Author: benmeadowcroft

Today I’ll share just a short snippet that I used to look for some specific scheduled tasks on a Windows system. Luckily windows creates XML files that are located somewhere like the C:\Windows\System32\Tasks folder. These files contain an XML representation of the scheduled tasks, and it is this that I am scanning with YARA.

Here’s a quick example of the rule:

// Detects the scheduled task on a Windows machine that runs Ransim
rule RansimTaskDetect : RansomwareScheduledTask {
    meta:
        author = "Ben Meadowcroft (@BenMeadowcroft)"
    strings:
        // Microsoft XML Task files are UTF-16 encoded so using wide strings
        $ransim = "ransim.ps1 -mode encrypt</Arguments>" ascii wide nocase
        $task1  = "<Task" ascii wide
        $task2  = "xmlns=\"http://schemas.microsoft.com/windows/2004/02/mit/task\">" ascii wide

    condition:
        all of them
}

The scheduled tasks runs a short PowerShell script that simulates some basic ransomware behavior, and this rule just matches the XML file for that task. This file is encoded in UTF-16, so the $task1 and $task2 strings simply reference some strings with the wide that are a part of the common strings found within the XML file (the start of the <Task element, and the XML namespace used to define the schema), the ascii wide modifiers searches for the string in both ascii and wide (double byte) form. The remaining string just looks for the invocation of the script as an argument to the task, and ignores the case used.

If I was looking for the presence of a task on live systems then I of course have other tools I could use, such as schtasks query. However, as I am often operating on the backups of a system being able to use this file based approach can be very helpful as it doesn’t rely on the availability of the primary system when I want to identify whether a scheduled task was present at some historical point in time.

My prior posts about examining ZIP archives have covered matching the file names within a ZIP archive, as well as matching the pre-compression CRC values of the files within the archive. In this blog I am going to reference an interesting example of parsing the OOXML format used by modern Microsoft Office products. This office format essentially is a ZIP archive that contains certain files within it (describing the office document).

Aaron Stephens at Mandiant wrote a blog called “Detecting Embedded Content in OOXML Documents“. In that blog Aaron shared a few different techniques used to detect and cluster Microsoft Office documents. One of these examples was detecting a specific PNG file embedded within documents, the image was using to guide the user towards enabling macros. The presence of the image in this phishing doc could be used to indicate a clustering of these attacks.

Given the image files CRC, size, and that it was a png file, the author was able to create a YARA rule that would match if this image file was located within the OOXML document (essentially a ZIP archive). This rule approached the ZIP file a little differently than we have in my prior couple of blogs. The author skips looking for the ZIP file entry and references the CRC ($crc) and uncompressed file size ($ufs) hex strings directly to narrow down the match. They also checked if the file name field entry ended with the ".png" extension.

rule png_397ba1d0601558dfe34cd5aafaedd18e {
    meta:
        author = "Aaron Stephens <[email protected]>"
        description = "PNG in OOXML document."

    strings:
        $crc = {f8158b40}
        $ext = ".png"
        $ufs = {b42c0000}

    condition:
        $ufs at @crc[1] + 8 and $ext at @crc[1] + uint16(@crc[1] + 12) + 16 - 4
}

In this example the condition is using the @crc[1] as the base from which the offsets are calculated, unlike our prior examples where the offsets were based from the start of the local file header. The use of the at operator tests for the presence of the other strings at a specific offset (to the CRC value in this case).

An alternative approach to consider is using the wildcard character ? in the hex string, this allows us to match on the CRC and uncompressed file size fields together while skipping over the 4 bytes used to store the compressed file size field. Then validating that the four letter .png extension is at the end of the file name field.

rule png_alt {
    strings:
        $crc_ufs = {f8158b40 ???????? b42c0000}
        $ext = ".png"

    condition:
        $ext at @crc_ufs[1] + uint16(@crc_ufs[1] + 12) + 16 - 4
}

In my first exploration of analyzing ZIP files with YARA I covered how we could create a YARA rule that matched on specific file names within the ZIP archive, in this post we’ll cover a few other fields that may be of interest.

One interesting example is looking for encrypted ZIP archives, here’s the Twitter post from Tyler McLellan that I read that showed how to do this with YARA:

This snippet is first checks if the file starts with a local file header record, uint16be(0) == 0x504B, it then tests whether the first bit in the “general purpose bit flag” is set by performing a bitwise and against the flag value (to get the value of the first bit) and seeing if that is set uint16(6) & 0x1 == 0x1. This first bit indicates whether the file is encrypted or not:

4.4.4 general purpose bit flag: (2 bytes)

Bit 0: If set, indicates that the file is encrypted.

section 4.4.4 of the .ZIP File Format Specification

Another interesting field in the ZIP Archive’s header is the CRC32 of the file. This is stored at offset 14 and is 4 bytes long. If you are looking for specific files then this can help narrow it down. It should be noted that the CRC is not a cryptographic calculation, but as it is stored in the header it is very simple to check for.

rule matchFileCRC {
   strings:
        $zip_header = {50 4B 03 04}
        $crc1 = {D8 39 6B 82}
        $crc2 = {B1 81 05 76}
        $crc3 = {C2 4A 02 94}
   condition:
        // iterate over local file zip headers
        for any i in (1..#zip_header):
        (
            // match any of the CRC32 values
            for any of ($crc*):
            (
                $ at @zip_header[i]+14
            )
        )
}

The line for any of ($crc*) checks all of the defined CRC values and the line $ at @zip_header[i]+14 causes the rule to be matched if any of the CRCs we are looking for are included in the ZIP header starting at offset 14.