Malformed user agent

Severity: medium
Time window: 1d
Group by: Account, AppDisplayName, ClientAppUsed, EventName, EventSource, Operation, OperationName, RecordType, SourceIP, Type, UserAgent, csMethod, csUriStem, host_s, httpMethod_s, requestQuery_s, requestUri_s, sSiteName
Author: Microsoft Security Research
Source: github.com/Azure/Azure-Sentinel

'Malware authors will sometimes hardcode user agent string values when writing the network communication component of their malware. Malformed user agents can be an indication of such malware.'

MITRE ATT&CK coverage

Tactic	Techniques
Initial Access	`T1189` Drive-by Compromise
Execution	`T1203` Exploitation for Client Execution
Command & Control	`T1071` Application Layer Protocol

Rule body kusto

id: a357535e-f722-4afe-b375-cff362b2b376
name: Malformed user agent
description: |
  'Malware authors will sometimes hardcode user agent string values when writing the network communication component of their malware. Malformed user agents can be an indication of such malware.'
severity: Medium
requiredDataConnectors:
  - connectorId: WAF
    dataTypes:
      - AzureDiagnostics
  - connectorId: Office365
    dataTypes:
      - OfficeActivity
  - connectorId: AzureActiveDirectory
    dataTypes:
      - SigninLogs
  - connectorId: AzureActiveDirectory
    dataTypes:
      - AADNonInteractiveUserSignInLogs
  - connectorId: AWS
    dataTypes:
      - AWSCloudTrail
  - connectorId: AzureMonitor(IIS)
    dataTypes:
      - W3CIISLog
queryFrequency: 1d
queryPeriod: 1d
triggerOperator: gt
triggerThreshold: 0
tactics:
  - InitialAccess
  - CommandAndControl
  - Execution
relevantTechniques:
  - T1189
  - T1071
  - T1203
query: |
  (union isfuzzy=true
  (OfficeActivity | where UserAgent != ""),
  (OfficeActivity
  | where RecordType in ("AzureActiveDirectory", "AzureActiveDirectoryStsLogon")
  | extend OperationName = Operation
  | parse ExtendedProperties with * 'User-Agent\\":\\"' UserAgent2 '\\' *
  | parse ExtendedProperties with * 'UserAgent",      "Value": "' UserAgent1 '"' *
  | where isnotempty(UserAgent1) or isnotempty(UserAgent2)
  | extend UserAgent = iff( RecordType == 'AzureActiveDirectoryStsLogon', UserAgent1, UserAgent2)
  | summarize StartTime = min(TimeGenerated), EndTime = max(TimeGenerated) by UserAgent, SourceIP = ClientIP, Account = UserId, Type, RecordType, Operation
  ),
  (AzureDiagnostics
  | where ResourceType =~ "APPLICATIONGATEWAYS"
  | where OperationName =~ "ApplicationGatewayAccess"
  | extend ClientIP = columnifexists("clientIP_s", "None"), UserAgent = columnifexists("userAgent_s", "None")
  | where UserAgent != '-'
  | summarize StartTime = min(TimeGenerated), EndTime = max(TimeGenerated) by UserAgent, SourceIP = ClientIP,  requestUri_s, httpMethod_s, host_s, requestQuery_s, Type
  ),
  (
  W3CIISLog
  | where isnotempty(csUserAgent)
  | summarize StartTime = min(TimeGenerated), EndTime = max(TimeGenerated) by UserAgent = csUserAgent, SourceIP = cIP, Account = csUserName, Type, sSiteName, csMethod, csUriStem
  ),
  (
  AWSCloudTrail
  | where isnotempty(UserAgent)
  | summarize StartTime = min(TimeGenerated), EndTime = max(TimeGenerated) by UserAgent, SourceIP = SourceIpAddress, Account = UserIdentityUserName, Type, EventSource, EventName
  ),
  (SigninLogs
  | where isnotempty(UserAgent)
  | summarize StartTime = min(TimeGenerated), EndTime = max(TimeGenerated) by UserAgent, SourceIP = IPAddress, Account = UserPrincipalName, Type, OperationName, tostring(LocationDetails), tostring(DeviceDetail), AppDisplayName, ClientAppUsed
  ),
  (AADNonInteractiveUserSignInLogs
  | where isnotempty(UserAgent)
  | summarize StartTime = min(TimeGenerated), EndTime = max(TimeGenerated) by UserAgent, SourceIP = IPAddress, Account = UserPrincipalName, Type, OperationName, tostring(LocationDetails), tostring(DeviceDetail), AppDisplayName, ClientAppUsed
  )
  )
  // Likely artefact of hardcoding
  | where UserAgent startswith "User" or UserAgent startswith '\"'
  // Incorrect casing
  or (UserAgent startswith "Mozilla" and not(UserAgent contains_cs "Mozilla"))
  // Incorrect casing
  or UserAgent contains_cs  "(Compatible;"
  // Missing MSIE version
  or UserAgent matches regex @"MSIE\s?;"
  // Incorrect spacing around MSIE version
  or UserAgent matches regex  @"MSIE(?:\d|.{1,5}?\d\s;)"
  | extend AccountName = split(Account, "@")[0], UPNSuffix = split(Account, "@")[1]
entityMappings:
  - entityType: Account
    fieldMappings:
      - identifier: FullName
        columnName: Account
      - identifier: Name
        columnName: AccountName
      - identifier: UPNSuffix
        columnName: UPNSuffix
  - entityType: IP
    fieldMappings:
      - identifier: Address
        columnName: SourceIP
version: 1.0.6
kind: Scheduled
metadata:
    source:
        kind: Community
    author:
        name: Microsoft Security Research
    support:
        tier: Community
    categories:
        domains: [ "Security - Threat Protection" ]

