Suppose i'm protection physically accessible RJ45 sockets in offices that are connected to a switch (Cisco SF220) with 802.1X port based authentication and a radius server.

Authorized devices have certificates that allow them to authenticate to the radius server. Assume such a device is installed somewhere and connected to a protected port.

An attacker disconnection the authorized device and plugging in his (malicious) device won't be able to authenticate without a valid certificate/login. But what if the attacker connects a switch/hub and connects both his malicious device and my authorized device to the port.

Won't the authorized device authenticate to the radius server and the switch will put the port into an access port for my protected VLAN. So in the end also his malicious device has access, because it's only port based authentication.

If so, what are recommended counter measures? I don't want to use MAC based authentication because MAC addresses can too easily be spoofed

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This is a well-known weakness of 802.1X. It protects well against casual misconduct but provides no full protection against a decided attack. You can somewhat reduce the weakness by combining 802.1X with MAC-port binding on the protected interfaces, but this can be overcome as well.

If you require better security take a look at MACsec defined by IEEE 802.1AE: https://ieeexplore.ieee.org/document/1678345/ (free after registration).

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