Fixes TCP Implementation

Microsoft released a set of fixes affecting Windows TCP/IP implementation that include two Critical Remote Code Execution (RCE) vulnerabilities (CVE-2021-24074, CVE-2021-24094) and an important Denial of Service (DoS) vulnerability (CVE-2021-24086).

Two of them expose unpatched systems to remote code execution (RCE) attacks, while the third one enables attackers to trigger a DoS state, taking down the targeted device.

“The DoS exploits for these CVEs would allow a remote attacker to cause a stop error. Customers might receive a blue screen on any Windows system that is directly exposed to the internet with minimal network traffic,” the Microsoft Security Response Center team said.

Microsoft also says, it is important to apply today’s security updates on all Windows devices as soon as possible, the company also provides workarounds for those who can’t immediately deploy them.

IPv4 and IPv6 Workarounds Available

The Internet Protocol version 4 (IPv4) workaround requires hardening against the use of Source Routing, normally disallowed in the Windows default state.

This workaround is documented in CVE-2021-24074 and can be applied through Group Policy or by running a NETSH command that does not require a reboot.

The Internet Protocol version 6 (IPv6) workarounds are documented in CVE-2021-24094 and CVE-2021-24086, and require blocking IPv6 fragments, which may negatively impact services with dependencies on IPv6.

IPv4 Source Routing requests and IPv6 fragments can be blocked on an edge device, such as a load balancer or a firewall. This option can be used to mitigate systems with high-risk exposure and then allow the systems to be patched following their standard cadence. 

Affected systems must be patched as quickly as possible because of the elevated risk associated with these vulnerabilities, and downloads for these can be found in the Microsoft Security Update Guide. 

Customers who have automatic updates enabled are automatically protected from these vulnerabilities.

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