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

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.

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 (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.

CAB File

The CAB file needs to be byte-patched to avoid extraction errors and to achieve the ZipSlip:

• 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)

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:

• 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 '../', we would need to increase this by 3 to point to the file (this is more like a guess)
• 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 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.
• Last but not least, the csum value should be recalculated. Luckily, as noted by j00sean and according to MS documentation, this value can be 0

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.

NOTE2: Microsoft Patch blocks arbitrary URI schemes, apparently using a blacklist approach (this is just a supposition)

CAB file parser

The utility cab_parser.py 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.

Usage

The generator is trivial to use, and has been tested with a number of different DLL payloads.

usage: generator.py [-h] -P PAYLOAD -u URL [-o OUTPUT] [--host] [-p LPORT] [-c COPY_TO]

[%] CVE-2021-40444 - MS Office Word RCE Exploit [%]

optional arguments:
  -h, --help            show this help message and exit
  -P PAYLOAD, --payload PAYLOAD
                        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
  -p LPORT, --lport LPORT
                        Port to use when hosting malicious payload
  -c COPY_TO, --copy-to COPY_TO
                        Copy payload to an alternate path

Credits

• RET2_pwn for the amazing blog
• j00sean for the good hints
• lockedbyte for the first decent poc