A client made a web request to a potentially harmful file (ASIM Web Session schema)

Severity: medium
Time window: 10m
Group by: SrcHostname, SrcIpAddr, SrcUsername, Url, requestedFileName
Author: Yaron
Source: github.com/Azure/Azure-Sentinel

'This rule identifies a web request to a URL that holds a file type, including .ps1, .bat, .vbs, and .scr that can be harmful if downloaded. This analytic rule uses ASIM and supports any built-in or custom source that supports the ASIM WebSession schema (ASIM WebSession Schema)'

MITRE ATT&CK coverage

Tactic	Techniques
Initial Access	`T1189` Drive-by Compromise

Rule body kusto

id: 09c49590-4e9d-4da9-a34d-17222d0c9e7e
name: A client made a web request to a potentially harmful file (ASIM Web Session schema)
description: |
  'This rule identifies a web request to a URL that holds a file type, including .ps1, .bat, .vbs, and .scr that can be harmful if downloaded. This analytic rule uses [ASIM](https://aka.ms/AboutASIM) and supports any built-in or custom source that supports the ASIM WebSession schema (ASIM WebSession Schema)'
severity: Medium
tags:
  - ParentAlert: https://github.com/Azure/Azure-Sentinel/blob/master/Detections/CiscoUmbrella/CiscoUmbrellaRequestBlocklistedFileType.yaml
    version: 1.0.0
  - Schema: ASimWebSession
    SchemaVersion: 0.2.1
requiredDataConnectors:
  - connectorId: SquidProxy
    dataTypes:
      - SquidProxy_CL
  - connectorId: Zscaler
    dataTypes:
      - CommonSecurityLog
queryFrequency: 10m
queryPeriod: 10m
triggerOperator: gt
triggerThreshold: 0
tactics:
  - InitialAccess
relevantTechniques:
  - T1189
query: |
  let default_file_ext_blocklist = dynamic(['.ps1', '.vbs', '.bat', '.scr']); // Update this list as per your requirement
  let custom_file_ext_blocklist=toscalar(_GetWatchlist('RiskyFileTypes')
    | extend Extension=column_ifexists("Extension", "")
    | where isnotempty(Extension)
    | summarize make_set(Extension)); // If you have an extensive list, you can also create a Watchlist that includes the file extensions you want to detect
  let file_ext_blocklist = array_concat(default_file_ext_blocklist, custom_file_ext_blocklist);
  _Im_WebSession(starttime=ago(10min), url_has_any=file_ext_blocklist, eventresult='Success')
  | extend requestedFileName=tostring(split(tostring(parse_url(Url)["Path"]), '/')[-1])
  | extend requestedFileExtension=extract(@'(\.\w+)$', 1, requestedFileName, typeof(string))
  | where requestedFileExtension in (file_ext_blocklist)
  | summarize
    EventStartTime=min(TimeGenerated),
    EventEndTime=max(TimeGenerated),
    EventCount=count()
    by SrcIpAddr, SrcUsername, SrcHostname, requestedFileName, Url
  | extend
    Name = iif(SrcUsername contains "@", tostring(split(SrcUsername, '@', 0)[0]), SrcUsername),
    UPNSuffix = iif(SrcUsername contains "@", tostring(split(SrcUsername, '@', 1)[0]), "")
alertDetailsOverride:
  alertDisplayNameFormat: Client {{SrcIpAddr}} accessed a URL with potentially harmful extension {{requestedFileExtension}}
  alertDescriptionFormat: The client at address {{SrcIpAddr}} accessed the URL {{Url}} that has the extension {{requestedFileExtension}}. Downloading a file with this extension may be harmful and may indicate malicious activity.
customDetails:
  requestedFileExt: requestedFileExtension
  Username: SrcUsername
  SrcHostname: SrcHostname
eventGroupingSettings:
  aggregationKind: AlertPerResult
entityMappings:
  - entityType: IP
    fieldMappings:
      - identifier: Address
        columnName: SrcIpAddr
  - entityType: URL
    fieldMappings:
      - identifier: Url
        columnName: Url
  - entityType: File
    fieldMappings:
      - identifier: Name
        columnName: requestedFileName
  - entityType: Account
    fieldMappings:
      - identifier: FullName
        columnName: SrcUsername
      - identifier: Name
        columnName: Name
      - identifier: UPNSuffix
        columnName: UPNSuffix
version: 1.1.3
kind: Scheduled
metadata:
    source:
        kind: Community
    author:
        name: Yaron
    support:
        tier: Community
    categories:
        domains: [ "Security - Others" ]

Stages and Predicates

Parameters

let default_file_ext_blocklist = dynamic(['.ps1', '.vbs', '.bat', '.scr']);
let file_ext_blocklist = array_concat(default_file_ext_blocklist, custom_file_ext_blocklist);

Let binding: `custom_file_ext_blocklist`

let custom_file_ext_blocklist = toscalar(_GetWatchlist('RiskyFileTypes')
  | extend Extension=column_ifexists("Extension", "")
  | where isnotempty(Extension)
  | summarize make_set(Extension));

Stage 1: `source`

_Im_WebSession(starttime=ago(10min), url_has_any=file_ext_blocklist, eventresult='Success')

Stage 2: `extend`

| extend requestedFileName=tostring(split(tostring(parse_url(Url)["Path"]), '/')[-1])

Stage 3: `extend`

| extend requestedFileExtension=extract(@'(\.\w+)$', 1, requestedFileName, typeof(string))

Stage 4: `where`

| where requestedFileExtension in (file_ext_blocklist)

Stage 5: `summarize`

| summarize
  EventStartTime=min(TimeGenerated),
  EventEndTime=max(TimeGenerated),
  EventCount=count()
  by SrcIpAddr, SrcUsername, SrcHostname, requestedFileName, Url

Stage 6: `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
`requestedFileExtension`	in	`file_ext_blocklist` transforms: `cased`

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
`EventCount`	`summarize`
`EventEndTime`	`summarize`
`EventStartTime`	`summarize`
`SrcHostname`	`summarize`
`SrcIpAddr`	`summarize`
`SrcUsername`	`summarize`
`Url`	`summarize`
`requestedFileName`	`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)

A client made a web request to a potentially harmful file (ASIM Web Session schema)

MITRE ATT&CK coverage

Rule body kusto

Stages and Predicates

Parameters

Let binding: `custom_file_ext_blocklist`

Stage 1: `source`

Stage 2: `extend`

Stage 3: `extend`

Stage 4: `where`

Stage 5: `summarize`

Stage 6: `extend`

Indicators

Output fields

Keyboard shortcuts

Search operators

A client made a web request to a potentially harmful file (ASIM Web Session schema)

MITRE ATT&CK coverage

Rule body kusto

Stages and Predicates

Parameters

Let binding: custom_file_ext_blocklist

Stage 1: source

Stage 2: extend

Stage 3: extend

Stage 4: where

Stage 5: summarize

Stage 6: extend

Indicators

Output fields

Let binding: `custom_file_ext_blocklist`

Stage 1: `source`

Stage 2: `extend`

Stage 3: `extend`

Stage 4: `where`

Stage 5: `summarize`

Stage 6: `extend`