Cheater Cheater Pumpkin Eater!

[Victor]

It was implemented in older ACR versions, but some lagging players resulted in false positives.
When I was testing with <100ms ping, I think it was able to detect 5% speedhacks (if you move at 1.05x the normal speed) after 5-10 seconds.

AC used to be vulnerable to lots of hacks because lots of stuff was client-side.
AC 1.2 is still vulnerable to shot hacks (shoot through walls) and nade nuke hacks (on some servers), but ACR fixes these vulnerabilities at the expense of making it harder to play with lag.
ACR is still vulnerable to speedhacks because movements are client-side. In older ACR versions, detected speedhacks results in killing the player.
In CoD 4, movements are server-sided, so only two hacks are possible: aimbots and wallhacks (assuming that the server password isn't hacked).

Aimbots are hard to detect because a player could move the mouse randomly and get lucky kills. Or maybe he's just pro.
Anti-aimbots could still detect improbable sequences of kills: multiple headshots (or kills) at significantly different angles in a short timeframe.

Anti-ESP is hard to implement and would consume lots of server resources.
Before the server sends a position packet to a client, it can do an advanced visibility check with prediction (check whether the client can see another player within a few seconds). It can withhold the positions of invisible clients, rendering ESP hacks unusable. But players are moving and there is lag, so it has to predict whether a player will become visible (to the client or any of its bots) soon.
But for N clients, this takes at best N(N-1)/2 comparisons, which takes quadratic time.

The server bandwidth requirements are quadratic, which results in requiring a large upload connection for more players.
However, AC's code has a trick for reducing memory usage back down to linear for the position packets.

The important part is that there is a trade-off: if you trust clients more, more hacks are possible, but then lag has less impact on gameplay.
ACR has a server command line option to trust client hit info more, making it easier to play for laggers, but then shot hacks ("shoot through walls" or "every shot gets headshots") are possible.