Name:Windows Exfiltration Over C2 Via Powershell UploadString id:59e8bf41-7472-412a-90d3-00f3afa452e9 version:3 date:2024-09-30 author:Teoderick Contreras, Splunk status:production type:TTP Description:The following analytic identifies potential data exfiltration using the PowerShell `net.webclient` command with the `UploadString` method. It leverages PowerShell Script Block Logging to detect instances where this command is executed. This activity is significant as it may indicate an attempt to upload sensitive data, such as desktop screenshots or files, to an external or internal URI, often associated with malware like Winter-Vivern. If confirmed malicious, this could lead to unauthorized data transfer, compromising sensitive information and potentially leading to further exploitation of the compromised host. Data_source:
-Powershell Script Block Logging 4104
search:`powershell` EventCode=4104 ScriptBlockText = "*Net.webclient*" AND ScriptBlockText = "*.UploadString*" | stats count min(_time) as firstTime max(_time) as lastTime by EventCode ScriptBlockText Computer UserID | `security_content_ctime(firstTime)` | `security_content_ctime(lastTime)` | `windows_exfiltration_over_c2_via_powershell_uploadstring_filter`
how_to_implement:To successfully implement this analytic, you will need to enable PowerShell Script Block Logging on some or all endpoints. Additional setup here https://docs.splunk.com/Documentation/UBA/5.0.4.1/GetDataIn/AddPowerShell#Configure_module_logging_for_PowerShell. known_false_positives:False positives should be limited. Filter as needed. References: -https://twitter.com/_CERT_UA/status/1620781684257091584 -https://cert.gov.ua/article/3761104 drilldown_searches: name:'View the detection results for - "$Computer$"' search:'%original_detection_search% | search Computer = "$Computer$"' earliest_offset:'$info_min_time$' latest_offset:'$info_max_time$' name:'View risk events for the last 7 days for - "$Computer$"' search:'| from datamodel Risk.All_Risk | search normalized_risk_object IN ("$Computer$") 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: - 'Winter Vivern' asset_type:Endpoint confidence:70 impact:70 message:A PowerShell script on $Computer$ is attempting to transfer files to a remote URL. mitre_attack_id: - 'T1041' observable: name:'Computer' type:'Hostname' - role: - 'Victim' product: - 'Splunk Enterprise' - 'Splunk Enterprise Security' - 'Splunk Cloud' required_fields: - '_time' - 'ScriptBlockText' - 'Computer' - 'UserID' - 'EventCode' risk_score:49 security_domain:endpoint