Stages and Predicates

union `isfuzzy=true` (7 sources)

Each leg below queries one source; the rule matches if any leg does. Sources: OfficeActivity, OfficeActivity, AzureDiagnostics, W3CIISLog, AWSCloudTrail, SigninLogs, AADNonInteractiveUserSignInLogs

Leg 1: `OfficeActivity`

OfficeActivity | where UserAgent != ""

Leg 2: `OfficeActivity`

OfficeActivity
| where RecordType in ("AzureActiveDirectory", "AzureActiveDirectoryStsLogon")
| extend OperationName = Operation
| parse ExtendedProperties with * 'User-Agent\\":\\"' UserAgent2 '\\' *
| parse ExtendedProperties with * 'UserAgent",      "Value": "' UserAgent1 '"' *
| where isnotempty(UserAgent1) or isnotempty(UserAgent2)
| extend UserAgent = iff( RecordType == 'AzureActiveDirectoryStsLogon', UserAgent1, UserAgent2)
| summarize StartTime = min(TimeGenerated), EndTime = max(TimeGenerated) by UserAgent, SourceIP = ClientIP, Account = UserId, Type, RecordType, Operation

Leg 3: `AzureDiagnostics`

AzureDiagnostics
| where ResourceType =~ "APPLICATIONGATEWAYS"
| where OperationName =~ "ApplicationGatewayAccess"
| extend ClientIP = columnifexists("clientIP_s", "None"), UserAgent = columnifexists("userAgent_s", "None")
| where UserAgent != '-'
| summarize StartTime = min(TimeGenerated), EndTime = max(TimeGenerated) by UserAgent, SourceIP = ClientIP,  requestUri_s, httpMethod_s, host_s, requestQuery_s, Type

Leg 4: `W3CIISLog`

W3CIISLog
| where isnotempty(csUserAgent)
| summarize StartTime = min(TimeGenerated), EndTime = max(TimeGenerated) by UserAgent = csUserAgent, SourceIP = cIP, Account = csUserName, Type, sSiteName, csMethod, csUriStem

Leg 5: `AWSCloudTrail`

AWSCloudTrail
| where isnotempty(UserAgent)
| summarize StartTime = min(TimeGenerated), EndTime = max(TimeGenerated) by UserAgent, SourceIP = SourceIpAddress, Account = UserIdentityUserName, Type, EventSource, EventName

