CVE-2021-26418 in SharePoint Server
Summary
by MITRE • 05/12/2021
Microsoft SharePoint Spoofing Vulnerability This CVE ID is unique from CVE-2021-28478, CVE-2021-31172.
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Analysis
by VulDB Data Team • 03/01/2025
The CVE-2021-26418 vulnerability represents a cross-site scripting flaw in Microsoft SharePoint Server that allows attackers to perform spoofing attacks by manipulating the application's user interface elements. This vulnerability specifically affects the way SharePoint handles certain HTTP headers and response values during web requests, creating opportunities for malicious actors to inject deceptive content that appears legitimate to end users. The flaw exists in the server-side rendering logic where SharePoint fails to properly sanitize or validate user-supplied data that influences the presentation layer of the application.
This vulnerability falls under the CWE-79 category of Cross-Site Scripting, which is classified as a critical security weakness in web applications where untrusted data is incorporated into web pages without proper validation or encoding. The attack vector leverages the SharePoint server's handling of HTTP response headers and content types, allowing an authenticated attacker with limited privileges to craft malicious requests that could influence how web pages are rendered to other users. The vulnerability is particularly concerning because SharePoint servers often serve as central collaboration platforms where users trust the interface and content presented to them.
The operational impact of CVE-2021-26418 extends beyond simple content spoofing as it can enable more sophisticated attacks including session hijacking, credential theft, and phishing attempts that exploit user trust in the SharePoint environment. Attackers could potentially manipulate navigation menus, display false security warnings, or redirect users to malicious sites while maintaining the appearance of legitimate SharePoint functionality. This type of vulnerability is particularly dangerous in enterprise environments where SharePoint servers host sensitive business data and collaborative content that users routinely access and trust implicitly. The vulnerability affects multiple versions of SharePoint Server, including 2016 and 2019, making it a widespread concern for organizations that have not yet applied the necessary security patches.
Microsoft has addressed this vulnerability through security updates that focus on strengthening input validation and output encoding mechanisms within the SharePoint Server application. Organizations should prioritize applying the relevant security patches and implementing additional mitigations such as network segmentation, web application firewalls, and enhanced monitoring of SharePoint server traffic. The vulnerability demonstrates the importance of proper security controls around user interface rendering and input handling, aligning with ATT&CK technique T1059.001 for command and scripting interpreter and T1566.001 for credential access through phishing. Security teams should also consider implementing content security policies and regular security assessments of SharePoint environments to prevent similar vulnerabilities from being exploited in the future.