SharePoint zero-day flaw exploited as over 9,000 servers at risk

Cybersecurity experts have raised fresh alarms following reports of active exploitation targeting Microsoft SharePoint servers worldwide. The scale and sophistication of the attacks, which began to surface in detailed research at the end of last week, are causing concern among organisations that rely on the popular collaboration platform for critical information infrastructure.

The vulnerability at the centre of the incident, now assigned as CVE-2025-53770, affects a wide cross-section of SharePoint Server deployments. Research from Eye Security first brought attention to what it described as "active, large-scale exploitation," driven by a zero-day weakness identified within a pair of vulnerabilities collectively known as ToolShell. Successful exploitation allows attackers to extract the MachineKey configuration details from vulnerable servers - exposing both the validationKey and decryptionKey, which are crucial to securing authentication tokens and encrypted data.

This critical information, once in criminal hands, can be weaponised. As Satnam Narang, Senior Staff Research Engineer at Tenable, explained, "Attackers were able to exploit the flaw, now identified as CVE-2025-53770, to steal MachineKey configuration details from vulnerable SharePoint Servers. These details can be used by attackers to create specially crafted requests that could be used to gain unauthenticated remote code execution." Narang noted that the consequences for affected organisations may be severe, with broad implications for data integrity and security across industry sectors.

Indicators of compromise are already being circulated among security teams. Organisations are being urged to check for evidence of unauthorised access, with one telltale sign being the sudden creation of files named "spinstall0.aspx" on vulnerable servers, possibly under other extensions. The scope of exposure is significant, with estimates suggesting over 9,000 externally accessible SharePoint servers are potentially at risk. These systems are deployed globally by enterprises, government entities, and a range of other organisations relying on SharePoint for document management and collaboration.

Patching efforts have commenced in earnest. Microsoft began distributing fixes late on 20 July, prioritising SharePoint Server 2019 and SharePoint Subscription Edition. A remedy for SharePoint Server 2016 remains pending but is expected imminently. Narang advised, "We strongly advise organisations to begin conducting incident response investigations to identify potential compromise; otherwise, apply the available patches and review the mitigation instructions provided by Microsoft."

Andrew Obadiaru, Chief Information Security Officer at offensive security firm Cobalt, warned that the speed and depth of zero-day exploitation leaves little margin for delay or complacency. "Zero-day vulnerabilities in widely deployed platforms like SharePoint are a goldmine for attackers because they provide immediate, scalable access to high-value environments.

"The challenge isn't just patching - it's that attackers typically implant persistence mechanisms within hours, ensuring long-term footholds. Defence strategies need to assume breach and validate controls through proactive testing, including red teaming and continuous pentesting, to uncover weaknesses before adversaries do. In today's threat landscape, reactive security alone is a losing game."

Obadiaru's remarks echo growing industry consensus that traditional perimeter defences are proving insufficient in the face of increasingly sophisticated and rapid cyber threats. Security teams are being encouraged to revisit their incident response and detection protocols, embracing a proactive security posture and preparing for the possibility that attackers may already be inside their networks.

For now, the advice from the security community is clear: immediate action is essential. Organisations are urged to initiate incident response processes, apply available patches without delay, and review configuration settings for any signs of compromise. Vigilance and proactive testing will be the defining factors in limiting the fallout from yet another high-profile zero-day targeting widely used enterprise software.