Detect web requests to potentially harmful files (ASIM Web Session)

Status: available
Severity: medium
Time window: 5m
Group by: DstIpAddr, DstPortNumber, SrcHostname, SrcIpAddr, SrcUsername, Url, requestedFileName
Source: github.com/Azure/Azure-Sentinel

'This rule detects web requests made to URLs containing file types such as .ps1, .bat, .vbs,.scr etc. which have the potential to be harmful if downloaded. This rule uses the Advanced Security Information Model (ASIM) and supports any web session source that complies with ASIM.'

MITRE ATT&CK coverage

Tactic	Techniques
Initial Access	`T1133` External Remote Services, `T1566` Phishing
Execution	`T1203` Exploitation for Client Execution
Persistence	`T1133` External Remote Services

Rule body kusto

id: c6608467-3678-45fe-b038-b590ce6d00fb
name: Detect web requests to potentially harmful files (ASIM Web Session)
description: |
  'This rule detects web requests made to URLs containing file types such as .ps1, .bat, .vbs,.scr etc. which have the potential to be harmful if downloaded. This rule uses the [Advanced Security Information Model (ASIM)](https://aka.ms/AboutASIM) and supports any web session source that complies with ASIM.'
severity: Medium
status: Available 
tags:
  - Schema: WebSession
    SchemaVersion: 0.2.6
requiredDataConnectors: []
queryFrequency: 5m
queryPeriod: 5m
triggerOperator: gt
triggerThreshold: 0
tactics:
  - InitialAccess
  - Persistence
  - Execution
relevantTechniques:
  - T1133
  - T1203
  - T1566
query: |
  let lookback = 5m;
  let RiskyFileExtensions = materialize(externaldata(Extensions: string)
      [@"https://raw.githubusercontent.com/Azure/Azure-Sentinel/master/Sample%20Data/Feeds/RiskyFileExtensionsInUrl.csv"]
      with(format="csv", ignoreFirstRecord=True));
  let CustomRiskyFileExtensions = (_ASIM_GetWatchlistRaw("Web_RiskyFileExtensions") // Create new Watchlist and add your custom indicators(Optional)
      | extend
          Extensions = tostring(WatchlistItem["Extensions"])
      | project Extensions
      | where isnotempty(Extensions));
  let CombinedRiskyFileExtensions = union RiskyFileExtensions, CustomRiskyFileExtensions;
  let knownRiskyFileExtensions=toscalar(CombinedRiskyFileExtensions
      | where isnotempty(Extensions)
      | summarize make_set(Extensions, 1000));
  let Whitelisted_Domains = materialize(externaldata(WhiteListedDomains: string)
      [@"https://raw.githubusercontent.com/Azure/Azure-Sentinel/master/Sample%20Data/Feeds/WhiteListedDomainsForWebSessionUseCases.csv"]
      with(format="csv", ignoreFirstRecord=True));
  let CustomWhiteListedDomains = (_ASIM_GetWatchlistRaw("Web_WhiteListedDomains") // Create new Watchlist and add your white listed domains(Optional)
      | extend
          WhiteListedDomains = tostring(WatchlistItem["WhiteListedDomains"])
      | project WhiteListedDomains
      | where isnotempty(WhiteListedDomains));
  let CombinedWhitelisted_Domains = union Whitelisted_Domains, CustomWhiteListedDomains;
  let knownWhitelisted_Domains=toscalar(CombinedWhitelisted_Domains
      | where isnotempty(WhiteListedDomains)
      | summarize make_set(WhiteListedDomains, 1000));
  _Im_WebSession (starttime=ago(lookback), url_has_any=knownRiskyFileExtensions, eventresult='Success')
  | project
      Url,
      SrcIpAddr,
      TimeGenerated,
      SrcUsername,
      SrcHostname,
      DstIpAddr,
      DstPortNumber
  | extend requestedFileName=tostring(split(tostring(parse_url(Url)["Path"]), '/')[-1])
  | extend requestedFileExt=extract(@'(\.\w+)$', 1, requestedFileName, typeof(string))
  | where requestedFileExt in~ (knownRiskyFileExtensions)
  | summarize
      EventCount=count(),
      EventStartTime=min(TimeGenerated),
      EventEndTime=max(TimeGenerated)
      by
      SrcUsername,
      SrcIpAddr,
      SrcHostname,
      DstIpAddr,
      DstPortNumber,
      Url,
      requestedFileName
  | extend FQDN = split(parse_url(Url)["Host"], '.')
  | extend Domain = iif(array_length(FQDN) > 1, strcat(FQDN[-2], '.', FQDN[-1]), FQDN)
  | where Domain !in~ (knownWhitelisted_Domains)
  | project-away FQDN
  | extend Name = iif(SrcUsername contains "@", tostring(split(SrcUsername,'@',0)[0]),SrcUsername), UPNSuffix = iif(SrcUsername contains "@",tostring(split(SrcUsername,'@',1)[0]),"")
entityMappings:
  - entityType: IP
    fieldMappings:
      - identifier: Address
        columnName: SrcIpAddr
  - entityType: URL
    fieldMappings:
      - identifier: Url
        columnName: Url
  - entityType: File
    fieldMappings:
      - identifier: Name
        columnName: requestedFileName
  - entityType: Host
    fieldMappings:
      - identifier: HostName
        columnName: SrcHostname
  - entityType: Account
    fieldMappings:
      - identifier: Name
        columnName: Name
      - identifier: UPNSuffix
        columnName: UPNSuffix
eventGroupingSettings:
  aggregationKind: AlertPerResult
customDetails:
  EventCount: EventCount
  EventStartTime: EventStartTime
  EventEndTime: EventEndTime
  DstIpAddr: DstIpAddr
alertDetailsOverride:
  alertDisplayNameFormat: "User '{{SrcUsername}}' with IP address '{{SrcIpAddr}}' accessed a potentially harmful URL"
  alertDescriptionFormat: "User accessed URL - '{{Url}}' that contains a file - '{{requestedFileName}}' with risky extension. Downloading this file could pose a potential risk"
version: 1.0.0
kind: Scheduled

