CVE-2021-26857 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-2021-26858, CVE-2021-27065, CVE-2021-27078.
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Analysis
by VulDB Data Team • 05/26/2026
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-based management interface and underlying web services. Attackers can exploit this weakness by sending specially crafted HTTP requests that bypass authentication mechanisms and directly invoke vulnerable code paths within the server's architecture. The flaw specifically affects Exchange Server versions 2016 and 2019, making it particularly dangerous for organizations that have not yet applied the necessary security patches.
The technical implementation of this vulnerability involves a combination of HTTP header manipulation and improper validation of user-supplied data within the Exchange Server's web application framework. When the server processes malicious requests containing crafted parameters, it fails to properly sanitize the inputs before passing them to internal processing functions. This allows attackers to inject malicious code that executes with the privileges of the Exchange Server process, typically running with high system privileges. The vulnerability is particularly concerning because it operates at the application layer and can be exploited through standard web protocols, making it accessible to attackers with minimal technical expertise.
From an operational perspective, successful exploitation of this vulnerability can lead to complete compromise of the Exchange Server infrastructure. Attackers can establish persistent backdoors, exfiltrate sensitive email data, deploy additional malware, and use the compromised server as a launch point for lateral movement within the network. The impact extends beyond the immediate server compromise as Exchange servers often serve as central points for email communication and may contain access to other enterprise systems. Organizations with unpatched Exchange servers face significant risk of data breaches, regulatory violations, and potential financial losses due to extended compromise periods.
Security professionals should implement multiple layers of defense to mitigate this vulnerability. Immediate patching of affected Exchange Server versions is critical, as Microsoft has released security updates specifically addressing this flaw. Network segmentation and firewall rules should be implemented to restrict access to Exchange server ports and services, particularly limiting external access to the web-based management interface. Monitoring for suspicious HTTP requests and unusual authentication patterns can help detect exploitation attempts. Organizations should also conduct thorough vulnerability assessments to identify any potential compromise indicators and establish incident response procedures tailored to Exchange server security incidents. This vulnerability aligns with CWE-20, which addresses improper input validation, and maps to attack techniques in the ATT&CK framework under initial access and execution phases.