Adversaries may be attempting to brute force credentials by generating a high volume of failed logon attempts against a user account on IIS. SOC teams should proactively hunt for this behavior to identify potential credential compromise and prevent unauthorized access to critical systems.
KQL Query
let timeBin = 10m;
let failedThreshold = 100;
W3CIISLog
| where scStatus in ("401","403")
| where csUserName != "-"
// Handling Exchange specific items in IIS logs to remove the unique log identifier in the URI
| extend csUriQuery = iff(csUriQuery startswith "MailboxId=", tostring(split(csUriQuery, "&")[0]) , csUriQuery )
| extend csUriQuery = iff(csUriQuery startswith "X-ARR-CACHE-HIT=", strcat(tostring(split(csUriQuery, "&")[0]),tostring(split(csUriQuery, "&")[1])) , csUriQuery )
| extend scStatusFull = strcat(scStatus, ".",scSubStatus)
// Map common IIS codes
| extend scStatusFull_Friendly = case(
scStatusFull == "401.0", "Access denied.",
scStatusFull == "401.1", "Logon failed.",
scStatusFull == "401.2", "Logon failed due to server configuration.",
scStatusFull == "401.3", "Unauthorized due to ACL on resource.",
scStatusFull == "401.4", "Authorization failed by filter.",
scStatusFull == "401.5", "Authorization failed by ISAPI/CGI application.",
scStatusFull == "403.0", "Forbidden.",
scStatusFull == "403.4", "SSL required.",
"See - https://support.microsoft.com/help/943891/the-http-status-code-in-iis-7-0-iis-7-5-and-iis-8-0")
// Mapping to Hex so can be mapped using website in comments above
| extend scWin32Status_Hex = tohex(tolong(scWin32Status))
// Map common win32 codes
| extend scWin32Status_Friendly = case(
scWin32Status_Hex =~ "775", "The referenced account is currently locked out and cannot be logged on to.",
scWin32Status_Hex =~ "52e", "Logon failure: Unknown user name or bad password.",
scWin32Status_Hex =~ "532", "Logon failure: The specified account password has expired.",
scWin32Status_Hex =~ "533", "Logon failure: Account currently disabled.",
scWin32Status_Hex =~ "2ee2", "The request has timed out.",
scWin32Status_Hex =~ "0", "The operation completed successfully.",
scWin32Status_Hex =~ "1", "Incorrect function.",
scWin32Status_Hex =~ "2", "The system cannot find the file specified.",
scWin32Status_Hex =~ "3", "The system cannot find the path specified.",
scWin32Status_Hex =~ "4", "The system cannot open the file.",
scWin32Status_Hex =~ "5", "Access is denied.",
scWin32Status_Hex =~ "8009030e", "SEC_E_NO_CREDENTIALS",
scWin32Status_Hex =~ "8009030C", "SEC_E_LOGON_DENIED",
"See - https://msdn.microsoft.com/library/cc231199.aspx")
// decode URI when available
| extend decodedUriQuery = url_decode(csUriQuery)
// Count of failed logons by a user
| summarize makeset(decodedUriQuery), makeset(cIP), makeset(sSiteName), makeset(sPort), makeset(csUserAgent), makeset(csMethod), makeset(csUriQuery), makeset(scStatusFull), makeset(scStatusFull_Friendly), makeset(scWin32Status_Hex), makeset(scWin32Status_Friendly), FailedConnectionsCount = count() by bin(TimeGenerated, timeBin), csUserName, Computer, sIP
| where FailedConnectionsCount >= failedThreshold
| project TimeGenerated, csUserName, set_decodedUriQuery, Computer, set_sSiteName, sIP, set_cIP, set_sPort, set_csUserAgent, set_csMethod, set_scStatusFull, set_scStatusFull_Friendly, set_scWin32Status_Hex, set_scWin32Status_Friendly, FailedConnectionsCount
| order by FailedConnectionsCount
| extend HostName = tostring(split(Computer, ".")[0]), DomainIndex = toint(indexof(Computer, '.'))
| extend HostNameDomain = iff(DomainIndex != -1, substring(Computer, DomainIndex + 1), Computer)
| extend AccountName = tostring(split(csUserName, "@")[0]), AccountUPNSuffix = tostring(split(csUserName, "@")[1])
id: 884c4957-70ea-4f57-80b9-1bca3890315b
name: High count of failed logons by a user
description: |
'Identifies when 100 or more failed attempts by a given user in 10 minutes occur on the IIS Server.
This could be indicative of attempted brute force based on known account information.
This could also simply indicate a misconfigured service or device.