Leg 6: `SigninLogs`

SigninLogs
| where isnotempty(UserAgent)
| summarize StartTime = min(TimeGenerated), EndTime = max(TimeGenerated) by UserAgent, SourceIP = IPAddress, Account = UserPrincipalName, Type, OperationName, tostring(LocationDetails), tostring(DeviceDetail), AppDisplayName, ClientAppUsed

Leg 7: `AADNonInteractiveUserSignInLogs`

AADNonInteractiveUserSignInLogs
| where isnotempty(UserAgent)
| summarize StartTime = min(TimeGenerated), EndTime = max(TimeGenerated) by UserAgent, SourceIP = IPAddress, Account = UserPrincipalName, Type, OperationName, tostring(LocationDetails), tostring(DeviceDetail), AppDisplayName, ClientAppUsed

Applied to the combined result

| where UserAgent startswith "User" or UserAgent startswith '\"'
or (UserAgent startswith "Mozilla" and not(UserAgent contains_cs "Mozilla"))
or UserAgent contains_cs  "(Compatible;"
or UserAgent matches regex @"MSIE\s?;"
or UserAgent matches regex  @"MSIE(?:\d|.{1,5}?\d\s;)"
| extend AccountName = split(Account, "@")[0], UPNSuffix = split(Account, "@")[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
`OperationName`	eq	`ApplicationGatewayAccess`
`RecordType`	in	`AzureActiveDirectory` transforms: `cased` `AzureActiveDirectoryStsLogon` transforms: `cased`
`ResourceType`	eq	`APPLICATIONGATEWAYS`
`UserAgent`	contains	`(Compatible;` transforms: `cased`
`UserAgent`	is_not_null	(no value, null check)
`UserAgent`	ne	`-` transforms: `cased`
`UserAgent`	regex_match	`MSIE(?:\d\|.{1,5}?\d\s;)` `MSIE\s?;`
`UserAgent`	starts_with	`Mozilla` `User` `\"`
`UserAgent1`	is_not_null	(no value, null check)
`UserAgent2`	is_not_null	(no value, null check)
`csUserAgent`	is_not_null	(no value, null check)

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
`Account`	`summarize`
`AppDisplayName`	`summarize`
`ClientAppUsed`	`summarize`
`EndTime`	`summarize`
`OperationName`	`summarize`
`SourceIP`	`summarize`
`StartTime`	`summarize`
`Type`	`summarize`
`UserAgent`	`summarize`
`AccountName`	`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)

Malformed user agent

MITRE ATT&CK coverage

Rule body kusto

Stages and Predicates

union `isfuzzy=true` (7 sources)

Leg 1: `OfficeActivity`

Leg 2: `OfficeActivity`

Leg 3: `AzureDiagnostics`

Leg 4: `W3CIISLog`

Leg 5: `AWSCloudTrail`

Leg 6: `SigninLogs`

Leg 7: `AADNonInteractiveUserSignInLogs`

Applied to the combined result

Indicators

Output fields

Keyboard shortcuts

Search operators

Malformed user agent

MITRE ATT&CK coverage

Rule body kusto

Stages and Predicates

union isfuzzy=true (7 sources)

Leg 1: OfficeActivity

Leg 2: OfficeActivity

Leg 3: AzureDiagnostics

Leg 4: W3CIISLog

Leg 5: AWSCloudTrail

Leg 6: SigninLogs

Leg 7: AADNonInteractiveUserSignInLogs

Applied to the combined result

Indicators

Output fields

union `isfuzzy=true` (7 sources)

Leg 1: `OfficeActivity`

Leg 2: `OfficeActivity`

Leg 3: `AzureDiagnostics`

Leg 4: `W3CIISLog`

Leg 5: `AWSCloudTrail`

Leg 6: `SigninLogs`

Leg 7: `AADNonInteractiveUserSignInLogs`