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Executive Summary
CVE-2021-34523 is a critical vulnerability in Microsoft Exchange Server that allows for elevation of privilege. This vulnerability is part of the ProxyShell exploit chain, which also includes CVE-2021-34473 and CVE-2021-31207. With a CVSS v3.1 base score of 9.8, this vulnerability poses a significant risk to organizations using affected versions of Exchange Server. The ProxyShell exploit chain has been actively exploited in the wild, making it imperative for organizations to understand the technical details, potential impacts, and mitigation strategies associated with CVE-2021-34523.
Technical Information
CVE-2021-34523 is an elevation of privilege vulnerability in Microsoft Exchange Server. The vulnerability allows attackers to gain elevated privileges on the Exchange Server, enabling them to execute arbitrary commands and potentially take over the server. The vulnerability is part of the ProxyShell exploit chain, which also includes CVE-2021-34473 and CVE-2021-31207. The CVSS v3.1 base score for CVE-2021-34523 is 9.8, indicating its critical nature.
The vulnerability affects the following versions of Microsoft Exchange Server: - Microsoft Exchange Server 2013 Cumulative Update 23 - Microsoft Exchange Server 2016 Cumulative Update 19 and 20 - Microsoft Exchange Server 2019 Cumulative Update 8 and 9
The attack vector for CVE-2021-34523 is network-based, with a low attack complexity and no required privileges or user interaction. The impact of a successful exploit includes complete confidentiality, integrity, and availability compromise of the affected Exchange Server.
Exploitation in the Wild
The ProxyShell exploit chain, which includes CVE-2021-34523, has been actively exploited in the wild. Attackers have used this vulnerability to gain elevated privileges on Exchange Servers, allowing them to execute arbitrary commands and potentially take over the server. Notable incidents include the exploitation of ProxyShell vulnerabilities to deploy ransomware and other malicious payloads.
Indicators of Compromise (IOCs) associated with the exploitation of CVE-2021-34523 include unusual PowerShell commands, unexpected changes in user privileges, and the presence of web shells on Exchange Servers. Organizations should monitor for these IOCs to detect potential exploitation attempts.
APT Groups using this vulnerability
While specific APT groups exploiting CVE-2021-34523 have not been publicly disclosed, the nature of the vulnerability and its inclusion in the ProxyShell exploit chain suggest that it could be leveraged by sophisticated threat actors. The ProxyShell vulnerabilities have been linked to attacks targeting various sectors, including government, healthcare, and financial services, across multiple countries.
Affected Product Versions
The following versions of Microsoft Exchange Server are affected by CVE-2021-34523: - Microsoft Exchange Server 2013 Cumulative Update 23 - Microsoft Exchange Server 2016 Cumulative Update 19 and 20 - Microsoft Exchange Server 2019 Cumulative Update 8 and 9
Organizations using these versions of Exchange Server should prioritize applying the necessary patches to mitigate the risk of exploitation.
Workaround and Mitigation
To mitigate the risk of exploitation of CVE-2021-34523, organizations should implement the following strategies:
Patch Management: Apply the latest cumulative updates provided by Microsoft for Exchange Server. The patches addressing CVE-2021-34523 are available in the following advisories: Microsoft Security Guidance.
Network Segmentation: Isolate Exchange servers from the rest of the network to limit the potential impact of a successful exploit.
Monitoring and Detection: Implement monitoring for unusual activity on Exchange servers, such as unexpected PowerShell commands or changes in user privileges.
Access Controls: Restrict access to Exchange servers to only authorized personnel and implement multi-factor authentication (MFA) to enhance security.
References
- NVD - CVE-2021-34523
- Microsoft Security Guidance
- CISA's Known Exploited Vulnerabilities Catalog
- SecPod Blog - Microsoft Exchange Servers Actively Under Exploitation via ProxyShell Vulnerabilities
- Packet Storm Security - Microsoft Exchange ProxyShell Remote Code Execution
- GitHub - ProxyShell POC
- Metasploit Framework - Exchange ProxyShell RCE
Rescana is here for you
At Rescana, we understand the critical importance of protecting your organization's digital assets. Our Continuous Threat and Exposure Management (CTEM) platform helps you stay ahead of emerging threats by providing real-time visibility into your security posture, identifying vulnerabilities, and offering actionable insights to mitigate risks. If you have any questions about this report or any other issue, please feel free to reach out to us at ops@rescana.com. We are here to help you navigate the complex landscape of cybersecurity and ensure the safety of your organization's information assets.
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