Webshells Observed in Post-Compromised Exchange Servers  

CISA has added two new Malware Analysis Reports (MARs) to Alert AA21-062A: Mitigate Microsoft Exchange Server Vulnerabilities. Each new MAR (AR21-084A and AR21-084B) identifies a webshell observed in post-compromised Microsoft Exchange Servers. After successful exploiting a Microsoft Exchange Server vulnerability for initial accesses, a malicious cyber actor can upload a webshell to enable remote administration of the affected system.

CISA has also updated seven previously released MARs. The updated MARs now include CISA-developed YARA rules to help network defenders detect associated malware.

CISA encourages users and administrators to review the following resources for more information:

• CISA Alert AA21-062A: Mitigate Microsoft Exchange Server Vulnerabilities
• MAR-10328877-1.v1: China Chopper Webshell
• MAR-10328923-1.v1: China Chopper Webshell
• MAR-10329107-1.v1: China Chopper Webshell
• MAR-10329297-1.v1: China Chopper Webshell
• MAR-10329298-1.v1: China Chopper Webshell
• MAR-10329301-1.v1: China Chopper Webshell
• MAR-10329494-1.v1: China Chopper Webshell
• MAR-10329499-1.v1: China Chopper Webshell
• MAR-10329496-1.v1: China Chopper Webshell
• CISA web page Remediating Microsoft Exchange Vulnerabilities
• CISA web page Ransomware Guidance and Resources