Stages and Predicates

Parameters

let lookback = 5m;
let CombinedRiskyFileExtensions = union RiskyFileExtensions, CustomRiskyFileExtensions;
let CombinedWhitelisted_Domains = union Whitelisted_Domains, CustomWhiteListedDomains;

Let binding: `RiskyFileExtensions`

let RiskyFileExtensions = materialize(externaldata(Extensions: string)
    [@"https://raw.githubusercontent.com/Azure/Azure-Sentinel/master/Sample%20Data/Feeds/RiskyFileExtensionsInUrl.csv"]
    with(format="csv", ignoreFirstRecord=True));

Let binding: `CustomRiskyFileExtensions`

let CustomRiskyFileExtensions = (_ASIM_GetWatchlistRaw("Web_RiskyFileExtensions")
    | extend
        Extensions = tostring(WatchlistItem["Extensions"])
    | project Extensions
    | where isnotempty(Extensions));

Let binding: `knownRiskyFileExtensions`

let knownRiskyFileExtensions = toscalar(CombinedRiskyFileExtensions
    | where isnotempty(Extensions)
    | summarize make_set(Extensions, 1000));

Derived from CombinedRiskyFileExtensions.

Let binding: `Whitelisted_Domains`

let Whitelisted_Domains = materialize(externaldata(WhiteListedDomains: string)
    [@"https://raw.githubusercontent.com/Azure/Azure-Sentinel/master/Sample%20Data/Feeds/WhiteListedDomainsForWebSessionUseCases.csv"]
    with(format="csv", ignoreFirstRecord=True));

Let binding: `CustomWhiteListedDomains`

let CustomWhiteListedDomains = (_ASIM_GetWatchlistRaw("Web_WhiteListedDomains")
    | extend
        WhiteListedDomains = tostring(WatchlistItem["WhiteListedDomains"])
    | project WhiteListedDomains
    | where isnotempty(WhiteListedDomains));

Let binding: `knownWhitelisted_Domains`

let knownWhitelisted_Domains = toscalar(CombinedWhitelisted_Domains
    | where isnotempty(WhiteListedDomains)
    | summarize make_set(WhiteListedDomains, 1000));

Derived from CombinedWhitelisted_Domains.

