Certutil Artifacts Analysis

certutil is a build-in tool on windows systems that is used to manage certificates. However certutil could be used to base64 encode/decode, calculate file hashes and download files from the internet. In this blog we will take a look at the artifacts generated by certutil when downloading a file, specifically the metadata file, analyze it's structure and write a parser for it.

Analysis

In this example we will download a file using certutil and observe the files written to the system. The following command will download mimkatz from github and save it to the current directory (C:\windows\temp):

certutil.exe -urlcache -split -f "https://github.com/gentilkiwi/mimikatz/releases/download/2.2.0-20200816/mimikatz_trunk.7z" m.7z

After executing the above command we can see in procmon that is writing to two files:

Both files have the same name but on different directories. The file name BC456035A9E2885290EDC953764CC761  is the MD5 hash for the URL UTF-16LE encode:

The file located on C:\Users\u0041\AppData\LocalLow\Microsoft\CryptnetUrlCache\Content\BC456035A9E2885290EDC953764CC761 contains the actual file:

However C:\Users\u0041\AppData\LocalLow\Microsoft\CryptnetUrlCache\MetaData\BC456035A9E2885290EDC953764CC761 contains UTF-16LE encode URL and MD5 hash for the downloaded file in addition to some binary data:

The URL and the MD5 hash can be extracted easily, But what about the header (116 bytes) ? the following summarize my findings:

As you can see the metadata file contains a lot of useful information, However there is a lot more data that is unknown (highlighted with red color). I am still researching what that data might be, If you want to help please leave a comment on this blog or send me a DM on twitter @A__ALFAIFI.

CryptnetURLCacheParser

I wrote a parser that output the above data in readable format (CSV, JSON or JSONL). You can find it here for the Python version CryptnetURLCacheParser and here CryptnetURLCacheParser-rs for the Rust version. The following is the parser output in JSON format:

{
    "Timestamp": "2020-09-11T16:53:07.297208",
    "URL": "https://github.com/gentilkiwi/mimikatz/releases/download/2.2.0-20200816/mimikatz_trunk.7z",
    "FileSize": 875252,
    "MetadataHash": "5abdd836d4458370a804f2974431c993",
    "FullPath": "C:\\Users\\u0041\\AppData\\LocalLow\\Microsoft\\CryptnetUrlCache\\MetaData\\BC456035A9E2885290EDC953764CC761",
    "MD5": "5abdd836d4458370a804f2974431c993"
}

Edit 1

I have identified a couple of stuff in the metadata structure, the table above is updated to the latest findings. The following is the summery:

• The MetadataHash field is actually the E-Tag header from the HTTP response.
• Another timestamp is recorded which is the LastModificationTime header from the HTTP response (Thanks to @mrAn61 on Twitter for pointing that out).
• Created another parser for this artifact bu in Rust! you can find it here CryptnetURLCacheParser-rs.

References 

• https://thinkdfir.com/2020/07/30/certutil-download-artefacts/amp/
• https://docs.microsoft.com/en-us/windows-server/administration/windows-commands/certutil