A potentially malicious web request was executed against a web server

let queryperiod = 1d;
let mode = dynamic(['Blocked', 'Detected']);
let successCode = dynamic(['200', '101','204', '400','504','304','401','500']);
let sessionBin = 30m;
AGWFirewallLogs
| where TimeGenerated > ago(queryperiod)
| where Action  in (mode)
| sort by Hostname asc, ClientIp asc, TimeGenerated asc
| extend SessionBlockedStarted = row_window_session(TimeGenerated, queryperiod, 10m, ((ClientIp != prev(ClientIp)) or (Hostname != prev(Hostname))))
| summarize SessionBlockedEnded = max(TimeGenerated), SessionBlockedCount = count() by Hostname, ClientIp, SessionBlockedStarted
| extend TimeKey = range(bin(SessionBlockedStarted, sessionBin), bin(SessionBlockedEnded, sessionBin), sessionBin)
| mv-expand TimeKey to typeof(datetime)
| join kind = inner(
    AGWAccessLogs
    | where TimeGenerated > ago(queryperiod)
    | where  (isempty(HttpStatus) or HttpStatus in (successCode))
    | extend TimeKey = bin(TimeGenerated, sessionBin)
    | extend Hostname = coalesce(Host,OriginalHost)
    | extend ClientIp = tostring(ClientIp)
) on TimeKey, Hostname, ClientIp
| where TimeGenerated between (SessionBlockedStarted..SessionBlockedEnded)
| extend
    OriginalRequestUriWithArgs = column_ifexists("OriginalRequestUriWithArgs", ""),
    ServerStatus = column_ifexists("ServerStatus", "")
| summarize
    SuccessfulAccessCount = count(),
    UserAgents = make_set(UserAgent, 250),
    RequestURIs = make_set(RequestUri, 250),
    OriginalRequestURIs = make_set(OriginalRequestUriWithArgs, 250),
    SuccessCodes = make_set(HttpStatus, 250),
    SuccessCodes_BackendServer = make_set(ServerStatus, 250),
    take_any(SessionBlockedEnded, SessionBlockedCount)
    by Hostname, ClientIp, SessionBlockedStarted 
| where SessionBlockedCount > SuccessfulAccessCount
| extend BlockvsSuccessRatio = SessionBlockedCount/toreal(SuccessfulAccessCount)
| sort by BlockvsSuccessRatio desc, SessionBlockedStarted asc
| project-reorder SessionBlockedStarted, SessionBlockedEnded, Hostname, ClientIp, SessionBlockedCount, SuccessfulAccessCount, BlockvsSuccessRatio, SuccessCodes, RequestURIs, OriginalRequestURIs, UserAgents

status: Available
OriginalUri: https://github.com/Azure/Azure-Sentinel/blob/master/Solutions/Azure Web Application Firewall (WAF)/Analytic Rules/MaliciousWAFSessions.yaml
query: |
  let queryperiod = 1d;
  let mode = dynamic(['Blocked', 'Detected']);
  let successCode = dynamic(['200', '101','204', '400','504','304','401','500']);
  let sessionBin = 30m;
  AGWFirewallLogs
  | where TimeGenerated > ago(queryperiod)
  | where Action  in (mode)
  | sort by Hostname asc, ClientIp asc, TimeGenerated asc
  | extend SessionBlockedStarted = row_window_session(TimeGenerated, queryperiod, 10m, ((ClientIp != prev(ClientIp)) or (Hostname != prev(Hostname))))
  | summarize SessionBlockedEnded = max(TimeGenerated), SessionBlockedCount = count() by Hostname, ClientIp, SessionBlockedStarted
  | extend TimeKey = range(bin(SessionBlockedStarted, sessionBin), bin(SessionBlockedEnded, sessionBin), sessionBin)
  | mv-expand TimeKey to typeof(datetime)
  | join kind = inner(
      AGWAccessLogs
      | where TimeGenerated > ago(queryperiod)
      | where  (isempty(HttpStatus) or HttpStatus in (successCode))
      | extend TimeKey = bin(TimeGenerated, sessionBin)
      | extend Hostname = coalesce(Host,OriginalHost)
      | extend ClientIp = tostring(ClientIp)
  ) on TimeKey, Hostname, ClientIp
  | where TimeGenerated between (SessionBlockedStarted..SessionBlockedEnded)
  | extend
      OriginalRequestUriWithArgs = column_ifexists("OriginalRequestUriWithArgs", ""),
      ServerStatus = column_ifexists("ServerStatus", "")
  | summarize
      SuccessfulAccessCount = count(),
      UserAgents = make_set(UserAgent, 250),
      RequestURIs = make_set(RequestUri, 250),
      OriginalRequestURIs = make_set(OriginalRequestUriWithArgs, 250),
      SuccessCodes = make_set(HttpStatus, 250),
      SuccessCodes_BackendServer = make_set(ServerStatus, 250),
      take_any(SessionBlockedEnded, SessionBlockedCount)
      by Hostname, ClientIp, SessionBlockedStarted 
  | where SessionBlockedCount > SuccessfulAccessCount
  | extend BlockvsSuccessRatio = SessionBlockedCount/toreal(SuccessfulAccessCount)
  | sort by BlockvsSuccessRatio desc, SessionBlockedStarted asc
  | project-reorder SessionBlockedStarted, SessionBlockedEnded, Hostname, ClientIp, SessionBlockedCount, SuccessfulAccessCount, BlockvsSuccessRatio, SuccessCodes, RequestURIs, OriginalRequestURIs, UserAgents  
requiredDataConnectors:
- dataTypes:
  - AzureDiagnostics
  connectorId: WAF
tactics:
- InitialAccess
name: A potentially malicious web request was executed against a web server
relevantTechniques:
- T1190
severity: Medium
entityMappings:
- fieldMappings:
  - identifier: Address
    columnName: ClientIp
  entityType: IP
kind: Scheduled
queryFrequency: 1d
description: |
  'Detects unobstructed Web Application Firewall (WAF) activity in sessions where the WAF blocked incoming requests by computing the ratio between blocked requests and unobstructed WAF requests in these sessions (BlockvsSuccessRatio metric).
  A high ratio value for a given client IP and hostname calls for further investigation of the WAF data in that session, due to the significantly high number of blocked requests and a few unobstructed logs that may be malicious but have passed undetected through the WAF. The successCode variable defines what the detection thinks is a successful status code and should be altered to fit the environment.'  
triggerThreshold: 0
triggerOperator: gt
version: 1.0.6
queryPeriod: 1d
id: 46ac55ae-47b8-414a-8f94-89ccd1962178