Detect potential presence of a malicious file with a double extension (ASIM Web Session)

Status: available
Severity: medium
Time window: 1h
Group by: DstHostname, DstIpAddr, DstPortNumber, FileWithdualextension, SrcHostname, SrcIpAddr, SrcUsername, Url
Source: github.com/Azure/Azure-Sentinel

'Double extension vulnerability is a significant concern in file uploads, as it can lead to various issues if an attacker successfully uploads a virus-infected file.'

MITRE ATT&CK coverage

Tactic	Techniques
Persistence	`T1505` Server Software Component
Stealth	`T1036` Masquerading
Command & Control	`T1071` Application Layer Protocol

Rule body kusto

id: 6a71687f-00cf-44d3-93fc-8cbacc7b5615
name: Detect potential presence of a malicious file with a double extension (ASIM Web Session)
description: |
  'Double extension vulnerability is a significant concern in file uploads, as it can lead to various issues if an attacker successfully uploads a virus-infected file.'
severity: Medium
status: Available 
tags:
  - Schema: WebSession
    SchemaVersion: 0.2.6
requiredDataConnectors: []
queryFrequency: 1h
queryPeriod: 1h
triggerOperator: gt
triggerThreshold: 0
tactics:
  - DefenseEvasion
  - Persistence
  - CommandAndControl
relevantTechniques:
  - T1036
  - T1505
  - T1071
query: |
  let common_file_ext_list = dynamic([".txt", ".xlsx", ".doc", ".docx", ".csv", ".pdf", ".png", ".jpg", ".jpeg"]); // Add list of common files as per your environment
  _Im_WebSession (starttime=ago(1h), eventresult='Success')
  | where HttpRequestMethod in~ ("POST", "PUT") 
  | project
      Url,
      SrcIpAddr,
      SrcUsername,
      SrcHostname,
      DstIpAddr,
      DstPortNumber,
      DstHostname,
      TimeGenerated
  | extend requestedFileName=tostring(split(tostring(parse_url(Url)["Path"]), '/')[-1])
  | extend FileWithdualextension = extract(@'([\w-]+\.\w+\.\w+)$', 1, requestedFileName, typeof(string))
  | extend SecondExt = tostring(split(FileWithdualextension, '.')[-1])
  | where strcat('.', SecondExt) in~ (common_file_ext_list) // Second extension is mostly from the common files
  | summarize
      EventCount=count(),
      EventStartTime=min(TimeGenerated),
      EventEndTime=max(TimeGenerated)
      by
      SrcIpAddr,
      Url,
      FileWithdualextension,
      SrcUsername,
      SrcHostname,
      DstIpAddr,
      DstPortNumber,
      DstHostname
  | extend Name = iif(SrcUsername contains "@", tostring(split(SrcUsername,'@',0)[0]),SrcUsername), UPNSuffix = iif(SrcUsername contains "@",tostring(split(SrcUsername,'@',1)[0]),"")
entityMappings:
  - entityType: File
    fieldMappings:
      - identifier: Name
        columnName: FileWithdualextension
  - entityType: URL
    fieldMappings:
      - identifier: Url
        columnName: Url
  - entityType: IP
    fieldMappings:
      - identifier: Address
        columnName: SrcIpAddr
  - entityType: Account
    fieldMappings:
      - identifier: Name
        columnName: Name
      - identifier: UPNSuffix
        columnName: UPNSuffix
  - entityType: Host
    fieldMappings:
      - identifier: HostName
        columnName: SrcHostname
eventGroupingSettings:
  aggregationKind: AlertPerResult
customDetails:
  EventCount: EventCount
  EventStartTime: EventStartTime
  EventEndTime: EventEndTime
  DstHostname: DstHostname
alertDetailsOverride:
  alertDisplayNameFormat: "User '{{SrcUsername}}' with IP address '{{SrcIpAddr}}' has been observed with posting potentially risky dual extension file"
  alertDescriptionFormat: "User posted file '{{FileWithdualextension}}' which potentially contain dual extensions. This type of activity could be malicious and performed to bypass file upload filters or security measures implemented by the application. Destination server name this request was targetted to - '{{DstHostname}}'"
version: 1.0.1
kind: Scheduled

Stages and Predicates

Parameters

let common_file_ext_list = dynamic([".txt", ".xlsx", ".doc", ".docx", ".csv", ".pdf", ".png", ".jpg", ".jpeg"]);

Stage 1: `source`

_Im_WebSession (starttime=ago(1h), eventresult='Success')

Stage 2: `where`

| where HttpRequestMethod in~ ("POST", "PUT")

Stage 3: `project`

| project
    Url,
    SrcIpAddr,
    SrcUsername,
    SrcHostname,
    DstIpAddr,
    DstPortNumber,
    DstHostname,
    TimeGenerated

Stage 4: `extend` (3 consecutive steps)

| extend requestedFileName=tostring(split(tostring(parse_url(Url)["Path"]), '/')[-1])
| extend FileWithdualextension = extract(@'([\w-]+\.\w+\.\w+)$', 1, requestedFileName, typeof(string))
| extend SecondExt = tostring(split(FileWithdualextension, '.')[-1])

Stage 5: `where`

| where strcat('.', SecondExt) in~ (common_file_ext_list)

Stage 6: `summarize`

| summarize
    EventCount=count(),
    EventStartTime=min(TimeGenerated),
    EventEndTime=max(TimeGenerated)
    by
    SrcIpAddr,
    Url,
    FileWithdualextension,
    SrcUsername,
    SrcHostname,
    DstIpAddr,
    DstPortNumber,
    DstHostname

Stage 7: `extend`

| extend Name = iif(SrcUsername contains "@", tostring(split(SrcUsername,'@',0)[0]),SrcUsername), UPNSuffix = iif(SrcUsername contains "@",tostring(split(SrcUsername,'@',1)[0]),"")

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
`HttpRequestMethod`	in	`POST` `PUT`

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
`DstHostname`	`summarize`
`DstIpAddr`	`summarize`
`DstPortNumber`	`summarize`
`EventCount`	`summarize`
`EventEndTime`	`summarize`
`EventStartTime`	`summarize`
`FileWithdualextension`	`summarize`
`SrcHostname`	`summarize`
`SrcIpAddr`	`summarize`
`SrcUsername`	`summarize`
`Url`	`summarize`
`Name`	`extend`
`UPNSuffix`	`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)

Detect potential presence of a malicious file with a double extension (ASIM Web Session)

MITRE ATT&CK coverage

Rule body kusto

Stages and Predicates

Parameters

Stage 1: `source`

Stage 2: `where`

Stage 3: `project`

Stage 4: `extend` (3 consecutive steps)

Stage 5: `where`

Stage 6: `summarize`

Stage 7: `extend`

Indicators

Output fields

Keyboard shortcuts

Search operators

Detect potential presence of a malicious file with a double extension (ASIM Web Session)

MITRE ATT&CK coverage

Rule body kusto

Stages and Predicates

Parameters

Stage 1: source

Stage 2: where

Stage 3: project

Stage 4: extend (3 consecutive steps)

Stage 5: where

Stage 6: summarize

Stage 7: extend

Indicators

Output fields

Stage 1: `source`

Stage 2: `where`

Stage 3: `project`

Stage 4: `extend` (3 consecutive steps)

Stage 5: `where`

Stage 6: `summarize`

Stage 7: `extend`