Analytics Story: ProxyShell

Date: 2021-08-24 ID: 413bb68e-04e2-11ec-a835-acde48001122 Author: Michael Haag, Teoderick Contreras, Mauricio Velazco, Splunk Product: Splunk Enterprise Security

Description

ProxyShell is a chain of exploits targeting on-premise Microsoft Exchange Server - CVE-2021-34473, CVE-2021-34523, and CVE-2021-31207.

Why it matters

During Pwn2Own April 2021, a security researcher demonstrated an attack chain targeting on-premise Microsoft Exchange Server. August 5th, the same researcher publicly released further details and demonstrated the attack chain. CVE-2021-34473 Pre-auth path confusion leads to ACL Bypass (Patched in April by KB5001779) CVE-2021-34523 - Elevation of privilege on Exchange PowerShell backend (Patched in April by KB5001779) . CVE-2021-31207 - Post-auth Arbitrary-File-Write leads to RCE (Patched in May by KB5003435) Upon successful exploitation, the remote attacker will have SYSTEM privileges on the Exchange Server. In addition to remote access/execution, the adversary may be able to run Exchange PowerShell Cmdlets to perform further actions.

1| tstats `security_content_summariesonly` min(_time) as firstTime max(_time) as lastTime sum(All_Risk.calculated_risk_score) as risk_score, count(All_Risk.calculated_risk_score) as risk_event_count, values(All_Risk.annotations.mitre_attack.mitre_tactic_id) as annotations.mitre_attack.mitre_tactic_id, dc(All_Risk.annotations.mitre_attack.mitre_tactic_id) as mitre_tactic_id_count, values(All_Risk.analyticstories) as analyticstories values(All_Risk.annotations.mitre_attack.mitre_technique_id) as annotations.mitre_attack.mitre_technique_id, dc(All_Risk.annotations.mitre_attack.mitre_technique_id) as mitre_technique_id_count, values(All_Risk.tag) as tag, values(source) as source, dc(source) as source_count dc(All_Risk.analyticstories) as dc_analyticstories from datamodel=Risk.All_Risk where All_Risk.analyticstories IN ("ProxyNotShell","ProxyShell") OR (All_Risk.analyticstories IN ("ProxyNotShell","ProxyShell") AND All_Risk.analyticstories="Cobalt Strike") All_Risk.risk_object_type="system" by _time span=1h All_Risk.risk_object All_Risk.risk_object_type | `drop_dm_object_name(All_Risk)` | `security_content_ctime(firstTime)` | `security_content_ctime(lastTime)`| where source_count >=5 | `proxyshell_proxynotshell_behavior_detected_filter`

Detections

Name ▲▼ Technique ▲▼ Type ▲▼
Detect Exchange Web Shell External Remote Services, Exploit Public-Facing Application, Web Shell TTP
Exchange PowerShell Abuse via SSRF Exploit Public-Facing Application, External Remote Services TTP
Exchange PowerShell Module Usage PowerShell TTP
MS Exchange Mailbox Replication service writing Active Server Pages External Remote Services, Exploit Public-Facing Application, Web Shell TTP
W3WP Spawning Shell Web Shell TTP
Windows MSExchange Management Mailbox Cmdlet Usage PowerShell Anomaly
Windows Suspicious Child Process Spawned From WebServer Web Shell TTP
Windows Exchange Autodiscover SSRF Abuse Exploit Public-Facing Application, External Remote Services TTP

Data Sources

Name ▲▼ Platform ▲▼ Sourcetype ▲▼ Source ▲▼
CrowdStrike ProcessRollup2 N/A crowdstrike:events:sensor crowdstrike
Powershell Script Block Logging 4104 Windows icon Windows xmlwineventlog XmlWinEventLog:Microsoft-Windows-PowerShell/Operational
Sysmon EventID 1 Windows icon Windows xmlwineventlog XmlWinEventLog:Microsoft-Windows-Sysmon/Operational
Sysmon EventID 11 Windows icon Windows xmlwineventlog XmlWinEventLog:Microsoft-Windows-Sysmon/Operational
Windows Event Log Security 4688 Windows icon Windows xmlwineventlog XmlWinEventLog:Security
Windows IIS Windows icon Windows IIS:Configuration:Operational IIS:Configuration:Operational

References

Source: GitHub | Version: 1