Microsoft’s May 2021 Patch Tuesday Addresses 55 CVEs (CVE-2021-31166)

CVE-2021-31166 | HTTP Protocol Stack Remote Code Execution Vulnerability

CVE-2021-31166 is a RCE vulnerability which can be exploited by a remote, unauthenticated attacker sending a crafted HTTP packet to a system utilizing the HTTP Protocol Stack (http.sys). The vulnerability is considered to be wormable, which means that a single infection could result in a chain reaction of systems impacted across an enterprise without any user interaction. Microsoft assigned this critical flaw with a 9.8 CVSSv3 score, emphasizing the severity of the vulnerability. While details have not been released, this vulnerability is rated as “Exploitation More Likely” according to Microsoft’s Exploitability Index and we strongly recommend ensuring this patch is applied as soon as possible.

CVE-2021-28476 | Hyper-V Remote Code Execution Vulnerability

CVE-2021-28476 is a RCE vulnerability in Hyper-V which could allow a remote, unauthenticated attacker to compromise a Hyper-V host via a guest virtual machine (VM). The critical flaw was assigned a CVSSv3 score of 9.9, however it is rated as “Exploitation Less Likely.” The advisory from Microsoft does point out that the likely exploitation scenario for this flaw would result in a denial of service (DoS) condition, though in some cases RCE is possible as a guest VM could cause the Hyper-V host’s kernel to read from an arbitrary address.

CVE-2021-31198, CVE-2021-31207, CVE-2021-31209, CVE-2021-31195 | Multiple Exchange Server Vulnerabilities

CVE-2021-31198, CVE-2021-31207, CVE-2021-31209 and CVE-2021-31195 are several flaws that impact Microsoft Exchange Server 2013, 2016, and 2019 and are all rated “Exploitation Less Likely,” ranging in severity from CVSSv3 6.5 to 7.8. Given the history of prior Exchange Server vulnerabilities in 2021 we felt it was important to highlight them and ensure administrators take action.

CVE-2021-31209 is a server spoofing vulnerability and received a CVSSv3 score of 6.5. CVE-2021-31195 and CVE-2021-31198 are both RCE vulnerabilities, but CVE-2021-31198, which received a CVSSv3 score of 7.8, is listed as a local attack vector. On the other hand, CVE-2021-31195 received a CVSSv3 score of 6.5 and is listed as having no impact on integrity or availability. Both RCEs require user interaction to exploit.

Only one of these vulnerabilities, CVE-2021-31207 — a security feature bypass which received a CVSSv3 score of 6.6, was publicly disclosed. According to Microsoft, it was one of the Exchange Server vulnerabilities found during Pwn2Own 2021. None of these vulnerabilities have been reported as exploited in the wild at the time of publication.

Additionally, Microsoft is introducing new security functionality to Exchange Servers as part of the May Security Update that will allow administrators to validate the version information of their Exchange Servers. Microsoft’s Exchange Team published a blog post highlighting this new functionality.

CVE-2021-28474 and CVE-2021-31181 | Microsoft SharePoint Server Remote Code Execution Vulnerability

CVE-2021-28474 and CVE-2021-31181 are a pair of RCE vulnerabilities in Microsoft SharePoint Server. Both were assigned a CVSSv3 score of 8.8 and a severity of Important. Microsoft rates these vulnerabilities as “Exploitation More Likely.” An attacker would need to be authenticated in order to exploit these flaws, though successful exploitation would grant an attacker remote code execution through the creation of a SharePoint site.