Proactive MDR Strategies for Rapid Response

On July 18, 2025, a critical cybersecurity incident laid bare a zero-day vulnerability in Microsoft SharePoint Server. These were the threats that slipped conventional defenses and struck at the core of enterprise operations before anyone even realized they were under attack. Our article recounts how human expertise and proactive detection capabilities identified and contained this threat a full day before it was publicly disclosed and emergency patches were released.

Detecting a SharePoint Zero-Day Vulnerability

At the start of the workday, Microsoft Defender flagged “Possible web shell installation” on a major manufacturer’s SharePoint server. This alert pointed to a previously unknown zero-day vulnerability, later cataloged as CVE‑2025‑53770. With no public advisories, no assigned CVE, and no available patch, the UnderDefense Managed Detection and Response (MDR) team found themselves squarely in a true zero-day scenario. Signature-based defenses and external threat feeds were useless against this novel exploit.

Crucially, although the alert was generated, no automated containment kicked in. This underlines a critical challenge: security tools alone don’t ensure protection. Without precise tuning, response playbooks, and human oversight, even high‑severity alerts can pass unnoticed. It was the MDR team’s continuous monitoring and rapid intervention that alone held the line against further compromise.

The Investigation Unfolds

At 8:47 PM UTC on July 18, analysts traced the malicious activity to the legitimate w3wp.exe process—the IIS application‑pool worker for SharePoint. By running a heavily obfuscated PowerShell command with the -EncodedCommand flag, attackers concealed their payload from basic inspection and many logging tools.

Decoding the payload revealed its aim: deploying a web shell named spinstall0.aspx under C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\16\TEMPLATE\LAYOUTS\spinstall0.aspx.

Embedding the shell in the standard LAYOUTS directory via a trusted process allowed the adversary to blend in with normal traffic, bypassing signature‑based and file‑blacklist defenses. This tactic demanded that defenders looked for anomalous behavior from legitimate sources, not just known bad files or processes.

Responding and Containing

Immediately upon confirmation, the UnderDefense MDR team alerted the manufacturer’s internal SOC, issuing a detailed incident summary by email and following up with an urgent hotline call. Their recommendations included:

• Remove the malicious file. Delete spinstall0.aspx from the affected server.
• Sweep for additional threats. Check LAYOUTS, TEMP, and inetpub folders for unauthorized .aspx files.
• Audit configurations. Examine all public directories and SharePoint/IIS config files for tampering or injected commands.
• Lock down access. Block internet‑facing connections via firewall rules and require VPN for remote users. 

By 10:00 PM UTC, the SOC team had executed these steps and shared the recovered shell with UnderDefense for deeper analysis.

That review exposed the shell’s capability to exfiltrate MachineKey settings—critical cryptographic keys governing authentication tokens and ViewState integrity.

At 10:39 PM UTC, additional guidance was issued:

• Regenerate MachineKey. Update the web.config across all servers to invalidate compromised keys.
• Rotate keys broadly. Ensure any systems sharing the old MachineKey generate new keys to thwart lateral movement.

The MDR team also notified other affected clients and circulated general hardening recommendations.

On July 19, reports confirmed active exploitation of on‑premises SharePoint Servers via CVE‑2025‑53770—a critical, unauthenticated RCE vulnerability (CVSS 9.8) affecting SharePoint Server Subscription Edition, 2019, and 2016. Attackers leveraged an unpatched deserialization bypass in the ToolPane.aspx endpoint, a variant of the earlier CVE‑2025‑49704, circumventing Microsoft’s July Patch Tuesday fixes.

Microsoft released out‑of‑band updates as follows:

Patches arrived on July 20 for Subscription Edition and 2019, with 2016 updates on July 21. SharePoint Online remained unaffected, as the flaw targeted on-premises architectures.

Lessons from the Front Lines

The SharePoint zero-day incident, involving CVE-2025-53770 and the “ToolShell” exploit chain, stands as a case study in the complexities of modern cybersecurity.

• Proactive detection is critical. Spotting the zero‑day a day before public disclosure narrowed attackers’ window of opportunity.
• Human expertise is irreplaceable. Automation scales, but only skilled analysts can decipher and respond to novel threats and evasive tactics.
• Collaboration amplifies impact. Ongoing forensic sharing between MDR and SOC teams revealed deeper compromises and spurred critical remediation.

What You Should Do Now

If you’ve been affected by the SharePoint vulnerability CVE-2025-53770, here’s what you should do:

Conclusion

This SharePoint zero‑day incident, centered on CVE‑2025‑53770 and the “ToolShell” exploit chain, reveals just how sophisticated and elusive modern cyber threats have become. In this case, proactive behavioral monitoring paired with the sharp instincts of the UnderDefense MDR team made all the difference, catching and containing the attack well before public disclosure or emergency patches were available.

To replicate this level of resilience, organizations must continuously analyze behavior across their environments, foster collaboration between automated defenses and human analysts, and partner with specialized teams. Only by combining these elements you can reliably detect, respond to, and neutralize even the most advanced zero‑day threats.