CVE-2026-55021 in SharePoint Server
Summary
by MITRE • 07/14/2026
Improper neutralization of input during web page generation ('cross-site scripting') in Microsoft Office SharePoint allows an authorized attacker to perform spoofing over a network.
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Analysis
by VulDB Data Team • 07/14/2026
This vulnerability represents a classic cross-site scripting flaw that exists within Microsoft Office SharePoint's web page generation functionality. The issue stems from inadequate input validation and sanitization mechanisms that fail to properly neutralize malicious user-supplied data before incorporating it into dynamically generated web content. When SharePoint processes user inputs through its rendering pipeline, it does not sufficiently escape or encode special characters that could be interpreted as executable script code by web browsers. This weakness creates a persistent security gap where authorized attackers with sufficient privileges can inject malicious JavaScript payloads into web pages that are subsequently served to other users.
The technical exploitation of this vulnerability occurs when an attacker leverages their authorized access to SharePoint to manipulate input fields, document libraries, or any area where user-generated content is processed and displayed. The flaw specifically manifests during the page generation phase where SharePoint fails to properly sanitize data before rendering it in HTML contexts. This allows attackers to inject script tags, event handlers, or other malicious code sequences that execute within the victim's browser context when they view affected pages. The vulnerability operates at the application layer and can be classified under CWE-79 as improper neutralization of input during web page generation, which directly maps to the well-known OWASP Top Ten category of XSS vulnerabilities.
From an operational impact perspective, this vulnerability enables sophisticated social engineering attacks where attackers can craft convincing spoofing campaigns that appear legitimate to end users. The authorized access requirement means that attackers typically need to have valid credentials or have already compromised accounts within the SharePoint environment. However, once exploited, the attack can escalate beyond simple data theft to full session hijacking, credential harvesting, and privilege escalation within the SharePoint ecosystem. The impact extends to all users who may encounter the malicious content, potentially affecting thousands of individuals depending on the scope of the compromised SharePoint deployment.
The security implications align with several ATT&CK techniques including T1566 for spearphishing with a link, T1059 for command and scripting interpreter, and T1531 for account access removal. Organizations should implement comprehensive input validation at multiple layers including client-side sanitization, server-side encoding of all user-supplied content, and regular security testing of SharePoint applications. Mitigation strategies include deploying web application firewalls, implementing strict content security policies, enabling automatic input encoding, and conducting regular security assessments. Additionally, organizations should enforce principle of least privilege access controls to limit the potential damage from compromised accounts, while ensuring proper monitoring and logging of all user activities within SharePoint environments to detect anomalous behavior patterns that might indicate exploitation attempts.