Name:Windows Unusual Count Of Invalid Users Failed To Auth Using NTLM id:15603165-147d-4a6e-9778-bd0ff39e668f version:4 date:2024-09-30 author:Mauricio Velazco, Splunk status:production type:Anomaly Description:The following analytic identifies a source endpoint failing to authenticate with multiple invalid users using the NTLM protocol. It leverages EventCode 4776 and calculates the standard deviation for each host, using the 3-sigma rule to detect anomalies. This behavior is significant as it may indicate a Password Spraying attack, where an adversary attempts to gain initial access or elevate privileges. If confirmed malicious, this activity could lead to unauthorized access or privilege escalation, posing a significant threat to the Active Directory environment. This detection is focused on domain controllers. Data_source:
-Windows Event Log Security 4776
search:`wineventlog_security` EventCode=4776 TargetUserName!=*$ Status=0xc0000064 | bucket span=2m _time | stats dc(TargetUserName) AS unique_accounts values(TargetUserName) as user by _time, Workstation | eventstats avg(unique_accounts) as comp_avg , stdev(unique_accounts) as comp_std by Workstation | eval upperBound=(comp_avg+comp_std*3) | eval isOutlier=if(unique_accounts > 10 and unique_accounts >= upperBound, 1, 0) | search isOutlier=1 | rename Workstation as src |`windows_unusual_count_of_invalid_users_failed_to_auth_using_ntlm_filter`
how_to_implement:To successfully implement this search, you need to be ingesting Domain Controller events. The Advanced Security Audit policy setting `Audit Credential Validation' within `Account Logon` needs to be enabled. known_false_positives:A host failing to authenticate with multiple invalid domain users is not a common behavior for legitimate systems. Possible false positive scenarios include but are not limited to vulnerability scanners and missconfigured systems. If this detection triggers on a host other than a Domain Controller, the behavior could represent a password spraying attack against the host's local accounts. References: -https://attack.mitre.org/techniques/T1110/003/ -https://docs.microsoft.com/en-us/windows/security/threat-protection/auditing/audit-credential-validation -https://docs.microsoft.com/en-us/windows/security/threat-protection/auditing/event-4776 drilldown_searches: name:'View the detection results for - "$user$"' search:'%original_detection_search% | search user = "$user$"' earliest_offset:'$info_min_time$' latest_offset:'$info_max_time$' name:'View risk events for the last 7 days for - "$user$"' search:'| from datamodel Risk.All_Risk | search normalized_risk_object IN ("$user$") starthoursago=168 | stats count min(_time) as firstTime max(_time) as lastTime values(search_name) as "Search Name" values(risk_message) as "Risk Message" values(analyticstories) as "Analytic Stories" values(annotations._all) as "Annotations" values(annotations.mitre_attack.mitre_tactic) as "ATT&CK Tactics" by normalized_risk_object | `security_content_ctime(firstTime)` | `security_content_ctime(lastTime)`' earliest_offset:'$info_min_time$' latest_offset:'$info_max_time$' tags: analytic_story: - 'Active Directory Password Spraying' - 'Volt Typhoon' asset_type:Endpoint confidence:70 impact:70 message:Potential NTLM based password spraying attack from $src$ mitre_attack_id: - 'T1110.003' - 'T1110' observable: name:'user' type:'User' - role: - 'Victim' name:'src' type:'Endpoint' - role: - 'Attacker' product: - 'Splunk Enterprise' - 'Splunk Enterprise Security' - 'Splunk Cloud' required_fields: - '_time' - 'EventCode' - 'TargetUserName' - 'Workstation' - 'Status' risk_score:49 security_domain:endpoint