CVE-2026-62829: Microsoft SharePoint Server XSS Vulnerability

CVE-2026-62829 Overview

CVE-2026-62829 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 over a network. Successful exploitation requires user interaction, such as clicking a crafted link or loading a manipulated SharePoint page. The vulnerability affects Microsoft SharePoint Server Subscription Edition and SharePoint Server 2019.

Critical Impact
An authorized attacker can inject malicious script into SharePoint pages to spoof content, hijack user sessions, or steal sensitive information rendered in a victim's browser.

Affected Products

Microsoft SharePoint Server Subscription Edition
Microsoft SharePoint Server 2019
Microsoft Office SharePoint (web-facing components)

Discovery Timeline

2026-08-11 - CVE-2026-62829 published to the National Vulnerability Database
2026-08-11 - Last updated in NVD database

Technical Details for CVE-2026-62829

Vulnerability Analysis

The vulnerability exists in SharePoint's web page generation logic, where user-controllable input is written to server-rendered HTML without adequate sanitization or output encoding. An attacker with authenticated access to a SharePoint site can submit crafted payloads through input fields, list items, or web part properties. When another user views the affected page, the browser interprets the injected script as legitimate content originating from the trusted SharePoint domain.

The scope change in the impact profile indicates that injected script executes beyond the vulnerable component's original security boundary. This behavior is consistent with reflected or stored XSS in a same-site context, which allows spoofed UI elements and manipulated content to appear authoritative to the victim.

Root Cause

The root cause is improper neutralization of input during web page generation, categorized under [CWE-79]. SharePoint fails to encode or strip HTML and JavaScript metacharacters before embedding attacker-supplied values into the DOM. This allows <script> blocks, event handler attributes, and other executable constructs to survive rendering.

Attack Vector

Exploitation requires an authenticated user with permission to submit content into the vulnerable SharePoint context. The attacker plants a script payload in a location visible to other users, then relies on a victim to visit or interact with the page. Because the payload executes under the SharePoint origin, attackers can read authenticated session data, perform actions as the victim, and present spoofed content that appears legitimate. No verified public proof-of-concept code is available for this vulnerability. See the Microsoft Security Update Guide for vendor-specific technical details.

Detection Methods for CVE-2026-62829

Indicators of Compromise

Unexpected <script> tags, javascript: URIs, or inline event handlers stored in SharePoint list items, page fields, or web part properties.
HTTP requests to SharePoint endpoints containing encoded script payloads such as %3Cscript%3E or onerror= in query parameters and form bodies.
Anomalous outbound requests from user browsers to attacker-controlled domains immediately after loading a SharePoint page.

Detection Strategies

Inspect SharePoint IIS logs and ULS logs for POST requests to list, page, and web part endpoints containing HTML tags or JavaScript keywords in parameter values.
Deploy Content Security Policy (CSP) reporting to capture inline-script and unsafe-eval violations on SharePoint sites.
Correlate authenticated user actions that modify page content with subsequent client-side alerts from web filtering or browser isolation tooling.

Monitoring Recommendations

Alert on newly created or modified SharePoint content that contains HTML or script markup in fields that should hold plaintext.
Monitor SharePoint audit logs for privileged actions performed shortly after a user views a page authored by a low-privileged account.
Track browser telemetry for script execution originating from SharePoint pages that reference external domains.

How to Mitigate CVE-2026-62829

Immediate Actions Required

Apply the Microsoft security update referenced in the Microsoft Security Update Guide to all SharePoint Server Subscription Edition and SharePoint Server 2019 instances.
Audit SharePoint content created or modified before patch deployment for stored script payloads and remove malicious entries.
Restrict content contribution permissions to trusted users while patch rollout is in progress.

Patch Information

Microsoft has published patch guidance for CVE-2026-62829 through the Microsoft Security Update Guide. Administrators should install the corresponding cumulative update for their SharePoint version and confirm patch status using Get-SPProduct in PowerShell. Refer to the official advisory for exact build numbers and prerequisites.

Workarounds

Enforce a strict Content Security Policy on SharePoint sites to block inline script execution where feasible.
Limit the Add and Customize Pages and Contribute permissions to reduce the population of users capable of injecting content.
Enable browser-based script filtering and warn users against following unsolicited SharePoint links until patching is complete.

bash

# Verify installed SharePoint build after patching
Get-SPProduct | Select-Object ProductName, PatchableUnitDisplayName, Patch