PUA - Nimgrab Execution

Severity: high
Type: Detection
Time window: 5m
Match by: hostname
Source: github.com/chronicle/detection-rules

Detects the usage of nimgrab, a tool bundled with the Nim programming framework and used for downloading files.

MITRE ATT&CK coverage

Tactic	Techniques
Command & Control	`T1105` Ingress Tool Transfer

References

https://github.com/SigmaHQ/sigma/blob/master/rules/windows/process_creation/proc_creation_win_pua_nimgrab.yml

Event coverage

Provider	Event	Title
Sysmon	Event ID 1	Process creation
Security-Auditing	Event ID 4688	A new process has been created.

Rule body yaral

/*
 * Copyright 2025 Google LLC
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

rule pua_nimgrab_execution {

  meta:
    author = "frack113"
    description = "Detects the usage of nimgrab, a tool bundled with the Nim programming framework and used for downloading files."
    reference = "https://github.com/SigmaHQ/sigma/blob/master/rules/windows/process_creation/proc_creation_win_pua_nimgrab.yml"
    license = "https://github.com/SigmaHQ/Detection-Rule-License/blob/main/LICENSE.Detection.Rules.md"
    rule_name = "PUA - Nimgrab Execution"
    sigma_uuid = "74a12f18-505c-4114-8d0b-8448dd5485c6"
    sigma_status = "test"
    rule_id = "mr_70b5f0f8-6bd3-42ab-be2f-928084b09942"
    tactic = "TA0011"
    technique = "T1105"
    type = "Detection"
    data_source = "Sysmon"
    platform = "Windows"
    severity = "High"
    priority = "High"
    false_positives = "Legitimate use of Nim on a developer systems"

  events:
    $process.metadata.event_type = "PROCESS_LAUNCH"
    (
        re.regex($process.target.process.file.full_path, `\\nimgrab\.exe$`) nocase or
        (
            strings.contains(strings.to_upper($process.target.process.file.md5), "2DD44C3C29D667F5C0EF5F9D7C7FFB8B") or
            strings.contains(strings.to_upper($process.target.process.file.sha256), "F266609E91985F0FE3E31C5E8FAEEEC4FFA5E0322D8B6F15FE69F4C5165B9559") or
            strings.contains(strings.to_upper($process.target.process.file.pe_file.imphash), "C07FDDD21D123EA9B3A08EEF44AAAC45")
        )
    )
    $process.principal.hostname = $hostname

  match:
    $hostname over 5m

  outcome:
    //example usage of specifying test user and hostname to adjust risk score
    $risk_score = max(if($process.principal.user.userid = "user" and $process.principal.hostname = "hostname", 0, 15))
    $principal_process_pid = array_distinct($process.principal.process.pid)
    $principal_process_command_line = array_distinct($process.principal.process.command_line)
    $principal_process_file_sha256 = array_distinct($process.principal.process.file.sha256)
    $principal_process_file_full_path = array_distinct($process.principal.process.file.full_path)
    $principal_process_product_specfic_process_id = array_distinct($process.principal.process.product_specific_process_id)
    $principal_process_parent_process_product_specfic_process_id = array_distinct($process.principal.process.parent_process.product_specific_process_id)
    $target_process_pid = array_distinct($process.target.process.pid)
    $target_process_command_line = array_distinct($process.target.process.command_line)
    $target_process_file_sha256 = array_distinct($process.target.process.file.sha256)
    $target_process_file_full_path = array_distinct($process.target.process.file.full_path)
    $target_process_product_specfic_process_id = array_distinct($process.target.process.product_specific_process_id)
    $principal_user_userid = array_distinct($process.principal.user.userid)
    $log_type = array_distinct(strings.concat($process.metadata.log_type,"/",$process.metadata.product_event_type))

  condition:
    $process
}

Stages and Predicates

Stage 0: `match + condition`

match:
    $hostname over 5m
condition:
    $process

Fires when at least one $process event in the 5m window.

Stage 1: `events: $process · PROCESS_LAUNCH`

$process.metadata.event_type = "PROCESS_LAUNCH"
(
    re.regex($process.target.process.file.full_path, `\\nimgrab\.exe$`) nocase or
    (
        strings.contains(strings.to_upper($process.target.process.file.md5), "2DD44C3C29D667F5C0EF5F9D7C7FFB8B") or
        strings.contains(strings.to_upper($process.target.process.file.sha256), "F266609E91985F0FE3E31C5E8FAEEEC4FFA5E0322D8B6F15FE69F4C5165B9559") or
        strings.contains(strings.to_upper($process.target.process.file.pe_file.imphash), "C07FDDD21D123EA9B3A08EEF44AAAC45")
    )
)
$process.principal.hostname = $hostname

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
`target.process.file.full_path`	regex_match	`\nimgrab.exe$` transforms: `nocase`
`target.process.file.md5`	contains	`2DD44C3C29D667F5C0EF5F9D7C7FFB8B` transforms: `toupper`
`target.process.file.pe_file.imphash`	contains	`C07FDDD21D123EA9B3A08EEF44AAAC45` transforms: `toupper`
`target.process.file.sha256`	contains	`F266609E91985F0FE3E31C5E8FAEEEC4FFA5E0322D8B6F15FE69F4C5165B9559` transforms: `toupper`

Output fields

Fields the rule emits when it matches. Chronicle authors list these in the outcome block; they appear on the detection and $risk_score drives alerting. Sentinel / Defender XDR rules build them up through project / summarize / extend stages. Sentinel maps these into alert fields via entityMappings and customDetails; Defender XDR custom detections surface them as alert fields directly.

Field	Source
`risk_score`	`max(if($process.principal.user.userid = "user" and $process.principal.hostname = "hostname", 0, 15))`
`principal_process_pid`	`array_distinct($process.principal.process.pid)`
`principal_process_command_line`	`array_distinct($process.principal.process.command_line)`
`principal_process_file_sha256`	`array_distinct($process.principal.process.file.sha256)`
`principal_process_file_full_path`	`array_distinct($process.principal.process.file.full_path)`
`principal_process_product_specfic_process_id`	`array_distinct($process.principal.process.product_specific_process_id)`
`principal_process_parent_process_product_specfic_process_id`	`array_distinct($process.principal.process.parent_process.product_specific_process_id)`
`target_process_pid`	`array_distinct($process.target.process.pid)`
`target_process_command_line`	`array_distinct($process.target.process.command_line)`
`target_process_file_sha256`	`array_distinct($process.target.process.file.sha256)`
`target_process_file_full_path`	`array_distinct($process.target.process.file.full_path)`
`target_process_product_specfic_process_id`	`array_distinct($process.target.process.product_specific_process_id)`
`principal_user_userid`	`array_distinct($process.principal.user.userid)`
`log_type`	`array_distinct(strings.concat($process.metadata.log_type,"/",$process.metadata.product_event_type))`

`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:`)
`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)

PUA - Nimgrab Execution

MITRE ATT&CK coverage

References

Event coverage

Rule body yaral

Stages and Predicates

Stage 0: `match + condition`

Stage 1: `events: $process · PROCESS_LAUNCH`

Indicators

Output fields

Keyboard shortcuts

Search operators

PUA - Nimgrab Execution

MITRE ATT&CK coverage

References

Event coverage

Rule body yaral

Stages and Predicates

Stage 0: match + condition

Stage 1: events: $process · PROCESS_LAUNCH

Indicators

Output fields

Stage 0: `match + condition`

Stage 1: `events: $process · PROCESS_LAUNCH`