References:
IIS status code mapping - https://support.microsoft.com/help/943891/the-http-status-code-in-iis-7-0-iis-7-5-and-iis-8-0
Win32 Status code mapping - https://msdn.microsoft.com/library/cc231199.aspx'
severity: Medium
requiredDataConnectors:
- connectorId: AzureMonitor(IIS)
dataTypes:
- W3CIISLog
queryFrequency: 1d
queryPeriod: 1d
triggerOperator: gt
triggerThreshold: 0
tactics:
- CredentialAccess
relevantTechniques:
- T1110
query: |
let timeBin = 10m;
let failedThreshold = 100;
W3CIISLog
| where scStatus in ("401","403")
| where csUserName != "-"
// Handling Exchange specific items in IIS logs to remove the unique log identifier in the URI
| extend csUriQuery = iff(csUriQuery startswith "MailboxId=", tostring(split(csUriQuery, "&")[0]) , csUriQuery )
| extend csUriQuery = iff(csUriQuery startswith "X-ARR-CACHE-HIT=", strcat(tostring(split(csUriQuery, "&")[0]),tostring(split(csUriQuery, "&")[1])) , csUriQuery )
| extend scStatusFull = strcat(scStatus, ".",scSubStatus)
// Map common IIS codes
| extend scStatusFull_Friendly = case(
scStatusFull == "401.0", "Access denied.",
scStatusFull == "401.1", "Logon failed.",
scStatusFull == "401.2", "Logon failed due to server configuration.",
scStatusFull == "401.3", "Unauthorized due to ACL on resource.",
scStatusFull == "401.4", "Authorization failed by filter.",
scStatusFull == "401.5", "Authorization failed by ISAPI/CGI application.",
scStatusFull == "403.0", "Forbidden.",
scStatusFull == "403.4", "SSL required.",
"See - https://support.microsoft.com/help/943891/the-http-status-code-in-iis-7-0-iis-7-5-and-iis-8-0")
// Mapping to Hex so can be mapped using website in comments above
| extend scWin32Status_Hex = tohex(tolong(scWin32Status))
// Map common win32 codes
| extend scWin32Status_Friendly = case(
scWin32Status_Hex =~ "775", "The referenced account is currently locked out and cannot be logged on to.",
scWin32Status_Hex =~ "52e", "Logon failure: Unknown user name or bad password.",
scWin32Status_Hex =~ "532", "Logon failure: The specified account password has expired.",
scWin32Status_Hex =~ "533", "Logon failure: Account currently disabled.",
scWin32Status_Hex =~ "2ee2", "The request has timed out.",
scWin32Status_Hex =~ "0", "The operation completed successfully.",
scWin32Status_Hex =~ "1", "Incorrect function.",
scWin32Status_Hex =~ "2", "The system cannot find the file specified.",
scWin32Status_Hex =~ "3", "The system cannot find the path specified.",
scWin32Status_Hex =~ "4", "The system cannot open the file.",
scWin32Status_Hex =~ "5", "Acces| Sentinel Table | Notes |
|---|---|
W3CIISLog | Ensure this data connector is enabled |
Scenario: Scheduled System Maintenance or Patching
Description: A system administrator runs a scheduled maintenance task that temporarily generates a high number of failed logon attempts due to incorrect credentials during a script execution.
Filter/Exclusion: Exclude events where the source IP is from the internal network and the user is a known admin account (e.g., domain\Administrator), or use a filter like src_ip in (10.0.0.0/8) and user = domain\Administrator.
Scenario: User Testing or Password Reset Process
Description: A user is testing a new password or attempting to reset their password, which results in multiple failed logon attempts before the correct credentials are entered.
Filter/Exclusion: Exclude events where the user is actively resetting their password (e.g., using PasswordResetTool.exe or Active Directory Password Reset tool), or filter by user = [specific user] and event_id = 4625 with a time window of 5 minutes.
Scenario: Automated Job or Script Execution
Description: A scheduled job or script (e.g., PowerShell.exe, Task Scheduler, or SQL Server Agent) attempts to connect to a service using incorrect credentials during testing or configuration.
Filter/Exclusion: Exclude events where the process name is known to be part of a scheduled job (e.g., PowerShell.exe with a specific command line or Task Scheduler task name), or filter by process_name = 'PowerShell.exe' and command_line contains 'Test-Connection'.
Scenario: Multi-Factor Authentication (MFA) Failures
Description: A user is attempting to log in with MFA enabled, and the system logs failed logon attempts due to MFA verification failure (e.g., incorrect code or expired