NRT Process executed from binary hidden in Base64 encoded file

Status: available
Severity: medium
Source: github.com/Azure/Azure-Sentinel

Encoding malicious software is a technique used to obfuscate files from detection. The first CommandLine component is looking for Python decoding base64. The second CommandLine component is looking for Bash/sh command line base64 decoding. The third one is looking for Ruby decoding base64.

MITRE ATT&CK coverage

Tactic	Techniques
Execution	`T1059` Command and Scripting Interpreter
Stealth	`T1027` Obfuscated Files or Information, `T1140` Deobfuscate/Decode Files or Information

Event coverage

Provider	Event	Title
Security-Auditing	Event ID 4688	A new process has been created.

Rule body kusto

id: 7ad4c32b-d0d2-411c-a0e8-b557afa12fce
name: NRT Process executed from binary hidden in Base64 encoded file
description: |
  'Encoding malicious software is a technique used to obfuscate files from detection.
  The first CommandLine component is looking for Python decoding base64.
  The second CommandLine component is looking for Bash/sh command line base64 decoding.
  The third one is looking for Ruby decoding base64.'
severity: Medium
status: Available
requiredDataConnectors:
  - connectorId: SecurityEvents
    dataTypes:
      - SecurityEvent
  - connectorId: WindowsSecurityEvents
    dataTypes:
      - SecurityEvent
tactics:
  - Execution
  - DefenseEvasion
relevantTechniques:
  - T1059
  - T1027
  - T1140
query: |
  SecurityEvent
  | where EventID==4688
  | where isnotempty(CommandLine)
  | project TimeGenerated, Computer, AccountName = SubjectUserName, AccountNTDomain = SubjectDomainName, FileName = Process, CommandLine, ParentProcessName, SubjectAccount
  | extend HostName = tostring(split(Computer, ".")[0]), DomainIndex = toint(indexof(Computer, '.'))
  | extend HostNameDomain = iff(DomainIndex != -1, substring(Computer, DomainIndex + 1), Computer)
  | where CommandLine contains ".decode('base64')"
          or CommandLine contains "base64 --decode"
          or CommandLine contains ".decode64("
entityMappings:
  - entityType: Account
    fieldMappings:
      - identifier: FullName
        columnName: SubjectAccount
      - identifier: Name
        columnName: AccountName
      - identifier: NTDomain
        columnName: AccountNTDomain
  - entityType: Host
    fieldMappings:
      - identifier: FullName
        columnName: Computer
      - identifier: HostName
        columnName: HostName
      - identifier: DnsDomain
        columnName: HostNameDomain
version: 1.0.2
kind: NRT

Stages and Predicates

Stage 1: `source`

SecurityEvent

Stage 2: `where`

| where EventID==4688

Stage 3: `where`

| where isnotempty(CommandLine)

Stage 4: `project`

| project TimeGenerated, Computer, AccountName = SubjectUserName, AccountNTDomain = SubjectDomainName, FileName = Process, CommandLine, ParentProcessName, SubjectAccount

Stage 5: `extend`

| extend HostName = tostring(split(Computer, ".")[0]), DomainIndex = toint(indexof(Computer, '.'))

Stage 6: `extend`

| extend HostNameDomain = iff(DomainIndex != -1, substring(Computer, DomainIndex + 1), Computer)

HostNameDomain =

ifDomainIndex != -1substring(Computer, (DomainIndex + 1))

elseComputer

Stage 7: `where`

| where CommandLine contains ".decode('base64')"
        or CommandLine contains "base64 --decode"
        or CommandLine contains ".decode64("

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`	contains	`.decode('base64')` corpus 2 (kusto 2) `.decode64(` corpus 2 (kusto 2) `base64 --decode` corpus 2 (kusto 2)
`CommandLine`	is_not_null	(no value, null check)
`EventID`	eq	`4688` transforms: `cased` corpus 313 (splunk 283, kusto 30)

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
`AccountNTDomain`	`project`
`AccountName`	`project`
`CommandLine`	`project`
`Computer`	`project`
`FileName`	`project`
`ParentProcessName`	`project`
`SubjectAccount`	`project`
`TimeGenerated`	`project`
`DomainIndex`	`extend`
`HostName`	`extend`
`HostNameDomain`	`extend`

`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)

NRT Process executed from binary hidden in Base64 encoded file

MITRE ATT&CK coverage

Event coverage

Rule body kusto

Stages and Predicates

Stage 1: `source`

Stage 2: `where`

Stage 3: `where`

Stage 4: `project`

Stage 5: `extend`

Stage 6: `extend`

Stage 7: `where`

Indicators

Output fields

Keyboard shortcuts

Search operators

NRT Process executed from binary hidden in Base64 encoded file

MITRE ATT&CK coverage

Event coverage

Rule body kusto

Stages and Predicates

Stage 1: source

Stage 2: where

Stage 3: where

Stage 4: project

Stage 5: extend

Stage 6: extend

Stage 7: where

Indicators

Output fields

Stage 1: `source`

Stage 2: `where`

Stage 3: `where`

Stage 4: `project`

Stage 5: `extend`

Stage 6: `extend`

Stage 7: `where`