Zero-Day: Microsoft SharePoint Flaw Allows Remote Code

In July 2025, Microsoft issued an urgent warning about a critical zero-day vulnerability affecting on-premises SharePoint servers. This flaw enables authenticated attackers to execute arbitrary code on vulnerable systems, posing a serious risk of data theft, system compromise, and ransomware deployment. Microsoft recommends immediate patching for hybrid and self-hosted SharePoint instances to prevent exploitation.

🔍 What Makes This a Zero-Day?

An official share zero‑day vulnerability refers to a security flaw being actively exploited before a public patch is available. In this case, Microsoft disclosed the SharePoint issue and released mitigations only after reports of active abuse targeting corporate and government networks.

This isn't just theoretical: during the Pwn2Own Berlin 2025 hacking event, researchers demonstrated a working exploit against SharePoint combining authentication bypass and insecure deserialization, taking home a $100K reward.

⚙️ Vulnerability Breakdown: How Remote Code Execution Works

• Attack vector: Requires at least authenticated "Site Owner" privileges to send crafted HTTP requests that deliver malicious serialized .NET objects.
• Deserialization flaw: Vulnerable SharePoint server deserializes attacker-controlled objects, running arbitrary code inside the w3wp.exe process.
• Impact: Attackers can write files, deploy web shells, run scripts, and escalate privileges within the SharePoint application pool.

The Pwn2Own demonstration chained an authentication bypass with a .NET deserialization exploit—demonstrating full system compromise.

🛡️ The Risk Landscape

Microsoft confirmed that this flaw affects only on-premises deployments; SharePoint Online (Microsoft 365) remains unaffected. Still, many enterprises still rely on hybrid or fully on-prem models for intranet and document management.

A compromised SharePoint server can become an initial foothold for attackers to:

1. Harvest internal data (such as documents or credentials).
2. Install web shells and schedule persistent backdoors.
3. Spread laterally via Active Directory or expose tokens to cloud services.
4. Trigger ransomware or destructive campaigns.

These outcomes match patterns seen in earlier SharePoint zero-day attacks.

🧩 Microsoft & Security Industry Respond

In July 2025’s Patch Tuesday, Microsoft released fixes for over 130 vulnerabilities—including this SharePoint RCE—accompanied by a "high priority" advisory urging immediate patching on self-hosted environments.

CISA and similar agencies have classified the flaw as a Known Exploited Vulnerability, requiring urgent remediation.

✅ What You Need to Do Now

• Patch immediately: Apply the July 8, 2025 security updates on all SharePoint Server instances.
• Restrict privileges: Ensure users with "Site Owner" access are limited to trusted personnel.
• Isolate servers: Block internet access or perimeter exposure of SharePoint servers.
• Enhance monitoring: Watch IIS logs for suspicious POST requests, serialized payloads, and unexpected child processes spawned by w3wp.exe.
• Rotate credentials: Reset service accounts and API tokens post-patch.
• Conduct forensics: Scan for web shells, scheduled tasks, or anomalous file activity.

🏗️ Long-Term Mitigation Strategies

1. Consider migrating to SharePoint Online for auto-patched security.
2. Review and eliminate legacy plugins relying on unsafe serialization.
3. Utilize a Web Application Firewall (WAF) to block suspicious requests.
4. Adopt secure serialization formats (e.g., JSON whitelisting, not binary deserialization).
5. Participate in bug bounty programs or security audits for critical assets.

📰 Broader Context: Patch Ecosystem Expands

Microsoft’s July Patch Tuesday featured a high volume of vulnerabilities across the ecosystem, including a wormable SPNEGO flaw and SQL Server zero-day disclosure.

SharePoint is just one high-profile target. With 41 RCEs and 137 total fixes this month, Windows and Office components remain in cybercriminal crosshairs.

📌 Final Takeaway

This SharePoint zero-day is a grave threat—already weaponized in high-profile hacking competitions and real-world attacks. For on-prem deployments, swift patching is non-negotiable. Monitoring and long-term hardening measures are essential to prevent next-generation exploitation.

For real-time updates, detection scripts, and in-depth guidance on SharePoint hardening and cloud migration, follow TechPickr123’s Insights series.