ProxyShell

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Description

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

• Product: Splunk Enterprise, Splunk Enterprise Security, Splunk Cloud
• Datamodel: Endpoint, Risk, Web
• Last Updated: 2021-08-24
• Author: Michael Haag, Teoderick Contreras, Mauricio Velazco, Splunk
• ID: 413bb68e-04e2-11ec-a835-acde48001122

Narrative

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.

Detections

Name	Technique	Type
Detect Exchange Web Shell	Server Software Component, Web Shell, Exploit Public-Facing Application	TTP
Exchange PowerShell Abuse via SSRF	Exploit Public-Facing Application	TTP
Exchange PowerShell Module Usage	Command and Scripting Interpreter, PowerShell	TTP
MS Exchange Mailbox Replication service writing Active Server Pages	Server Software Component, Web Shell, Exploit Public-Facing Application	TTP
ProxyShell ProxyNotShell Behavior Detected	Exploit Public-Facing Application	Correlation
W3WP Spawning Shell	Server Software Component, Web Shell	TTP
Windows Exchange Autodiscover SSRF Abuse	Exploit Public-Facing Application	TTP
Windows Exchange PowerShell Module Usage	Command and Scripting Interpreter, PowerShell	TTP
Windows MSExchange Management Mailbox Cmdlet Usage	Command and Scripting Interpreter, PowerShell	Anomaly

Reference

• https://y4y.space/2021/08/12/my-steps-of-reproducing-proxyshell/
• https://www.zerodayinitiative.com/blog/2021/8/17/from-pwn2own-2021-a-new-attack-surface-on-microsoft-exchange-proxyshell
• https://www.youtube.com/watch?v=FC6iHw258RI
• https://www.huntress.com/blog/rapid-response-microsoft-exchange-servers-still-vulnerable-to-proxyshell-exploit#what-should-you-do
• https://i.blackhat.com/USA21/Wednesday-Handouts/us-21-ProxyLogon-Is-Just-The-Tip-Of-The-Iceberg-A-New-Attack-Surface-On-Microsoft-Exchange-Server.pdf
• https://www.inversecos.com/2022/07/hunting-for-apt-abuse-of-exchange.html

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