September 2020 Patch Tuesday: Microsoft fixes over 110 CVEs again

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.

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.

CVE-2020-0688 is a remote code execution vulnerability in Microsoft Exchange Server caused by the validation key not being uniquely created at install time, leaving deployments with a predictable, shared key (CWE-287, improper authentication). A remote attacker who can reach an affected Exchange server and knows the common install-time key can supply maliciously crafted, cryptographically signed payloads that the server trusts, triggering code execution without needing per-server secrets. Successful exploitation gives the attacker code execution on the Exchange server, which can be used to access mail data, move laterally, and stage follow-on activity; CISA notes known use in ransomware campaigns. All organizations running the affected on-premises Microsoft Exchange Server are in scope per CISA's listing, though the affected version range is not specified in the source data. Exploitation is confirmed in the wild: the flaw was added to CISA's KEV on 2021-11-03 with known ransomware use, and EPSS rates 30-day exploitation probability at 100% (top percentile).

Do: Apply Microsoft's Exchange security updates addressing CVE-2020-0688 (released in February 2020) to every on-premises Exchange server, per CISA's required action. As an interim mitigation, configure a unique ASP.NET machineKey in each Exchange server's web.config instead of the default shared install-time key, and hunt for indicators of exploitation given the known ransomware use.

A remote code execution vulnerability exists in the way that Microsoft COM for Windows handles objects in memory. An attacker who successfully exploited the vulnerability could execute arbitrary code on a target system. To exploit the vulnerability, a user would have to open a specially crafted file or lure the target to a website hosting malicious JavaScript. The security update addresses the vulnerability by correcting how Microsoft COM for Windows handles objects in memory.

A security feature bypass vulnerability exists in Windows Defender Application Control (WDAC) which could allow an attacker to bypass WDAC enforcement. An attacker who successfully exploited this vulnerability could execute PowerShell commands that would be blocked by WDAC. To exploit the vulnerability, an attacker need administrator access on a local machine where PowerShell is running. The attacker could then connect to a PowerShell session and send commands to execute arbitrary code. The update addresses the vulnerability by correcting how PowerShell commands are validated when WDAC protection is enabled.

A remote code execution vulnerability exists in Microsoft SharePoint when the software fails to check the source markup of an application package. An attacker who successfully exploited the vulnerability could run arbitrary code in the context of the SharePoint application pool and the SharePoint server farm account. Exploitation of this vulnerability requires that a user uploads a specially crafted SharePoint application package to an affected version of SharePoint. The security update addresses the vulnerability by correcting how SharePoint checks the source markup of application packages.

A remote code execution vulnerability exists in Microsoft Dynamics 365 for Finance and Operations (on-premises) version 10.0.11. An attacker who successfully exploited this vulnerability could gain remote code execution via server-side script execution on the victim server. An authenticated attacker with privileges to import and export data could exploit this vulnerability by sending a specially crafted file to a vulnerable Dynamics server. The security update addresses the vulnerability by correcting how Microsoft Dynamics 365 for Finance and Operations (on-premises) version 10.0.11 handles user input.

A remote code execution vulnerability exists in Microsoft Dynamics 365 (on-premises) when the server fails to properly sanitize web requests to an affected Dynamics server. An attacker who successfully exploited the vulnerability could run arbitrary code in the context of the SQL service account. An authenticated attacker could exploit this vulnerability by sending a specially crafted request to a vulnerable Dynamics server. The security update addresses the vulnerability by correcting how Microsoft Dynamics 365 (on-premises) validates and sanitizes user input.

A remote code execution vulnerability exists in Microsoft Exchange server due to improper validation of cmdlet arguments. An attacker who successfully exploited the vulnerability could run arbitrary code in the context of the System user. Exploitation of the vulnerability requires an authenticated user in a certain Exchange role to be compromised. The security update addresses the vulnerability by correcting how Microsoft Exchange handles cmdlet arguments.