the file (this is more like a guess)
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* **cCfData**: As there is only 1 file, we should have just 1 CFDATA, I'm not too sure why this has to be set to 2
* **cbFile**: Interestingly, if the CAB extraction concludes without any error, the INF file will be marked for deletion
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by WORD, ruining the exploit. The only way to prevent this is to make WORD believe the extraction failed. If the
cbFile value is defined as greater than the cabinet file itself, the extractor will reach an EOF before reading
all the bytes defined in cbFile, raising an extraction error.
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* Last but not least, the **csum** value should be recalculated. Luckily, as noted by [j00sean](
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and according to [MS documentation]([ms-cab].pdf),
this value can be 0
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**NOTE1**: Defender now detects if the CAB file contains a PE by using the `_IMAGE_DOS_HEADER.e_magic` value as a
signature, potentially avoiding PE files to be embedded in the CAB. Can this signature be bypassed?
I'm not sure but, as observed before, this is a patched vulnerability, so I'm not planning to invest much more time
on this. Up to the curious reader to develop this further.
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**NOTE2**: Microsoft Patch blocks arbitrary URI schemes, apparently using a blacklist approach (this is just a supposition)
# DLL Attack
The main attack chain associated with CVE-2021-40444 is the DLL attack loaded via the `.cpl` URI scheme. In order to
exploit that, an attacker needs to generate a specially crafted DLL. If you want to test it out, try my [evildll-gen](
# JScript, VBScript, Javaw, MSIexec, ...
As noted by [Max Maluin](, it is possible to interact with several filetypes abusing IE and
the associated file extension based URI. While this is might be a good way to exploit IE, it has limitations.
Indeed, irtshould be noted that the method used in the exploit to download files is based on ActiveX control updates,
and cannot be used to download arbitrary files.
As per Microsoft [documentation](, the `codebase` tag
can point just to a few filetypes: OCX, INF and CAB.
Even if we can directly download an OCX or INF file, we still can't be sure to download the file in the right location
within the system. With the cab exploit, it is possible to move the `.inf` file in a well-known path using the path traversal,
but in any other case the file will be stored in a random directory, making it virtually impossible to reference it.
As of today, I didn't find a way to chain download and execution WITHOUT a CAB file.
Note: Talking about IE alone, HTML smuggling could be a possible scenario to exploit the vulnerability.
# Cab-less file attack using hybrid RAR file
This technique was firstly disclosed by [Eduardo Braun]( on Twitter and further explained in [this]('Ext2Prot'%20Vulnerability%20'CABless'%20version.pdf) paper.
Please note that using this technique, the attack chain is a bit different. This attack requires the user to download
a specially crafted RAR file, obtained by chaining a valid WSF script and a valid RAR file. Once opened, the RAR will contain
a DOCX with a reference to an HTML, which in turn will try to load the RAR file as a WSF script.
To summarise:
1. Specially crafted RAR file is downloaded (likely Download folder)
2. DOCX extracted and opened
3. Relationship stored in document.xml.rels points to malicious html
4. IE preview is launched to open the HTML link
5. JScript within the HTML contains a script/iframe pointing to the RAR file, prefixed with the ".wsf:" URI scheme
6. As the RAR was designed to be contemporaneously a valid RAR and a valid WSF script, the script is executed
# What are the exploits PoC implemented by the tool
The generator utility can currently reproduce the following attacks:
| Attack | HTML Templates | Target | Delivery Method | Execution Method | Working |
| Original version of the attack | cab-orig-* | WORD | DOCX | CAB + DLL | YES |
| j00sean IE-only attack | cab-orig-j00san | IE | HTML | CAB + DLL | YES |
| My version without DLL | cab-uri-* | WORD | DOCX | CAB + JS/VBS | NO<sup>1</sup> |
| Eduardo B. "CABless" attack using RAR | cabless-rar-* | WORD | RAR | WSF | YES |
| Modified j00sean attack + HTML smuggling | cabless-smuggling-* | IE | HTML | JS/VBS | YES<sup>2</sup> |
_<sup>1</sup>The CAB is not downloaded properly in some environments_
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_<sup>2</sup>The user needs to click on "Save" to download the file on IE_
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# CAB file parser
The utility `` can be used to see the headers of the exploit file, but don't consider this a full
parser. It's a very quick and dirty CAB header viewer I developed to understand what was going on.
# Install
The generator is designed to work on Windows, as it uses the `makecab` utility. Before usage, be sure to install required dependencies:
* With Virtualenv
git clone
cd CVE-2021-40444
pip install virtualenv
python -m virtualenv venv
pip install -r requirements
* Without Virtualenv
git clone
cd CVE-2021-40444
pip install -r requirements
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# Usage
The generator is trivial to use, and even if it has been tested with a number of different payloads and Windows
versions, it is not fail-proof. I'm encountering different behaviours across different Windows builds. As soon as
I have more details to share, I'll post them here.
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usage: [-h] -P PAYLOAD -u URL [-o OUTPUT] [--host] [-c COPY_TO] [-nc] [-t]
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[%] CVE-2021-40444 - MS Office Word RCE Exploit [%]
optional arguments:
-h, --help show this help message and exit
DLL payload to use for the exploit
-u URL, --url URL Server URL for malicious references (CAB->INF)
-o OUTPUT, --output OUTPUT
Output files basename (no extension)
--host If set, will host the payload after creation
-c COPY_TO, --copy-to COPY_TO
Copy payload to an alternate path
-nc, --no-cab Use the CAB-less version of the exploit
-t, --test Open IExplorer to test the final HTML file
# Example
* Generate the original exploit and test it locally
python -u -P test\calc.dll --host
_Note: the port is selected by the URL, and the exploit is generated basing on the payload file extension_
* Generate the CABless exploit with RAR and test it locally via IE
python -u -P test\job-jscript.wsf --no-cab --host -t
* Generate the CABless exploit (IE-only) with HTML smuggling and test it locally via IE
python -u -P test\calc.js --no-cab --host -t
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# Credits
* [RET2_pwn]( for the amazing blog
* [j00sean]( for the good hints
* [lockedbyte]( for the first decent poc
* [Max_Mal]( for the hint on the alternate URI schemes
* [wdormann]( for the hint on the HTML file size restrictions
* [Edu_Braun_0day]( for the cool CAB-less version of the exploit