Sort answer: It is not possible.
To put a trojan horse in every node connected to Ethereum you need to exploit a buffer overflow vulnerability. (https://en.wikipedia.org/wiki/Buffer_overflow) This is only possible in C/C++ languages, provided that the programmers do not properly audit their code.
geth is written in
Go protects against buffer overflow at language level.
Parity is written in
Rust, which also protects against buffer overflows.
It would be only possible to deploy a trojan on a node that runs Ethereum C++, but a buffer overflow vulnerability must exist. Not all C code has buffer overflows, it dependes on the developer who writes it, for example,
qmail has been never hacked, despite many contests.
It could be , however that
Rust have a vulnerability in protecting against buffer overflows, and somehow, under some conditions,
Rust fail on this, and then , probably, you could exploit this. But the probability of this event is almost nil.
Also, why would you want to hack an Ethereum node ? Nobody nowadays have their private keys on the net. All transactions are submitted in raw format , already signed by cold wallet or some computer that only connects to the network when it needs to send something.