Invocation Of Crypto-Classes From The "Cryptography" PowerShell Namespace

Status: test
Severity: medium
Log source: product windows, category process_creation
Author: Andreas Braathen (mnemonic.io)
Source: github.com/SigmaHQ/sigma

Detects the invocation of PowerShell commands with references to classes from the "System.Security.Cryptography" namespace. The PowerShell namespace "System.Security.Cryptography" provides classes for on-the-fly encryption and decryption. These can be used for example in decrypting malicious payload for defense evasion.

MITRE ATT&CK coverage

Tactic	Techniques
Execution	`T1059.001` Command and Scripting Interpreter: PowerShell
Stealth	`T1027.010` Obfuscated Files or Information: Command Obfuscation

Event coverage

Provider	Event	Title
Sysmon	Event ID 1	Process creation

Rule body yaml

title: Invocation Of Crypto-Classes From The "Cryptography" PowerShell Namespace
id: ad856965-f44d-42a8-945e-bbf7bd03d05a
status: test
description: |
    Detects the invocation of PowerShell commands with references to classes from the "System.Security.Cryptography" namespace.
    The PowerShell namespace "System.Security.Cryptography" provides classes for on-the-fly encryption and decryption.
    These can be used for example in decrypting malicious payload for defense evasion.
references:
    - https://learn.microsoft.com/en-us/dotnet/api/system.security.cryptography?view=net-8.0
    - https://blogs.vmware.com/security/2023/11/jupyter-rising-an-update-on-jupyter-infostealer.html
    - https://www.virustotal.com/gui/file/39102fb7bb6a74a9c8cb6d46419f9015b381199ea8524c1376672b30fffd69d2
author: Andreas Braathen (mnemonic.io)
date: 2023-12-01
tags:
    - attack.execution
    - attack.stealth
    - attack.t1059.001
    - attack.t1027.010
    - detection.threat-hunting
logsource:
    product: windows
    category: process_creation
detection:
    selection_img:
        - Image|endswith:
              - '\powershell.exe'
              - '\pwsh.exe'
        - OriginalFileName:
              - 'PowerShell.EXE'
              - 'pwsh.dll'
    selection_cmdlet_namespace:
        CommandLine|contains: 'System.Security.Cryptography.'
    selection_cmdlet_classes:
        CommandLine|contains:
            - '.AesCryptoServiceProvider'
            - '.DESCryptoServiceProvider'
            - '.DSACryptoServiceProvider'
            - '.RC2CryptoServiceProvider'
            - '.Rijndael'
            - '.RSACryptoServiceProvider'
            - '.TripleDESCryptoServiceProvider'
    condition: all of selection_*
falsepositives:
    - Classes are legitimately used, but less so when e.g. parents with low prevalence or decryption of content in temporary folders.
level: medium

Stages and Predicates

Stage 0: `condition`

all of selection_*

Stage 1: `selection_img`

selection_img:
    - Image|endswith:
          - '\powershell.exe'
          - '\pwsh.exe'
    - OriginalFileName:
          - 'PowerShell.EXE'
          - 'pwsh.dll'

Stage 2: `selection_cmdlet_namespace`

selection_cmdlet_namespace:
    CommandLine|contains: 'System.Security.Cryptography.'

Stage 3: `selection_cmdlet_classes`

selection_cmdlet_classes:
    CommandLine|contains:
        - '.AesCryptoServiceProvider'
        - '.DESCryptoServiceProvider'
        - '.DSACryptoServiceProvider'
        - '.RC2CryptoServiceProvider'
        - '.Rijndael'
        - '.RSACryptoServiceProvider'
        - '.TripleDESCryptoServiceProvider'

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	`.AesCryptoServiceProvider` `.DESCryptoServiceProvider` `.DSACryptoServiceProvider` `.RC2CryptoServiceProvider` `.RSACryptoServiceProvider` `.Rijndael` `.TripleDESCryptoServiceProvider` `System.Security.Cryptography.`
`Image`	ends_with	`\powershell.exe` corpus 182 (sigma 182) `\pwsh.exe` corpus 168 (sigma 168)
`OriginalFileName`	eq	`PowerShell.EXE` corpus 120 (sigma 84, splunk 30, elastic 6) `pwsh.dll` corpus 112 (sigma 79, splunk 30, elastic 3)

