CVE-2021-26855 in Exchange Server (ProxyLogon)
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-2021-26857, CVE-2021-26858, CVE-2021-27065, CVE-2021-27078.
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Analysis
by VulDB Data Team • 06/06/2026
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 application layer, specifically in the handling of HTTP requests that process email messages and web services. The flaw exists in the server's parsing mechanisms for specific HTTP headers and request parameters, creating a path for malicious actors to inject and execute arbitrary code within the Exchange Server environment. This vulnerability is particularly concerning because it operates at the application layer and can be exploited through standard web traffic without requiring any prior authentication credentials or privileged access. The attack surface is broad as it affects multiple Exchange Server versions including 2016 and 2019, making it a significant threat across enterprise environments that rely on Microsoft's email infrastructure.
The technical implementation of this vulnerability involves a specific pattern of HTTP request manipulation that causes the Exchange Server to improperly handle certain input sequences, leading to code execution in the context of the Exchange Server application. Attackers can leverage this flaw by crafting malicious HTTP requests that trigger the vulnerable code path, potentially allowing them to gain full control over the Exchange Server instance. The vulnerability's exploitation requires no user interaction and can be executed entirely through network-based attacks, making it particularly dangerous for organizations with exposed Exchange Server instances. This type of vulnerability maps directly to CWE-121, which addresses stack-based buffer overflow conditions, and aligns with ATT&CK technique T1190 for exploitation of remote services. The flaw's nature suggests it may also relate to CWE-77 and CWE-78, indicating potential command injection vulnerabilities within the server's processing pipeline.
The operational impact of this vulnerability extends far beyond simple remote code execution, as successful exploitation can lead to complete compromise of the Exchange Server environment and potentially broader network infiltration. Once an attacker gains control of the Exchange Server, they can access email data, modify email content, create new email accounts, and potentially use the compromised server as a pivot point for attacking other systems within the network. The vulnerability's ability to execute code without authentication makes it particularly attractive to threat actors seeking persistent access to enterprise email systems, which often contain sensitive corporate data and serve as a critical communication infrastructure. Organizations may face significant data breaches, regulatory compliance violations, and reputational damage if this vulnerability is exploited successfully, especially since Exchange Server instances are commonly deployed in environments with high-value email data and privileged user accounts.
Mitigation strategies for this vulnerability should include immediate deployment of Microsoft's security patches and updates, which address the specific input validation flaws in the Exchange Server web application layer. Organizations should also implement network segmentation to limit access to Exchange Server instances and deploy intrusion detection systems to monitor for suspicious HTTP traffic patterns that may indicate exploitation attempts. Additional protective measures include disabling unnecessary Exchange Server web services, implementing strict firewall rules, and conducting thorough network monitoring for anomalous behavior that could indicate compromise. Security teams should also consider implementing application control policies and restricting write permissions to Exchange Server directories to limit the potential impact of successful exploitation. The vulnerability's classification as a remote code execution flaw necessitates immediate action, as the window for exploitation is typically short-lived once patches are released, but the potential for widespread compromise makes it a critical priority for enterprise security teams to address promptly.