Dynamic .NET Compilation Via Csc.EXE

 1title: Dynamic .NET Compilation Via Csc.EXE
 2id: dcaa3f04-70c3-427a-80b4-b870d73c94c4
 3status: test
 4description: 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.
 5references:
 6    - https://securityboulevard.com/2019/08/agent-tesla-evading-edr-by-removing-api-hooks/
 7    - https://www.clearskysec.com/wp-content/uploads/2018/11/MuddyWater-Operations-in-Lebanon-and-Oman.pdf
 8    - https://app.any.run/tasks/c6993447-d1d8-414e-b856-675325e5aa09/
 9    - https://twitter.com/gN3mes1s/status/1206874118282448897
10    - https://github.com/redcanaryco/atomic-red-team/blob/b27a3cb25025161d49ac861cb216db68c46a3537/atomics/T1027.004/T1027.004.md#atomic-test-1---compile-after-delivery-using-cscexe
11author: Florian Roth (Nextron Systems), X__Junior (Nextron Systems)
12date: 2019-08-24
13modified: 2024-05-27
14tags:
15    - attack.defense-evasion
16    - attack.t1027.004
17logsource:
18    category: process_creation
19    product: windows
20detection:
21    selection_img:
22        Image|endswith: '\csc.exe'
23    selection_susp_location_1:
24        CommandLine|contains:
25            - ':\Perflogs\'
26            - ':\Users\Public\'
27            - '\AppData\Local\Temp\' # User execution
28            - '\Temporary Internet'
29            - '\Windows\Temp\' # Admin execution
30    selection_susp_location_2:
31        - CommandLine|contains|all:
32              - ':\Users\'
33              - '\Favorites\'
34        - CommandLine|contains|all:
35              - ':\Users\'
36              - '\Favourites\'
37        - CommandLine|contains|all:
38              - ':\Users\'
39              - '\Contacts\'
40        - CommandLine|contains|all:
41              - ':\Users\'
42              - '\Pictures\'
43    selection_susp_location_3:
44        CommandLine|re: '([Pp]rogram[Dd]ata|%([Ll]ocal)?[Aa]pp[Dd]ata%|\\[Aa]pp[Dd]ata\\([Ll]ocal([Ll]ow)?|[Rr]oaming))\\[^\\]{1,256}$'
45    filter_main_programfiles:
46        # Note: this is a generic filter. You could baseline execution in your env for a more robust rule
47        ParentImage|startswith:
48            - 'C:\Program Files (x86)\' # https://twitter.com/gN3mes1s/status/1206874118282448897
49            - 'C:\Program Files\' # https://twitter.com/gN3mes1s/status/1206874118282448897
50    filter_main_sdiagnhost:
51        ParentImage: 'C:\Windows\System32\sdiagnhost.exe' # https://twitter.com/gN3mes1s/status/1206874118282448897
52    filter_main_w3p:
53        ParentImage: 'C:\Windows\System32\inetsrv\w3wp.exe' # https://twitter.com/gabriele_pippi/status/1206907900268072962
54    filter_optional_chocolatey:
55        ParentImage: # Chocolatey https://chocolatey.org/
56            - 'C:\ProgramData\chocolatey\choco.exe'
57            - 'C:\ProgramData\chocolatey\tools\shimgen.exe'
58    filter_optional_defender:
59        ParentCommandLine|contains: '\ProgramData\Microsoft\Windows Defender Advanced Threat Protection'
60    filter_optional_ansible:
61        # Note: As ansible is widely used we exclude it with this generic filter.
62        # A better option would be to filter based on script content basis or other marker while hunting
63        ParentCommandLine|contains:
64            # '{"failed":true,"msg":"Ansible requires PowerShell v3.0 or newer"}'
65            - 'JwB7ACIAZgBhAGkAbABlAGQAIgA6AHQAcgB1AGUALAAiAG0AcwBnACIAOgAiAEEAbgBzAGkAYgBsAGUAIAByAGUAcQB1AGkAcgBlAHMAIABQAG8AdwBlAHIAUwBoAGUAbABsACAAdgAzAC4AMAAgAG8AcgAgAG4AZQB3AGUAcgAiAH0AJw'
66            - 'cAewAiAGYAYQBpAGwAZQBkACIAOgB0AHIAdQBlACwAIgBtAHMAZwAiADoAIgBBAG4AcwBpAGIAbABlACAAcgBlAHEAdQBpAHIAZQBzACAAUABvAHcAZQByAFMAaABlAGwAbAAgAHYAMwAuADAAIABvAHIAIABuAGUAdwBlAHIAIgB9ACcA'
67            - 'nAHsAIgBmAGEAaQBsAGUAZAAiADoAdAByAHUAZQAsACIAbQBzAGcAIgA6ACIAQQBuAHMAaQBiAGwAZQAgAHIAZQBxAHUAaQByAGUAcwAgAFAAbwB3AGUAcgBTAGgAZQBsAGwAIAB2ADMALgAwACAAbwByACAAbgBlAHcAZQByACIAfQAnA'
68    condition: selection_img and 1 of selection_susp_location_* and not 1 of filter_main_* and not 1 of filter_optional_*
69falsepositives:
70    - Legitimate software from program files - https://twitter.com/gN3mes1s/status/1206874118282448897
71    - Legitimate Microsoft software - https://twitter.com/gabriele_pippi/status/1206907900268072962
72    - Ansible
73level: medium