CVE-2026-55019 Overview
CVE-2026-55019 is a cross-site scripting (XSS) vulnerability in Microsoft Office SharePoint Server. The flaw stems from improper neutralization of user-supplied input during web page generation [CWE-79]. An authenticated attacker can inject script content that executes in the context of another user's browser session, enabling spoofing attacks over a network.
Exploitation requires low privileges and user interaction, and the scope changes when the payload executes across trust boundaries. Microsoft published the advisory on July 14, 2026, and rates the issue as medium severity.
Critical Impact
An authorized attacker can inject malicious script into SharePoint pages to spoof content, hijack user sessions, or steal information rendered within the victim's browser.
Affected Products
- Microsoft SharePoint Server Subscription Edition
- Microsoft SharePoint Server 2019
- Microsoft SharePoint Server 2016 Enterprise
Discovery Timeline
- 2026-07-14 - CVE-2026-55019 published to NVD
- 2026-07-14 - Microsoft releases security update for CVE-2026-55019
- 2026-07-15 - Last updated in NVD database
Technical Details for CVE-2026-55019
Vulnerability Analysis
The vulnerability is a stored or reflected cross-site scripting flaw within SharePoint Server web page generation logic. SharePoint fails to properly sanitize or encode attacker-controlled input before rendering it in HTML output. When another authenticated user loads the affected page, the browser executes the injected script under the SharePoint origin.
Because the CVSS scope is changed, the injected script can affect resources beyond the vulnerable component. This allows the attacker to spoof interface elements, exfiltrate session tokens, or trigger unauthorized actions on behalf of the victim within the SharePoint site collection.
Root Cause
The root cause is improper neutralization of input during web page generation, classified under [CWE-79]. SharePoint accepts input from an authenticated user and reflects it into a response without adequate HTML encoding, attribute encoding, or content security controls. The absence of contextual output escaping allows script tags or event handlers to survive in the rendered DOM.
Attack Vector
The attack vector is network-based. An attacker with low-privilege access to a SharePoint site submits crafted input into a field that is later rendered to other users. When a targeted user with valid credentials interacts with the poisoned page, the payload executes in their browser context.
Because user interaction is required, phishing links, shared documents, or comments referencing the malicious page can be used to lure victims. The changed scope permits impact on resources managed by a different security authority than the vulnerable SharePoint component.
No verified public proof-of-concept code is available. Refer to the Microsoft Security Update Guide for CVE-2026-55019 for authoritative technical details.
Detection Methods for CVE-2026-55019
Indicators of Compromise
- Unexpected <script> tags, javascript: URIs, or inline event handlers stored in SharePoint list items, document metadata, or page content
- Outbound requests from user browsers to unfamiliar domains immediately after loading SharePoint pages
- Anomalous session activity or actions performed by users who recently viewed content authored by low-privileged accounts
Detection Strategies
- Inspect SharePoint content databases and audit logs for HTML markup embedded in fields expected to hold plain text
- Monitor Internet Information Services (IIS) web logs for URL parameters containing encoded script payloads targeting SharePoint endpoints
- Deploy a web application firewall with rules that flag XSS patterns in requests to SharePoint sites
Monitoring Recommendations
- Enable SharePoint auditing for content modifications and review changes made by newly created or low-privilege accounts
- Correlate browser telemetry with SharePoint access logs to identify script execution originating from internal pages
- Track user-reported UI anomalies such as unexpected login prompts or altered page layouts within SharePoint sites
How to Mitigate CVE-2026-55019
Immediate Actions Required
- Apply the Microsoft security update for CVE-2026-55019 to all SharePoint Server 2016, 2019, and Subscription Edition deployments
- Review recently authored SharePoint content for embedded script markup and remove suspicious entries
- Rotate credentials and session tokens for users who may have interacted with affected pages
Patch Information
Microsoft has released updates addressing CVE-2026-55019. Administrators should consult the Microsoft Security Update Guide for CVE-2026-55019 for the specific KB articles and cumulative updates that apply to each SharePoint Server version in their environment.
Workarounds
- Restrict contribute and edit permissions on SharePoint sites to trusted users until patches are applied
- Enforce a strict Content Security Policy at the reverse proxy or load balancer layer to limit inline script execution
- Require multi-factor authentication for SharePoint access to reduce the value of stolen session artifacts
# Example: query installed SharePoint updates on a server
Get-HotFix | Where-Object { $_.Description -like '*SharePoint*' } | Sort-Object InstalledOn -Descending
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
