Microsoft has released patches for 58 bugs and one advisory as part of its December 2020 Patch Tuesday security updates. Out of the vulnerabilities addressed in this month’s batch, 9 have been rated as Critical, 48 as Important, and 2 as Moderate.

Luckily, this year ends with a quite light Patch Tuesday compared to the previous ones. This month brings no zero-days or formerly revealed vulnerabilities. However, this doesn’t mean that administrators can dismiss Microsoft’s updates and should still address them at their earliest convenience.

Microsoft provides DNS cache poisoning security advice

Regarding the aforementioned security advisory included in the December 2020 Patch Tuesday, it relates to a DNS poisoning flaw found by researchers from Tsinghua University and the University of California.

Microsoft stated that they were aware of “a vulnerability involving DNS cache poisoning caused by IP fragmentation that affects Windows DNS Resolver. An attacker who successfully exploited this vulnerability could spoof the DNS packet which can be cached by the DNS Forwarder or the DNS Resolver.”

Administrators should adjust the Registry to change the maximum UDP packet size to 1,221 bytes in order to overcome this vulnerability. Consequently, the DNS resolver will turn over to TCP connections for DNS requests larger than 1,221 bytes.

Here is the workaround proposed by Microsoft:

  1. Run regedit.exe as Administrator.
  2. In Registry Editor, navigate to the HKLM\SYSTEM\CurrentControlSet\Services\DNS\Parameters subkey and set the following parameters:
    • Value: MaximumUdpPacketSize
    • Type: DWORD
    • Data: 4C5 Hexadecimal or 1221 Decimal
  3. Close Registry Editor and restart the DNS service.

What’s more, the company also warns that the incorrect usage of the Registry Editor can cause serious problems that may necessitate the reinstallation of the OS and can’t guarantee that complications resulted from the incorrect use of Registry Editor can be resolved. Registry Editor should be used at the users’ own risk.

For details on how the registry can be edited, users should view the “Changing Keys and Values” Help topic in Registry Editor (Regedit.exe) or the “Add and Delete Information in the Registry” and “Edit Registry Data” Help topics in Regedt32.exe.

Other noteworthy patched vulnerabilities

Even though this month there were no zero-days, there are some software flaws worth mentioning.

  • Hyper-V RCE Vulnerability (CVE-2020-17095): Enables malware running on a Hyper-V VM to execute arbitrary code.
  • Windows NTFS RCE Vulnerability (CVE-2020-17096): A local attacker might run a specially designed program which would elevate the rights of the attacker. “A remote attacker with SMBv2 access to a vulnerable system could send specially crafted requests over a network to exploit this vulnerability and execute code on the target system.”, notes Microsoft.
  • Windows Lock Screen Security Feature Bypass Vulnerability (CVE-2020-17099): During an active user session, attackers with physical access may be able to execute code directly from the Windows lock screen. However, only after a user has already signed in and locked their session could this flaw be abused.
  • Microsoft Exchange Server (CVE-2020-17143, CVE-2020-17144, CVE-2020-17141, CVE-2020-17141, CVE-2020-17117, CVE-2020-17132, CVE-2020-17142) – All of these flaws allow remote code execution.
  • Microsoft SharePoint (CVE-2020-17118, CVE-2020-17121) – Also RCE bugs.
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