Stage 1: `source`

_Im_WebSession (starttime=ago(lookback), url_has_any=knownRiskyFileExtensions, eventresult='Success')

Stage 2: `project`

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

Stage 3: `extend`

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

Stage 4: `extend`

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

Stage 5: `where`

| where requestedFileExt in~ (knownRiskyFileExtensions)

References knownRiskyFileExtensions (defined above).

Stage 6: `summarize`

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

Stage 7: `extend`

| extend FQDN = split(parse_url(Url)["Host"], '.')

Stage 8: `extend`

| extend Domain = iif(array_length(FQDN) > 1, strcat(FQDN[-2], '.', FQDN[-1]), FQDN)

Stage 9: `where`

| where Domain !in~ (knownWhitelisted_Domains)

References knownWhitelisted_Domains (defined above).

Stage 10: `project-away`

| project-away FQDN

Stage 11: `extend`

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

Exclusions

Top-level NOT(...) conjuncts: predicates this rule actively suppresses.

Field	Kind	Excluded values
`Domain`	eq	`knownWhitelisted_Domains`

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
`requestedFileExt`	in	`knownRiskyFileExtensions`

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
`DstIpAddr`	`summarize`
`DstPortNumber`	`summarize`
`EventCount`	`summarize`
`EventEndTime`	`summarize`
`EventStartTime`	`summarize`
`SrcHostname`	`summarize`
`SrcIpAddr`	`summarize`
`SrcUsername`	`summarize`
`Url`	`summarize`
`requestedFileName`	`summarize`
`Domain`	`extend`
`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 web requests to potentially harmful files (ASIM Web Session)

MITRE ATT&CK coverage

Rule body kusto

Stages and Predicates

Parameters

Let binding: `RiskyFileExtensions`

Let binding: `CustomRiskyFileExtensions`

Let binding: `knownRiskyFileExtensions`

Let binding: `Whitelisted_Domains`

Let binding: `CustomWhiteListedDomains`

Let binding: `knownWhitelisted_Domains`

Stage 1: `source`

Stage 2: `project`

Stage 3: `extend`

Stage 4: `extend`

Stage 5: `where`

Stage 6: `summarize`

Stage 7: `extend`

Stage 8: `extend`

Stage 9: `where`

Stage 10: `project-away`

Stage 11: `extend`

Exclusions

Indicators

Output fields

Keyboard shortcuts

Search operators

Detect web requests to potentially harmful files (ASIM Web Session)

MITRE ATT&CK coverage

Rule body kusto

Stages and Predicates

Parameters

Let binding: RiskyFileExtensions

Let binding: CustomRiskyFileExtensions

Let binding: knownRiskyFileExtensions

Let binding: Whitelisted_Domains

Let binding: CustomWhiteListedDomains

Let binding: knownWhitelisted_Domains

Stage 1: source

Stage 2: project

Stage 3: extend

Stage 4: extend

Stage 5: where

Stage 6: summarize

Stage 7: extend

Stage 8: extend

Stage 9: where

Stage 10: project-away

Stage 11: extend

Exclusions

Indicators

Output fields

Let binding: `RiskyFileExtensions`

Let binding: `CustomRiskyFileExtensions`

Let binding: `knownRiskyFileExtensions`

Let binding: `Whitelisted_Domains`

Let binding: `CustomWhiteListedDomains`

Let binding: `knownWhitelisted_Domains`

Stage 1: `source`

Stage 2: `project`

Stage 3: `extend`

Stage 4: `extend`

Stage 5: `where`

Stage 6: `summarize`

Stage 7: `extend`

Stage 8: `extend`

Stage 9: `where`

Stage 10: `project-away`

Stage 11: `extend`