CVE-2026-64900 Overview
CVE-2026-64900 is a cross-site scripting (XSS) vulnerability in Microsoft Office SharePoint Server. The flaw stems from improper neutralization of user input during web page generation [CWE-79]. An authenticated attacker can inject crafted script content that executes in the context of another user's browser session, enabling spoofing attacks over the network. The vulnerability affects SharePoint Server Subscription Edition, SharePoint Server 2019, and SharePoint Server 2016 Enterprise. Exploitation requires low privileges and user interaction, and the scope changes because injected script runs beyond the vulnerable component. Microsoft published an advisory in the Security Update Guide addressing the issue.
Critical Impact
Authenticated attackers can inject malicious scripts into SharePoint pages to impersonate legitimate users, hijack sessions, and manipulate collaboration content across affected servers.
Affected Products
- Microsoft SharePoint Server Subscription Edition
- Microsoft SharePoint Server 2019
- Microsoft SharePoint Server 2016 Enterprise
Discovery Timeline
- 2026-08-11 - CVE-2026-64900 published to NVD
- 2026-08-12 - Last updated in NVD database
Technical Details for CVE-2026-64900
Vulnerability Analysis
The vulnerability resides in SharePoint's web page generation logic, where user-supplied input is rendered without adequate output encoding. An attacker with authenticated SharePoint access can submit crafted payloads containing HTML or JavaScript through fields that are later reflected or stored in rendered pages. When another user views the affected page, the browser executes the attacker's script in the SharePoint origin context.
Because SharePoint hosts collaborative content across many users, injected script can read authenticated session data, forge requests as the victim, and present spoofed interface elements. The advisory classifies the impact as spoofing over a network with limited confidentiality and integrity effects, and a scope change indicating the script escapes the vulnerable component boundary. The EPSS score of 0.363% reflects a low predicted probability of active exploitation within 30 days.
Root Cause
The root cause is missing or incomplete output encoding on attacker-controllable input during HTML rendering. SharePoint pages generate markup that embeds user-provided values without neutralizing metacharacters such as <, >, ", and '. This condition maps to [CWE-79], Improper Neutralization of Input During Web Page Generation.
Attack Vector
Exploitation requires the attacker to hold valid SharePoint credentials with permission to submit content, and a target user must interact with the poisoned page. The attacker crafts input containing script payloads, stores or reflects that input through a vulnerable SharePoint feature, and waits for a victim to load the resulting page. Successful execution allows the attacker to impersonate the victim within the SharePoint application.
No public proof-of-concept exploit is currently available. See the Microsoft Security Update Guide entry for CVE-2026-64900 for vendor technical details.
Detection Methods for CVE-2026-64900
Indicators of Compromise
- Unexpected <script>, javascript:, or event-handler attributes (onerror, onload) stored in SharePoint list items, document metadata, or web part properties.
- Outbound requests from user browsers to unfamiliar domains immediately after loading SharePoint pages.
- Anomalous SharePoint session activity such as requests originating from a user's session but performed at machine speed or from unusual geolocations.
Detection Strategies
- Inspect SharePoint Unified Logging Service (ULS) logs and IIS access logs for POST or PUT requests containing HTML control characters and script keywords.
- Deploy web application firewall rules that flag payloads containing <script, onerror=, or javascript: targeting SharePoint endpoints such as /Lists/, /_layouts/, and /_api/.
- Correlate authenticated content-modification events with subsequent page views by other users to surface stored XSS chains.
Monitoring Recommendations
- Monitor for privilege changes, permission grants, or delegation actions performed shortly after a user views crafted SharePoint content.
- Alert on Content Security Policy (CSP) violation reports from browsers accessing SharePoint origins.
- Track modifications to high-traffic SharePoint pages, wikis, and web parts made by accounts that do not normally author collaboration content.
How to Mitigate CVE-2026-64900
Immediate Actions Required
- Apply the Microsoft security update referenced in the MSRC advisory for CVE-2026-64900 to all affected SharePoint Server instances.
- Inventory SharePoint Server Subscription Edition, 2019, and 2016 Enterprise deployments and prioritize internet-facing servers for patching.
- Review recent user-generated content on high-value sites for embedded script or suspicious HTML.
Patch Information
Microsoft has issued security updates for the affected SharePoint Server versions. Refer to the Microsoft Security Update Guide entry for CVE-2026-64900 for the specific KB articles, build numbers, and installation prerequisites that apply to each supported SharePoint release.
Workarounds
- Restrict SharePoint content-authoring permissions to trusted users where feasible until patches are deployed.
- Enforce a strict Content Security Policy that disallows inline scripts on SharePoint origins to reduce the impact of injected payloads.
- Require multi-factor authentication for SharePoint access to raise the cost of credential-driven exploitation.
- Educate users to avoid opening unexpected SharePoint links, especially those shared by low-trust internal accounts.
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
