CVE-2020-16898: “Bad Neighbor”
CVSS Score: 9.0
Microsoft announced a critical vulnerability in the Windows IPv6 stack, which allows an attacker to send maliciously crafted packets to potentially execute arbitrary code on a remote system. The proof-of-concept shared with MAPP (Microsoft Active Protection Program) members is both extremely simple and perfectly reliable. It results in an immediate BSOD (Blue Screen of Death), but more so, indicates the likelihood of exploitation for those who can manage to bypass Windows 10 and Windows Server 2019 mitigations. The effects of an exploit that would grant remote code execution would be widespread and highly impactful, as this type of bug could be made wormable. For ease of reference, we nicknamed the vulnerability “Bad Neighbor” because it is located within an ICMPv6 Neighbor Discovery “Protocol”, using the Router Advertisement type.
A remote code execution vulnerability exists when the Windows TCP/IP stack improperly handles ICMPv6 Router Advertisement packets that use Option Type 25 (Recursive DNS Server Option) and a length field value that is even. In this Option, the length is counted in increments of 8 bytes, so an RDNSS option with a length of 3 should have a total length of 24 bytes. The option itself consists of five fields: Type, Length, Reserved, Lifetime, and Addresses of IPv6 Recursive DNS Servers. The first four fields always total 8 bytes, but the last field can contain a variable number of IPv6 addresses, which are 16 bytes each. As a result, the length field should always be an odd value of at least 3, per RFC 8106: