Discord CDN Risky File Download (ASIM Web Session Schema)

Severity: medium
Time window: 1d
Group by: DiscordServerId, dcount_Url, max_TimeGenerated, min_TimeGenerated, set_EventResult, set_SrcIpAddr, set_SrcUsername, set_Url
Author: Microsoft Security Research
Source: github.com/Azure/Azure-Sentinel

'Identifies callouts to Discord CDN addresses for risky file extensions. This detection will trigger when a callout for a risky file is made to a discord server that has only been seen once in your environment. Unique discord servers are identified using the server ID that is included in the request URL (DiscordServerId in query). Discord CDN has been used in multiple campaigns to download additional payloads. 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
Command & Control	`T1071.001` Application Layer Protocol: Web Protocols

Rule body kusto

id: 01e8ffff-dc0c-43fe-aa22-d459c4204553
name: Discord CDN Risky File Download  (ASIM Web Session Schema)
description: |
  'Identifies callouts to Discord CDN addresses for risky file extensions. This detection will trigger when a callout for a risky file is made to a discord server that has only been seen once in your environment. 
   Unique discord servers are identified using the server ID that is included in the request URL (DiscordServerId in query). Discord CDN has been used in multiple campaigns to download additional payloads.
   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
requiredDataConnectors:
  - connectorId: SquidProxy
    dataTypes:
      - SquidProxy_CL
  - connectorId: Zscaler
    dataTypes:
      - CommonSecurityLog
queryFrequency: 1d
queryPeriod: 1d
triggerOperator: gt
triggerThreshold: 0
tactics:
  - CommandAndControl
relevantTechniques:
  - T1071.001
tags:
  - Discord
query: |
  let discord=dynamic(["cdn.discordapp.com", "media.discordapp.com"]);
    _Im_WebSession(url_has_any=discord, eventresult='Success')
    | where Url has "attachments"
    | extend DiscordServerId = extract(@"\/attachments\/([0-9]+)\/", 1, Url)
    | summarize dcount(Url), make_set(SrcUsername), make_set(SrcIpAddr), make_set(Url), min(TimeGenerated), max(TimeGenerated), make_set(EventResult) by DiscordServerId
    | mv-expand set_SrcUsername to typeof(string), set_Url to typeof(string), set_EventResult to typeof(string), set_SrcIpAddr to typeof(string)
    | summarize by DiscordServerId, dcount_Url, set_SrcUsername, min_TimeGenerated, max_TimeGenerated, set_EventResult, set_SrcIpAddr, set_Url
    | project StartTime=min_TimeGenerated, EndTime=max_TimeGenerated, Result=set_EventResult, SourceUser=set_SrcUsername, SourceIP=set_SrcIpAddr, RequestURL=set_Url
    | where RequestURL has_any (".bin",".exe",".dll",".bin",".msi")
    | extend AccountName = tostring(split(SourceUser, "@")[0]), AccountUPNSuffix = tostring(split(SourceUser, "@")[1])
entityMappings:
  - entityType: Account
    fieldMappings:
      - identifier: FullName
        columnName: SourceUser
      - identifier: Name
        columnName: AccountName
      - identifier: UPNSuffix
        columnName: AccountUPNSuffix
  - entityType: IP
    fieldMappings:
      - identifier: Address
        columnName: SourceIP
  - entityType: URL
    fieldMappings:
      - identifier: Url
        columnName: RequestURL
version: 1.1.4
kind: Scheduled
metadata:
    source:
        kind: Community
    author:
        name: Microsoft Security Research
    support:
        tier: Community
    categories:
        domains: [ "Security - Threat Protection" ]

Stages and Predicates

Parameters

let discord = dynamic(["cdn.discordapp.com", "media.discordapp.com"]);

Stage 1: `source`

_Im_WebSession(url_has_any=discord, eventresult='Success')

Stage 2: `where`

| where Url has "attachments"

Stage 3: `extend`

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

Stage 4: `summarize`

| summarize dcount(Url), make_set(SrcUsername), make_set(SrcIpAddr), make_set(Url), min(TimeGenerated), max(TimeGenerated), make_set(EventResult) by DiscordServerId

Stage 5: `mv-expand`

| mv-expand set_SrcUsername to typeof(string), set_Url to typeof(string), set_EventResult to typeof(string), set_SrcIpAddr to typeof(string)

Stage 6: `summarize`

| summarize by DiscordServerId, dcount_Url, set_SrcUsername, min_TimeGenerated, max_TimeGenerated, set_EventResult, set_SrcIpAddr, set_Url

Stage 7: `project`

| project StartTime=min_TimeGenerated, EndTime=max_TimeGenerated, Result=set_EventResult, SourceUser=set_SrcUsername, SourceIP=set_SrcIpAddr, RequestURL=set_Url

Stage 8: `where`

| where RequestURL has_any (".bin",".exe",".dll",".bin",".msi")

Stage 9: `extend`

| extend AccountName = tostring(split(SourceUser, "@")[0]), AccountUPNSuffix = tostring(split(SourceUser, "@")[1])

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
`RequestURL`	match	`.bin` `.dll` `.exe` `.msi`
`Url`	match	`attachments` transforms: `term`

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
`EndTime`	`project`
`RequestURL`	`project`
`Result`	`project`
`SourceIP`	`project`
`SourceUser`	`project`
`StartTime`	`project`
`AccountName`	`extend`
`AccountUPNSuffix`	`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)

Discord CDN Risky File Download (ASIM Web Session Schema)

MITRE ATT&CK coverage

Rule body kusto

Stages and Predicates

Parameters

Stage 1: `source`

Stage 2: `where`

Stage 3: `extend`

Stage 4: `summarize`

Stage 5: `mv-expand`

Stage 6: `summarize`

Stage 7: `project`

Stage 8: `where`

Stage 9: `extend`

Indicators

Output fields

Keyboard shortcuts

Search operators

Discord CDN Risky File Download (ASIM Web Session Schema)

MITRE ATT&CK coverage

Rule body kusto

Stages and Predicates

Parameters

Stage 1: source

Stage 2: where

Stage 3: extend

Stage 4: summarize

Stage 5: mv-expand

Stage 6: summarize

Stage 7: project

Stage 8: where

Stage 9: extend

Indicators

Output fields

Stage 1: `source`

Stage 2: `where`

Stage 3: `extend`

Stage 4: `summarize`

Stage 5: `mv-expand`

Stage 6: `summarize`

Stage 7: `project`

Stage 8: `where`

Stage 9: `extend`