The download of potentially risky files from the Discord Content Delivery Network (CDN) (ASIM Web Session)

Status: available
Severity: medium
Time window: 1d
Group by: DiscordServerId, EventEndTime, EventStartTime, SrcHostname, SrcIpAddr, SrcUsername, Url
Source: github.com/Azure/Azure-Sentinel

'This detection mechanism identifies instances where requests are made to Discord CDN addresses for file extensions that are considered risky. It triggers when a callout is made to a Discord server that has only been encountered once in your environment. The uniqueness of Discord servers is determined based on the server ID present in the request URL (DiscordServerId in the query). Discord CDN has been utilized in numerous campaigns to download additional payloads, highlighting the importance of monitoring such activities. The query includes a sample set of popular web script extensions (scriptExtensions), which should be customized to align with the specific requirements of your environment'

MITRE ATT&CK coverage

Tactic	Techniques
Command & Control	`T1071.001` Application Layer Protocol: Web Protocols

Rule body kusto

id: b7fe8f27-7010-404b-aec5-6e5245cea580
name: The download of potentially risky files from the Discord Content Delivery Network (CDN) (ASIM Web Session)
description: |
  'This detection mechanism identifies instances where requests are made to Discord CDN addresses for file extensions that are considered risky.
    It triggers when a callout is made to a Discord server that has only been encountered once in your environment. The uniqueness of Discord servers is determined based on the server ID present in the request URL (DiscordServerId in the query).
    Discord CDN has been utilized in numerous campaigns to download additional payloads, highlighting the importance of monitoring such activities.
    The query includes a sample set of popular web script extensions (scriptExtensions), which should be customized to align with the specific requirements of your environment'
severity: Medium
status: Available 
tags:
  - Schema: WebSession
    SchemaVersion: 0.2.6
requiredDataConnectors: []
queryFrequency: 1d
queryPeriod: 1d
triggerOperator: gt
triggerThreshold: 0
tactics:
  - CommandAndControl
relevantTechniques:
  - T1071.001
query: |
  let lookback = 1d;
  let connectionThreshold = 1;
  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 discord=dynamic(["cdn.discordapp.com", "media.discordapp.com"]);
  let WebData = _Im_WebSession(starttime=ago(lookback), url_has_any=discord, eventresult='Success')
      | where isnotempty(Url)
      | project Url, SrcUsername, SrcIpAddr, TimeGenerated, SrcHostname
      | where Url has "attachments"
      | extend DiscordServerId = extract(@"\/attachments\/([0-9]+)\/", 1, Url);
  WebData
  | summarize
      dcount(Url),
      min(TimeGenerated),
      max(TimeGenerated)
      by DiscordServerId
  | join kind=inner (WebData) on DiscordServerId
  | where dcount_Url <= connectionThreshold and Url has_any (knownRiskyFileExtensions)
  | summarize EventCount = count()
      by
      EventStartTime=min_TimeGenerated,
      EventEndTime=max_TimeGenerated,
      SrcUsername,
      SrcHostname,
      SrcIpAddr,
      Url
  | extend
      Name = iif(SrcUsername contains "@", tostring(split(SrcUsername, '@', 0)[0]), SrcUsername),
      UPNSuffix = iif(SrcUsername contains "@", tostring(split(SrcUsername, '@', 1)[0]), "")
entityMappings:
  - entityType: Account
    fieldMappings:
      - identifier: Name
        columnName: Name
      - identifier: UPNSuffix
        columnName: UPNSuffix
  - entityType: IP
    fieldMappings:
      - identifier: Address
        columnName: SrcIpAddr
  - entityType: URL
    fieldMappings:
      - identifier: Url
        columnName: Url
  - entityType: Host
    fieldMappings:
      - identifier: HostName
        columnName: SrcHostname
eventGroupingSettings:
  aggregationKind: AlertPerResult
customDetails:
  EventStartTime: EventStartTime
  EventEndTime: EventEndTime
alertDetailsOverride:
  alertDisplayNameFormat: "User '{{SrcUsername}}' with the IP address '{{SrcIpAddr}}' has been detected downloading potentially risky files from the Discord CDN"
  alertDescriptionFormat: " Client requested for URL '{{Url}}' that contains a files hosted on a recognized Discord Content Delivery Network (CDN) which are considered to be potentially risky. It is essential to investigate further to determine the nature of the files being requested and the intent of the users involved"
version: 1.0.0
kind: Scheduled

Stages and Predicates

Parameters

let lookback = 1d;
let connectionThreshold = 1;
let CombinedRiskyFileExtensions = union RiskyFileExtensions, CustomRiskyFileExtensions;
let discord = dynamic(["cdn.discordapp.com", "media.discordapp.com"]);

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.

The stages below define let WebData (the rule's main pipeline source).

Stage 1: `source`

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

Stage 2: `where`

| where isnotempty(Url)

Stage 3: `project`

| project Url, SrcUsername, SrcIpAddr, TimeGenerated, SrcHostname

Stage 4: `where`

| where Url has "attachments"

Stage 5: `extend`

| extend DiscordServerId = extract(@"\/attachments\/([0-9]+)\/", 1, Url)

The stages below run on WebData (the outer pipeline).

Stage 6: `summarize`

WebData
| summarize
    dcount(Url),
    min(TimeGenerated),
    max(TimeGenerated)
    by DiscordServerId

Stage 7: `join`

| join kind=inner (WebData) on DiscordServerId

Stage 8: `where`

| where dcount_Url <= connectionThreshold and Url has_any (knownRiskyFileExtensions)

References knownRiskyFileExtensions (defined above).

Stage 9: `summarize`

| summarize EventCount = count()
    by
    EventStartTime=min_TimeGenerated,
    EventEndTime=max_TimeGenerated,
    SrcUsername,
    SrcHostname,
    SrcIpAddr,
    Url

Stage 10: `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
`Url`	is_not_null	(no value, null check)
`Url`	match	`attachments` transforms: `term` `knownRiskyFileExtensions`
`dcount_Url`	le	`1` 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`
`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)

The download of potentially risky files from the Discord Content Delivery Network (CDN) (ASIM Web Session)

MITRE ATT&CK coverage

Rule body kusto

Stages and Predicates

Parameters

Let binding: `RiskyFileExtensions`

Let binding: `CustomRiskyFileExtensions`

Let binding: `knownRiskyFileExtensions`

Stage 1: `source`

Stage 2: `where`

Stage 3: `project`

Stage 4: `where`

Stage 5: `extend`

Stage 6: `summarize`

Stage 7: `join`

Stage 8: `where`

Stage 9: `summarize`

Stage 10: `extend`

Indicators

Output fields

Keyboard shortcuts

Search operators

The download of potentially risky files from the Discord Content Delivery Network (CDN) (ASIM Web Session)

MITRE ATT&CK coverage

Rule body kusto

Stages and Predicates

Parameters

Let binding: RiskyFileExtensions

Let binding: CustomRiskyFileExtensions

Let binding: knownRiskyFileExtensions

Stage 1: source

Stage 2: where

Stage 3: project

Stage 4: where

Stage 5: extend

Stage 6: summarize

Stage 7: join

Stage 8: where

Stage 9: summarize

Stage 10: extend

Indicators

Output fields

Let binding: `RiskyFileExtensions`

Let binding: `CustomRiskyFileExtensions`

Let binding: `knownRiskyFileExtensions`

Stage 1: `source`

Stage 2: `where`

Stage 3: `project`

Stage 4: `where`

Stage 5: `extend`

Stage 6: `summarize`

Stage 7: `join`

Stage 8: `where`

Stage 9: `summarize`

Stage 10: `extend`