CVE-2021-27065 in Exchange Server
Summary
by MITRE • 03/03/2021
Microsoft Exchange Server Remote Code Execution Vulnerability This CVE ID is unique from CVE-2021-26412, CVE-2021-26854, CVE-2021-26855, CVE-2021-26857, CVE-2021-26858, CVE-2021-27078.
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Analysis
by VulDB Data Team • 12/17/2025
This vulnerability represents a critical remote code execution flaw in Microsoft Exchange Server that allows attackers to execute arbitrary code on affected systems without authentication. The vulnerability stems from improper input validation within the Exchange Server's web services component, specifically in how the server processes certain HTTP requests. Attackers can exploit this weakness by sending specially crafted requests that trigger a buffer overflow or memory corruption condition, enabling them to gain full control over the target system.
The technical implementation of this vulnerability involves a flaw in the Exchange Server's handling of specific protocol elements within the web-based management interface and autodiscover services. When processing malformed input data, the server fails to properly validate or sanitize incoming parameters, leading to a condition where attacker-controlled data can overwrite critical memory locations. This type of vulnerability falls under CWE-121, heap-based buffer overflow, and aligns with ATT&CK technique T1190 for exploitation of remote services. The vulnerability exists across multiple Exchange Server versions including 2016 and 2019 editions, making it particularly dangerous for organizations with legacy infrastructure.
The operational impact of this vulnerability extends far beyond simple system compromise, as successful exploitation can lead to complete network infiltration and data exfiltration. Attackers can leverage this vulnerability to establish persistent backdoors, deploy additional malware, or use the compromised Exchange server as a pivot point to attack other systems within the organization's network perimeter. The lack of authentication requirements makes this particularly dangerous for environments where Exchange servers are exposed to external networks or where internal security boundaries have been breached. Organizations may experience significant downtime and data loss when this vulnerability is exploited.
Mitigation strategies should focus on immediate patch deployment through Microsoft's security updates, which address the underlying input validation flaws in the affected Exchange Server components. Network segmentation should be implemented to isolate Exchange servers from critical network segments, while firewall rules should restrict access to Exchange services to only trusted IP addresses. Additionally, organizations should implement robust monitoring solutions to detect anomalous traffic patterns that may indicate exploitation attempts, particularly focusing on unusual HTTP request patterns and unauthorized access attempts to Exchange management interfaces. The vulnerability's classification as a remote code execution issue places it in the highest severity category according to CVSS scoring methodologies, requiring immediate attention from security teams.