^{CVE-2021-31207:}Microsoft Exchange Server Security Feature Bypass Vulnerability (CVE-2021-31207) that allows attackers to bypass security features and potentially execute code remotely.

Overview

This vulnerability is one of three vulnerabilities collectively known as "ProxyShell" that affect Microsoft Exchange Server. The security feature bypass vulnerability allows attackers to circumvent normal security controls in Exchange Server. When exploited as part of the ProxyShell attack chain, it enables attackers to upload arbitrary files to the server, potentially leading to remote code execution. The vulnerability affects various versions of Microsoft Exchange Server and has been actively exploited in the wild. It has a CVSS base score that indicates high severity due to its potential impact on confidentiality, integrity, and availability of affected systems.

Remediation

To remediate this vulnerability, organizations should:

1. Apply the security updates provided by Microsoft for affected Exchange Server versions immediately.
2. Ensure that all Exchange servers are updated to the latest cumulative update and security update.
3. Monitor for signs of exploitation, including unusual file creation or modification in Exchange directories.
4. Implement network segmentation to limit access to Exchange servers.
5. Consider implementing additional security controls such as web application firewalls that can detect and block exploitation attempts.
6. Review logs for indicators of compromise, particularly focusing on unusual authentication attempts and file operations.
7. If exploitation is suspected, conduct a thorough investigation of affected systems and consider rebuilding from known good sources.

References

1. Microsoft Security Advisory: https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-31207
2. Zero Day Initiative Advisory: https://www.zerodayinitiative.com/advisories/ZDI-21-819/
3. Exploit Details: http://packetstormsecurity.com/files/163895/Microsoft-Exchange-ProxyShell-Remote-Code-Execution.html
4. Common Weakness Enumeration: CWE-434 (Unrestricted Upload of File with Dangerous Type)
5. ProxyShell technical analysis from security researchers (search for latest publications)
6. Microsoft Exchange Server security updates page for patch information