Dynamic .NET Compilation Via Csc.EXE

Severity: medium
Author: Florian Roth (Nextron Systems), X__Junior (Nextron Systems)
Source: upstream

Detects execution of "csc.exe" to compile .NET code. Attackers often leverage this to compile code on the fly and use it in other stages.

MITRE ATT&CK coverage

Tactic	Techniques
Defense Evasion	`T1027.004` Obfuscated Files or Information: Compile After Delivery

Event coverage

Provider	Event ID	Title
Sysmon	1	Process creation

Stages and Predicates

Stage 1: `selection_img`

Image|endswith: '\csc.exe'

Stage 2: `1 of selection_susp_location_1`

or:
CommandLine|contains: ':\Perflogs\'
CommandLine|contains: ':\Users\Public\'
CommandLine|contains: '\AppData\Local\Temp\'
CommandLine|contains: '\Temporary Internet'
CommandLine|contains: '\Windows\Temp\'

Stage 3: `1 of selection_susp_location_2`

or:
CommandLine|contains: ':\Users\'
CommandLine|contains: '\Contacts\'
CommandLine|contains: ':\Users\'
CommandLine|contains: '\Favorites\'
CommandLine|contains: ':\Users\'
CommandLine|contains: '\Favourites\'
CommandLine|contains: ':\Users\'
CommandLine|contains: '\Pictures\'

Stage 4: `1 of selection_susp_location_3`

CommandLine|re: '([Pp]rogram[Dd]ata|%([Ll]ocal)?[Aa]pp[Dd]ata%|\\[Aa]pp[Dd]ata\\([Ll]ocal([Ll]ow)?|[Rr]oaming))\\[^\\]{1,256}$'

Stage 5: `not 1 of filter_main_*`

or:
ParentImage: 'C:\Windows\System32\inetsrv\w3wp.exe'
ParentImage: 'C:\Windows\System32\sdiagnhost.exe'
ParentImage|startswith: 'C:\Program Files (x86)\'
ParentImage|startswith: 'C:\Program Files\'

Stage 6: `not 1 of filter_optional_*`

or:
ParentCommandLine|contains: 'JwB7ACIAZgBhAGkAbABlAGQAIgA6AHQAcgB1AGUALAAiAG0AcwBnACIAOgAiAEEAbgBzAGkAYgBsAGUAIAByAGUAcQB1AGkAcgBlAHMAIABQAG8AdwBlAHIAUwBoAGUAbABsACAAdgAzAC4AMAAgAG8AcgAgAG4AZQB3AGUAcgAiAH0AJw'
ParentCommandLine|contains: '\ProgramData\Microsoft\Windows Defender Advanced Threat Protection'
ParentCommandLine|contains: 'cAewAiAGYAYQBpAGwAZQBkACIAOgB0AHIAdQBlACwAIgBtAHMAZwAiADoAIgBBAG4AcwBpAGIAbABlACAAcgBlAHEAdQBpAHIAZQBzACAAUABvAHcAZQByAFMAaABlAGwAbAAgAHYAMwAuADAAIABvAHIAIABuAGUAdwBlAHIAIgB9ACcA'
ParentCommandLine|contains: 'nAHsAIgBmAGEAaQBsAGUAZAAiADoAdAByAHUAZQAsACIAbQBzAGcAIgA6ACIAQQBuAHMAaQBiAGwAZQAgAHIAZQBxAHUAaQByAGUAcwAgAFAAbwB3AGUAcgBTAGgAZQBsAGwAIAB2ADMALgAwACAAbwByACAAbgBlAHcAZQByACIAfQAnA'
ParentImage: 'C:\ProgramData\chocolatey\choco.exe'
ParentImage: 'C:\ProgramData\chocolatey\tools\shimgen.exe'

Indicators

Each row is a field, operator, and value that the rule matches. The corpus column counts how many other rules in the catalog look for the same combination: high numbers point to widely-used, community-vetted indicators. Blank or 1 shows that the indicator is specific to this rule.

Field	Kind	Values
`CommandLine`	match	`:\Perflogs\` corpus 4 (sigma 4) `:\Users\` corpus 6 (sigma 6) `:\Users\Public\` corpus 14 (sigma 14) `\AppData\Local\Temp\` corpus 16 (sigma 16) `\Contacts\` corpus 6 (sigma 6) `\Favorites\` corpus 6 (sigma 6) `\Favourites\` corpus 6 (sigma 6) `\Pictures\` corpus 3 (sigma 3) `\Temporary Internet` corpus 6 (sigma 6) `\Windows\Temp\` corpus 10 (sigma 10)
`CommandLine`	regex_match	`([Pp]rogram[Dd]ata\|%([Ll]ocal)?[Aa]pp[Dd]ata%\|\\[Aa]pp[Dd]ata\\([Ll]ocal([Ll]ow)?\|[Rr]oaming))\\[^\\]{1,256}$`
`Image`	ends_with	`\csc.exe` corpus 5 (sigma 5)
`ParentCommandLine`	match	`JwB7ACIAZgBhAGkAbABlAGQAIgA6AHQAcgB1AGUALAAiAG0AcwBnACIAOgAiAEEAbgBzAGkAYgBsAGUAIAByAGUAcQB1AGkAcgBlAHMAIABQAG8AdwBlAHIAUwBoAGUAbABsACAAdgAzAC4AMAAgAG8AcgAgAG4AZQB3AGUAcgAiAH0AJw` corpus 2 (sigma 2) `\ProgramData\Microsoft\Windows Defender Advanced Threat Protection` corpus 2 (sigma 2) `cAewAiAGYAYQBpAGwAZQBkACIAOgB0AHIAdQBlACwAIgBtAHMAZwAiADoAIgBBAG4AcwBpAGIAbABlACAAcgBlAHEAdQBpAHIAZQBzACAAUABvAHcAZQByAFMAaABlAGwAbAAgAHYAMwAuADAAIABvAHIAIABuAGUAdwBlAHIAIgB9ACcA` corpus 2 (sigma 2) `nAHsAIgBmAGEAaQBsAGUAZAAiADoAdAByAHUAZQAsACIAbQBzAGcAIgA6ACIAQQBuAHMAaQBiAGwAZQAgAHIAZQBxAHUAaQByAGUAcwAgAFAAbwB3AGUAcgBTAGgAZQBsAGwAIAB2ADMALgAwACAAbwByACAAbgBlAHcAZQByACIAfQAnA` corpus 2 (sigma 2)
`ParentImage`	eq	`C:\ProgramData\chocolatey\choco.exe` corpus 2 (sigma 2) `C:\ProgramData\chocolatey\tools\shimgen.exe` `C:\Windows\System32\inetsrv\w3wp.exe` corpus 2 (sigma 2) `C:\Windows\System32\sdiagnhost.exe` corpus 2 (sigma 2)
`ParentImage`	starts_with	`C:\Program Files (x86)\` corpus 2 (sigma 2) `C:\Program Files\` corpus 2 (sigma 2)

Dynamic .NET Compilation Via Csc.EXE

MITRE ATT&CK coverage

Event coverage

Stages and Predicates

Stage 1: selection_img

Stage 2: 1 of selection_susp_location_1

Stage 3: 1 of selection_susp_location_2

Stage 4: 1 of selection_susp_location_3

Stage 5: not 1 of filter_main_*