>"If the sequence number is completely out of range then the receiver ignores the segment entirely. If, however, it is within the window of expected sequence numbers, then the receiver sends back a “challenge ACK”. This is a segment which tells the sender that the RST segment had the wrong sequence number. It also tells the sender actual sequence number that the receiver was expecting. The sender can use the information in the challenge ACK to re-construct and re-send its RST.
Before 2010, the TCP protocol did not impose these additional restrictions on RST segments. RST segments were accepted or rejected according to the same rules as any other segment. However, this made blind TCP reset attacks too easy."
If the "challenge ACK" communicates that sequence number that was expected doesn't this in fact still aid reset attacker? Or am I reading this passage incorrectly and the "challenge ACK" procedure is no longer in more modern TCP stacks?