As per my understanding, with a successful 51% attack, an attacker can confirm invalid transactions and do double spending of BTC. They can probably reverse recent transactions too.
Are there any capabilities built into the code of Bitcoin or Bitcoin mining node softwares to tackle a scenario like this?
Since majority of the hashing power is controlled by the adversary in the case of 51% attack, they can literally do anything for as long as they can sustain the attack.
They can't do
anything.
- A miner cannot confirm invalid transactions because that would split the chain.
- The ability to double spend is limited because it requires mining an additional number of old blocks faster than everyone else can mine just new blocks.
- 51% of the hash power also allows the attacker to censor transactions for as long as they have 51%