Proofpoint High Impostor Score Detected

Status: Experimental
Severity: medium
Log types: Proofpoint.Event
Tags: Proofpoint, Email Security, Business Email Compromise, BEC, Impersonation, Phishing, Initial Access:Phishing
Reference: https://www.proofpoint.com/us/threat-reference/business-email-compromise
Source: github.com/panther-labs/panther-analysis

This rule alerts when Proofpoint detects a high impostor score (50+), indicating potential Business Email Compromise (BEC) or impersonation attacks. The impostor score measures the likelihood that the sender is impersonating a trusted entity. Severity is dynamic based on the score: CRITICAL (80+), HIGH (65+), MEDIUM (50+).

MITRE ATT&CK coverage

Tactic	Techniques
Initial Access	`T1566` Phishing

Rule body yaml

AnalysisType: rule
Filename: proofpoint_high_impostor_score.py
RuleID: "Proofpoint.HighImpostorScore"
DisplayName: "Proofpoint High Impostor Score Detected"
Enabled: true
LogTypes:
  - Proofpoint.Event
Status: Experimental
Tags:
  - Proofpoint
  - Email Security
  - Business Email Compromise
  - BEC
  - Impersonation
  - Phishing
  - Initial Access:Phishing
Severity: Medium
Description: >
  This rule alerts when Proofpoint detects a high impostor score (50+),
  indicating potential Business Email Compromise (BEC) or impersonation
  attacks. The impostor score measures the likelihood that the sender is
  impersonating a trusted entity. Severity is dynamic based on the score:
  CRITICAL (80+), HIGH (65+), MEDIUM (50+).
Runbook: |
  1. Review sender details for lookalike domains and verify if recipients took action on the email within the last 24 hours
  2. If BEC is confirmed, immediately notify finance/accounting teams and block the sender domain
  3. Report to law enforcement within 24 hours if financial fraud was attempted or executives were impersonated
Reference: https://www.proofpoint.com/us/threat-reference/business-email-compromise
Reports:
  MITRE ATT&CK:
    - TA0001:T1566 # Initial Access: Phishing
Tests:
  - Name: Critical Impostor Score
    ExpectedResult: true
    Log:
      messageTime: "2026-01-08T14:00:00Z"
      sender: "ceo@comp4ny.com"
      senderIP: "192.0.2.150"
      fromAddress:
        - "ceo@comp4ny.com"
      toAddresses:
        - "finance@company.com"
      recipient:
        - "finance@company.com"
      subject: "Urgent Wire Transfer Request"
      malwareScore: 0
      phishScore: 60
      spamScore: 10
      impostorScore: 95
      headerFrom: "CEO <ceo@comp4ny.com>"
      messageID: "<bec-urgent@comp4ny.com>"
  - Name: High Impostor Score
    ExpectedResult: true
    Log:
      messageTime: "2026-01-08T15:30:00Z"
      sender: "it-support@company-support.net"
      senderIP: "198.51.100.175"
      fromAddress:
        - "it-support@company-support.net"
      toAddresses:
        - "employee@company.com"
      recipient:
        - "employee@company.com"
      subject: "Password Reset Required"
      malwareScore: 0
      phishScore: 45
      spamScore: 15
      impostorScore: 70
      headerFrom: "IT Support <it-support@company-support.net>"
      messageID: "<fake-it@company-support.net>"
  - Name: Medium Impostor Score at Threshold
    ExpectedResult: true
    Log:
      messageTime: "2026-01-08T16:00:00Z"
      sender: "hr@c0mpany.com"
      senderIP: "203.0.113.88"
      fromAddress:
        - "hr@c0mpany.com"
      toAddresses:
        - "employee@company.com"
      recipient:
        - "employee@company.com"
      subject: "Update Your Benefits"
      malwareScore: 0
      phishScore: 30
      spamScore: 20
      impostorScore: 50
      headerFrom: "Human Resources <hr@c0mpany.com>"
      messageID: "<fake-hr@c0mpany.com>"
  - Name: Boundary Case - Score 49 Below Threshold
    ExpectedResult: false
    Log:
      messageTime: "2026-01-08T16:30:00Z"
      sender: "suspicious@example.com"
      senderIP: "198.51.100.99"
      fromAddress:
        - "suspicious@example.com"
      toAddresses:
        - "employee@company.com"
      recipient:
        - "employee@company.com"
      subject: "Account Verification"
      malwareScore: 0
      phishScore: 40
      spamScore: 25
      impostorScore: 49
      headerFrom: "Support Team <suspicious@example.com>"
      messageID: "<boundary49@example.com>"
  - Name: Low Impostor Score - No Alert
    ExpectedResult: false
    Log:
      messageTime: "2026-01-08T17:00:00Z"
      sender: "marketing@partner.com"
      senderIP: "203.0.113.20"
      fromAddress:
        - "marketing@partner.com"
      toAddresses:
        - "employee@company.com"
      recipient:
        - "employee@company.com"
      subject: "New Product Launch"
      malwareScore: 0
      phishScore: 0
      spamScore: 30
      impostorScore: 15
      headerFrom: "Marketing Team <marketing@partner.com>"
      messageID: "<newsletter@partner.com>"
  - Name: Zero Impostor Score - No Alert
    ExpectedResult: false
    Log:
      messageTime: "2026-01-08T18:00:00Z"
      sender: "colleague@company.com"
      senderIP: "203.0.113.5"
      fromAddress:
        - "colleague@company.com"
      toAddresses:
        - "employee@company.com"
      recipient:
        - "employee@company.com"
      subject: "Project Update"
      malwareScore: 0
      phishScore: 0
      spamScore: 0
      impostorScore: 0
      headerFrom: "Jane Doe <colleague@company.com>"
      messageID: "<project-update@company.com>"

Detection logic

Condition

impostorScore ge "50"

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
`impostorScore`	ge	`50`

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
`sender`
`senderIP`
`recipients`	`recipient`
`subject`
`messageID`
`quarantineFolder`
`quarantineRule`
`malwareScore`
`phishScore`
`impostorScore`

`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)