TOR by design is vulnerable to Sybil attacks. In fact, there have been attempts to exploit this vulnerability “in the field”. It is not clear how successful they were. There are some measures taken to prevent such attacks, but none of them guarantee safety. I2p and other p2p networks also suffer from the same problem.

In fact there is only one known way to mitigate Sybil (and alike) attacks. It is to expand the cost of operating in the network so much, that it would not be financially viable to perform it. There are two major way to achieve that: proof-of-work and proof-of-stake.

PoW is what majority of cryptocurrencies do. To operate in the network you need to perform significant calculations. The more calculations you perform the “stronger” your position is. For that you have to invest huge amount of money in hardware and energy to “outperform” other actors. That is what mining basically is.

PoS requires you instead to invest a crypto (or whatever, does not actually matter). The more crypto you invest “the bigger your ‘bank’ account is”, the “stronger” your position is as well. This is what nym and lokinet (technology behind session messenger) do.