CVE-2019-0604

Microsoft SharePoint fails to check the source markup of an application package, an improper input validation flaw (CWE-20) that allows maliciously crafted markup to be processed by the server. An attacker triggers the flaw by getting an affected SharePoint server to handle a crafted application package, without any special privileges described in the disclosure. Successful exploitation lets the attacker run remote code in the context of the SharePoint application pool and the SharePoint server farm account, providing control of the web server and access to a highly privileged farm-level identity. Any organization running an affected on-premises Microsoft SharePoint deployment is exposed, with internet-facing SharePoint servers at greatest risk. Exploitation is confirmed in the wild: the flaw was added to CISA's Known Exploited Vulnerabilities catalog on 2021-11-03 with known ransomware use and EPSS puts the probability of exploitation at 99.9%, although no public proof-of-concept is catalogued.

What to do: Apply Microsoft's SharePoint security updates per vendor instructions immediately, prioritizing internet-exposed SharePoint servers as CISA's required action directs. Given known in-the-wild and ransomware use, hunt for signs of compromise such as unexpected .aspx or webshell files in SharePoint directories and anomalous use of the SharePoint farm account. Restrict or firewall internet exposure of SharePoint servers until patches are confirmed applied.