`j` / `k`	Scroll down / up
`d` / `u`	Half-page down / up
`gg` / `G`	Top / bottom
`h` / `l`	History back / forward
`f`	Follow link (`Shift` = new tab)
`/`	Focus search
`?`	Toggle this help
`↑` / `↓`	Navigate search results
`Enter`	Open highlighted result
`Esc`	Close results / dialog

`type:`	`events` / `rules` / `providers`
`vendor:`	`sigma` / `elastic` / `splunk` / `kusto` / `chronicle` (vendor name alone also works: `sigma:`, `kql:`, `secops:`…)
`tactic:`	TA-id, slug, or name: `credential_access`, `TA0006`
`technique:`	technique or sub-technique ID: `T1003`, `T1003.001` (alias `tech:`)
`severity:`	`critical` / `high` / `medium` / `low` / `informational` (alias `sev:`)
`risk_score`	Numeric comparison on the Elastic risk score (0 to 100): `risk_score>50`, `risk_score<=20`, `risk_score=99` (alias `risk`; Elastic rules only)
`stages:`	Rules with exactly N pipeline stages
`correlation:`	`single_event` / `sequence` / `alternatives` / `alternatives_cross_log` / `all_required` / `correlated`
`with:`	Co-occurrence event-id; stacks (`with:4624 with:4769`) to require all, while a comma list in one occurrence (`with:4624,4769`) is an either-or group. Implies multi-event
`like:`	Structural neighbors of a rule slug (equivalents + subsumption stricter / broader): `like:comsvcs_lsass_memory_dump-splunk-sysmon`
`groupby:`	Entity-grouping substring match against `group_by_keys`: `groupby:user`, `groupby:host`
`uses:`	Rules whose predicate tree touches the field (any kind, any value): `uses:CommandLine`
`excludes:`	Rules with top-level `not()` clauses on the field (FP whitelists): `excludes:ParentImage`
`field:` / `value:`	Predicate search; narrows rule cards to those with a matching leaf and drives the indicator tier. Unquoted = substring, wildcards allowed (`value:mimikatz`)
`indicator:`	Shorthand for `field:F value:V`: `indicator:Image=*\powershell.exe`
`kind:`	Filter by predicate kind. Narrows rule cards to those carrying a matching predicate leaf (`vendor:elastic kind:cidr_match`) and drives the indicator tier: `contains` / `starts_with` / `ends_with` / `regex` / `cidr` / `eq` / `in` … (operator aliases `op:`/`match:`)
`has:` / `no:`	`sample`, `field`, `notes`, `refs`, `trace`, `thirdparty`, `rule`, `pattern`, `timewindow`, `threshold`, `newterms`, `sigma`/`elastic`/`splunk`/`kusto`/`chronicle`
`-op:val`	Exclude matches; works on most operators but not `type:`/`like:`/`has:`/`no:` (use `no:<flag>` to exclude a rule flag): `tactic:execution -vendor:splunk`. Standalone `-kind:`/`-field:`/`-value:` drop every rule carrying a matching predicate leaf (`type:rules -kind:is_null`)
`field:"…"` / `value:"…"`	Quoted value = anchored exact match (also allows spaces): `value:"net user"`
`a,b`	Comma = OR inside one operator (`vendor:sigma,elastic`, `severity:high,critical`); repeating a facet merges the same way. `field:`/`value:` never split (literal commas)
`vendors:` / `stage:`	Singular and plural spellings fold to the canonical operator and value: `tactics:` = `tactic:`, `type:event` = `type:events`, `correlation:sequences` = `correlation:sequence`, `has:thresholds` = `has:threshold`
`"quoted phrase"`	Exact-match a multi-word phrase (free text)

Invocation Of Crypto-Classes From The "Cryptography" PowerShell Namespace

MITRE ATT&CK coverage

Event coverage

Rule body yaml

Stages and Predicates

Stage 0: `condition`

Stage 1: `selection_img`

Stage 2: `selection_cmdlet_namespace`

Stage 3: `selection_cmdlet_classes`

Indicators

Keyboard shortcuts

Search operators

Invocation Of Crypto-Classes From The "Cryptography" PowerShell Namespace

MITRE ATT&CK coverage

Event coverage

Rule body yaml

Stages and Predicates

Stage 0: condition

Stage 1: selection_img

Stage 2: selection_cmdlet_namespace

Stage 3: selection_cmdlet_classes

Indicators

Stage 0: `condition`

Stage 1: `selection_img`

Stage 2: `selection_cmdlet_namespace`

Stage 3: `selection_cmdlet_classes`