As the Conversation summarizes in its article Ukraine invasion: should Russia lose its seat on the UN Security Council?:

“The Security Council was established by the 1945 UN Charter and comprises 15 members. Ten rotating non-permanent countries are elected by the UN General Assembly to do a two-year term on the Security Council. Five members – the USSR (now Russia), Republic of China (now People’s Republic of China), the US, UK and France – have the status of permanent members and so have a veto on any vote before the Council.”

Article 23 of the UN Charter says that the USSR, not Russia, is the permanent member of the UN Security Council, as Ukraine’s ambassador, Sergiy Kyslytsya, has suggested in February 2022. This could be used as an argument that Russia is not, in fact, a legitimate permanent member of the Security Council.