Exchange OAB Virtual Directory Attribute Containing Potential Webshell

Status: available
Severity: high
Time window: 1h
Source: github.com/Azure/Azure-Sentinel

This query uses Windows Event ID 5136 in order to detect potential webshell deployment by exploitation of CVE-2021-27065. This query looks for changes to the InternalHostName or ExternalHostName properties of Exchange OAB Virtual Directory objects in AD Directory Services where the new objects contain potential webshell objects.

MITRE ATT&CK coverage

Tactic	Techniques
Initial Access	`T1190` Exploit Public-Facing Application

Event coverage

Provider	Event	Title
Security-Auditing	Event ID 5136	A directory service object was modified.

Rule body kusto

id: faf1a6ff-53b5-4f92-8c55-4b20e9957594
name: Exchange OAB Virtual Directory Attribute Containing Potential Webshell
description: |
  'This query uses Windows Event ID 5136 in order to detect potential webshell deployment by exploitation of CVE-2021-27065.
  This query looks for changes to the InternalHostName or ExternalHostName properties of Exchange OAB Virtual Directory objects in AD Directory Services where the new objects contain potential webshell objects.'
severity: High
requiredDataConnectors:
  - connectorId: SecurityEvents
    dataTypes:
      - SecurityEvent
  - connectorId: WindowsSecurityEvents
    dataTypes:
      - SecurityEvent
queryFrequency: 1h
queryPeriod: 1h
triggerOperator: gt
triggerThreshold: 0
status: Available
tactics:
  - InitialAccess
relevantTechniques:
  - T1190
query: |
  SecurityEvent
  // Look for specific Directory Service Changes and parse data
  | where EventID == 5136
  | extend EventData = parse_xml(EventData).EventData.Data
  | mv-expand bagexpansion = array EventData
  | evaluate bag_unpack(EventData)
  | extend Key = tostring(column_ifexists('@Name', "")), Value = column_ifexists('#text', "")
  | evaluate pivot(Key, any(Value),TimeGenerated, EventID, Computer, Account, AccountType, EventSourceName, Activity, SubjectAccount)
  // Where changes relate to Exchange OAB
  | extend ObjectClass = column_ifexists("ObjectClass", "")
  | where ObjectClass =~ "msExchOABVirtualDirectory"
  // Look for InternalHostName or ExternalHostName properties being changed
  | extend AttributeLDAPDisplayName = column_ifexists("AttributeLDAPDisplayName", "")
  | where AttributeLDAPDisplayName in~ ("msExchExternalHostName", "msExchInternalHostName")
  // Look for suspected webshell activity
  | extend AttributeValue = column_ifexists("AttributeValue", "")
  | where AttributeValue has "script"
  | project-rename LastSeen = TimeGenerated
  | extend ObjectDN = column_ifexists("ObjectDN", "")
  | project-reorder LastSeen, Computer, Account, ObjectDN, AttributeLDAPDisplayName, AttributeValue
  | extend timestamp = LastSeen
  | extend HostName = tostring(split(Computer, ".")[0]), DomainIndex = toint(indexof(Computer, '.'))
  | extend HostNameDomain = iff(DomainIndex != -1, substring(Computer, DomainIndex + 1), Computer)
  | extend AccountName = tostring(split(Account, @'\')[1]), AccountNTDomain = tostring(split(Account, @'\')[0])
entityMappings:
  - entityType: Account
    fieldMappings:
      - identifier: FullName
        columnName: Account
      - identifier: Name
        columnName: AccountName
      - identifier: NTDomain
        columnName: AccountNTDomain
  - entityType: Host
    fieldMappings:
      - identifier: FullName
        columnName: Computer
      - identifier: HostName
        columnName: HostName
      - identifier: DnsDomain
        columnName: HostNameDomain
version: 1.0.4
kind: Scheduled

Stages and Predicates

Stage 1: `source`

SecurityEvent

Stage 2: `where`

| where EventID == 5136

Stage 3: `extend`

| extend EventData = parse_xml(EventData).EventData.Data

Stage 4: `mv-expand`

| mv-expand bagexpansion = array EventData

Stage 5: `evaluate`

| evaluate bag_unpack(EventData)

Stage 6: `extend`

| extend Key = tostring(column_ifexists('@Name', "")), Value = column_ifexists('#text', "")

Stage 7: `evaluate`

| evaluate pivot(Key, any(Value),TimeGenerated, EventID, Computer, Account, AccountType, EventSourceName, Activity, SubjectAccount)

Stage 8: `extend`

| extend ObjectClass = column_ifexists("ObjectClass", "")

Stage 9: `where`

| where ObjectClass =~ "msExchOABVirtualDirectory"

Stage 10: `extend`

| extend AttributeLDAPDisplayName = column_ifexists("AttributeLDAPDisplayName", "")

Stage 11: `where`

| where AttributeLDAPDisplayName in~ ("msExchExternalHostName", "msExchInternalHostName")

Stage 12: `extend`

| extend AttributeValue = column_ifexists("AttributeValue", "")

Stage 13: `where`

| where AttributeValue has "script"

Stage 14: `project-rename`

| project-rename LastSeen = TimeGenerated

Stage 15: `extend`

| extend ObjectDN = column_ifexists("ObjectDN", "")

Stage 16: `project-reorder`

| project-reorder LastSeen, Computer, Account, ObjectDN, AttributeLDAPDisplayName, AttributeValue

Stage 17: `extend` (4 consecutive steps)

| extend timestamp = LastSeen
| extend HostName = tostring(split(Computer, ".")[0]), DomainIndex = toint(indexof(Computer, '.'))
| extend HostNameDomain = iff(DomainIndex != -1, substring(Computer, DomainIndex + 1), Computer)
| extend AccountName = tostring(split(Account, @'\')[1]), AccountNTDomain = tostring(split(Account, @'\')[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
`AttributeLDAPDisplayName`	in	`msExchExternalHostName` `msExchInternalHostName`
`AttributeValue`	match	`script` transforms: `term`
`EventID`	eq	`5136` transforms: `cased` corpus 30 (splunk 24, kusto 5, elastic 1)
`ObjectClass`	eq	`msExchOABVirtualDirectory`

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
`EventData`	`extend`
`Key`	`extend`
`Value`	`extend`
`ObjectClass`	`extend`
`AttributeLDAPDisplayName`	`extend`
`AttributeValue`	`extend`
`LastSeen`	`project-rename`
`ObjectDN`	`extend`
`timestamp`	`extend`
`DomainIndex`	`extend`
`HostName`	`extend`
`HostNameDomain`	`extend`
`AccountNTDomain`	`extend`
`AccountName`	`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)

Exchange OAB Virtual Directory Attribute Containing Potential Webshell

MITRE ATT&CK coverage

Event coverage

Rule body kusto

Stages and Predicates

Stage 1: `source`

Stage 2: `where`

Stage 3: `extend`

Stage 4: `mv-expand`

Stage 5: `evaluate`

Stage 6: `extend`

Stage 7: `evaluate`

Stage 8: `extend`

Stage 9: `where`

Stage 10: `extend`

Stage 11: `where`

Stage 12: `extend`

Stage 13: `where`

Stage 14: `project-rename`

Stage 15: `extend`

Stage 16: `project-reorder`

Stage 17: `extend` (4 consecutive steps)

Indicators

Output fields

Keyboard shortcuts

Search operators

Exchange OAB Virtual Directory Attribute Containing Potential Webshell

MITRE ATT&CK coverage

Event coverage

Rule body kusto

Stages and Predicates

Stage 1: source

Stage 2: where

Stage 3: extend

Stage 4: mv-expand

Stage 5: evaluate

Stage 6: extend

Stage 7: evaluate

Stage 8: extend

Stage 9: where

Stage 10: extend

Stage 11: where

Stage 12: extend

Stage 13: where

Stage 14: project-rename

Stage 15: extend

Stage 16: project-reorder

Stage 17: extend (4 consecutive steps)

Indicators

Output fields

Stage 1: `source`

Stage 2: `where`

Stage 3: `extend`

Stage 4: `mv-expand`

Stage 5: `evaluate`

Stage 6: `extend`

Stage 7: `evaluate`

Stage 8: `extend`

Stage 9: `where`

Stage 10: `extend`

Stage 11: `where`

Stage 12: `extend`

Stage 13: `where`

Stage 14: `project-rename`

Stage 15: `extend`

Stage 16: `project-reorder`

Stage 17: `extend` (4 consecutive steps)