Microsoft announced yesterday that a newly discovered critical remote code execution vulnerability in SharePoint is being exploited. There is no patch available. As a workaround, Microsoft suggests using Microsoft Defender to detect any attacks. To use Defender, you must first configure the AMSI integration to give Defender visibility into SharePoint. Recent versions of SharePoint have the AMSI integration enabled by default.

Microsoft also states: "If you cannot enable AMSI, we recommend you consider disconnecting your server from the internet until a security update is available."

Defender will just detect the post-exploit activity. Currently, webshells are observed as a payload being deployed, taking advantage of the vulnerability.

The best write-up and details I found so far come from the Eye Security research team. They initially used CVE-2025-49704 and CVE-2025-49706 to identify the vulnerability. Later, Microsoft confirmed that this is a new issue and started using CVE-2025-53770. This latest issue appears to be a variation of the older vulnerabilities patched in this month's Patch Tuesday.

The vulnerability exploits an authentication bypass issue triggered by setting the "Referer" header to "/_layouts/SignOut.aspx". This vulnerability is then exploited to trigger remote code execution via "/_layouts/15/ToolPane.aspx". 

In our honeypot data, we observed two instances of the "ToolPane.aspx" URL, first on July 16th (on individual hit, I am waiting to hear from the submitter to see if there are details available). Today, we received additional reports, but they originated from p55001.probes.atlas.ripe.net:9000 and are likely related to scanning for research purposes. These hits did not include the Referer header to trigger the vulnerabiliy.

The hit on July 16th originated from 172.174.82.132. This IP address appears to be owned by Microsoft.

Microsoft Advisory:
https://msrc.microsoft.com/blog/2025/07/customer-guidance-for-sharepoint-vulnerability-cve-2025-53770/
Eye Security Blog:
https://research.eye.security/sharepoint-under-siege/

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Johannes B. Ullrich, Ph.D. , Dean of Research, SANS.edu
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