Amsi.DLL Load By Uncommon Process

Status: test
Severity: low
Log source: product windows, category image_load
Author: frack113
Source: github.com/SigmaHQ/sigma

Detects loading of Amsi.dll by uncommon processes

MITRE ATT&CK coverage

Tactic	Techniques
Impact	`T1490` Inhibit System Recovery

Event coverage

Provider	Event	Title
Sysmon	Event ID 7	Image loaded

Rule body yaml

title: Amsi.DLL Load By Uncommon Process
id: facd1549-e416-48e0-b8c4-41d7215eedc8
status: test
description: Detects loading of Amsi.dll by uncommon processes
references:
    - https://infosecwriteups.com/amsi-bypass-new-way-2023-d506345944e9
    - https://github.com/TheD1rkMtr/AMSI_patch
    - https://github.com/surya-dev-singh/AmsiBypass-OpenSession
author: frack113
date: 2023-03-12
modified: 2025-02-24
tags:
    - attack.impact
    - attack.t1490
    - detection.threat-hunting
logsource:
    category: image_load
    product: windows
detection:
    selection:
        ImageLoaded|endswith: '\amsi.dll'
    filter_main_exact:
        Image|endswith:
            - ':\Windows\explorer.exe'
            - ':\Windows\Sysmon64.exe'
    filter_main_generic:
        Image|contains:
            - ':\Program Files (x86)\'
            - ':\Program Files\'
            - ':\Windows\System32\'
            - ':\Windows\SysWOW64\'
            - ':\Windows\WinSxS\'
    filter_optional_defender:
        Image|contains: ':\ProgramData\Microsoft\Windows Defender\Platform\'
        Image|endswith: '\MsMpEng.exe'
    filter_main_dotnet:
        Image|contains:
            - ':\Windows\Microsoft.NET\Framework\'
            - ':\Windows\Microsoft.NET\Framework64\'
            - ':\Windows\Microsoft.NET\FrameworkArm\'
            - ':\Windows\Microsoft.NET\FrameworkArm64\'
        Image|endswith: '\ngentask.exe'
    filter_main_null:
        Image: null
    filter_main_empty:
        Image: ''
    condition: selection and not 1 of filter_main_* and not 1 of filter_optional_*
falsepositives:
    - Legitimate third party apps installed in "ProgramData" and "AppData" might generate some false positives. Apply additional filters accordingly
level: low

Stages and Predicates

Stage 0: `condition`

selection and not 1 of filter_main_* and not 1 of filter_optional_*

Stage 1: `selection`

selection:
    ImageLoaded|endswith: '\amsi.dll'

Stage 2: `not filter_main_*`

filter_main_exact:
    Image|endswith:
        - ':\Windows\explorer.exe'
        - ':\Windows\Sysmon64.exe'
filter_main_generic:
    Image|contains:
        - ':\Program Files (x86)\'
        - ':\Program Files\'
        - ':\Windows\System32\'
        - ':\Windows\SysWOW64\'
        - ':\Windows\WinSxS\'
filter_main_dotnet:
    Image|contains:
        - ':\Windows\Microsoft.NET\Framework\'
        - ':\Windows\Microsoft.NET\Framework64\'
        - ':\Windows\Microsoft.NET\FrameworkArm\'
        - ':\Windows\Microsoft.NET\FrameworkArm64\'
    Image|endswith: '\ngentask.exe'
filter_main_null:
    Image: null
filter_main_empty:
    Image: ''

Stage 3: `not filter_optional_defender`

filter_optional_defender:
    Image|contains: ':\ProgramData\Microsoft\Windows Defender\Platform\'
    Image|endswith: '\MsMpEng.exe'

Exclusions

Top-level NOT(...) conjuncts: predicates this rule actively suppresses.

Field	Kind	Excluded values
`Image`	match	`:\Windows\Microsoft.NET\Framework64\`
`Image`	match	`:\Windows\Microsoft.NET\FrameworkArm64\`
`Image`	match	`:\Windows\Microsoft.NET\FrameworkArm\`
`Image`	match	`:\Windows\Microsoft.NET\Framework\`
`Image`	ends_with	`\ngentask.exe`
`Image`	ends_with	`:\Windows\Sysmon64.exe`
`Image`	ends_with	`:\Windows\explorer.exe`
`Image`	is_null	(no value, null check)
`Image`	match	`:\Program Files (x86)\`
`Image`	match	`:\Program Files\`
`Image`	match	`:\Windows\SysWOW64\`
`Image`	match	`:\Windows\System32\`
`Image`	match	`:\Windows\WinSxS\`
`Image`	ends_with	`\MsMpEng.exe`
`Image`	match	`:\ProgramData\Microsoft\Windows Defender\Platform\`

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
`ImageLoaded`	ends_with	`\amsi.dll` corpus 3 (sigma 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)

Amsi.DLL Load By Uncommon Process

MITRE ATT&CK coverage

Event coverage

Rule body yaml

Stages and Predicates

Stage 0: `condition`

Stage 1: `selection`

Stage 2: `not filter_main_*`

Stage 3: `not filter_optional_defender`

Exclusions

Indicators

Keyboard shortcuts

Search operators

Amsi.DLL Load By Uncommon Process

MITRE ATT&CK coverage

Event coverage

Rule body yaml

Stages and Predicates

Stage 0: condition

Stage 1: selection

Stage 2: not filter_main_*

Stage 3: not filter_optional_defender

Exclusions

Indicators

Stage 0: `condition`

Stage 1: `selection`

Stage 2: `not filter_main_*`

Stage 3: `not filter_optional_defender`