I was wondering, if I have a Server that accepts TCP connections from clients, assuming the initial TCP connection is established without attackers, can the client assume that communication received on that socket is actually from the server? Thanks in advance.
No. At any point an interceptor can double proxy between you and the other side. It is also possible to hijack the connection completely and simulate the other side.
The network stack matches TCP/IP packets to sessions using the 5-tuple of source and destination IP and port number plus the protocol number. It doesn't (and can't) care how such a packet actually gets to the receiver.
So by spoofing the source address, it's trivial to send packets to your target host. However, to have them actually accepted, the sequence numbers need to be right (or right enough) to slot that packet into the ongoing stream.
But ... packets with bad sequence numbers don't break a connection, they're just assumed to be retransmissions of something that already arrived (if the number is low) or an indication that a bunch of packets were lost (if high). This is a little complicated by the fact that the session is bi-directional, but not too much.
So, especially if you're able to monitor the packets of the session, it's fairly simple to hijack it by sending sequence numbers a bit ahead of the legitimate sender, causing its packets to be discarded as duplicates, and yours to be accepted.
How micro is your micro? There are embedded TLS stacks such as wolfSSL[1]. If you carefully select the cipher suite and certificate requirements, and perhaps limit TLS payload sizes, you may be able to fit on a lot more systems than you initially suspect. x.509 is expensive in code space though, if that's the constraint, you may do better with an application specific certificate replacement of some sort.
If you're able to intercept traffic anywhere in the path between the two parties, interception and hijacking is trivial. So yes, client LAN, server LAN, transit link, or any router on the path. And there's a bunch of routers, switches, etc, on that path, all of which have firmware that can be exploited ...
If you're not able to directly intercept the traffic, you can generally spoof the source address, but getting a copy of the return packets (other than on the client's local network) is also possible if you can inject a bogus route, for instance.
The only reason this doesn't happen a lot more than it does is that most valuable stuff uses TLS/SSL.
