It's not SSN's that are the concern. In a war setting between the CBG's own SSN screen, it's own ASW aircraft and the ASW escorts the odds of an SSN getting into position for a more convetional torpedo attack are very low. Simply put, SSN's make noise and aside from some of the most modern British and Russian types they make quite a bit of noise by modern standards.
Now SSGN attack is a bit more of a threat since an SSGN could fire it's cruise missiles from outside the defensive perimeter of the CBG. The problem is that you're going to try lobbing about ~24 or less cruise missiles into a battlegroup with at least some combination of three Ticonderoga class cruisers or Arleigh Burke destroyers along with several other surface ships with their associated AA assets. In other words, you're pissing in the wind. Massed SSGN attack would have a better chance but the number of dedicated SSGN's in the world, well you're talking about using most of them to even have a chance.
No, the real concern is an SSK. Diesel powered subs on batteries on minimal power are holes in the water. You don't find them by the noise they make, you have to try and find them by where background noise isn't. Ships like the later flight Kilo class (which Russia was willing to sell to anyone) or the German 212's are incredibly quite, an order of magnitude quieter than even a Seawolf or Virginia. The concern is that an SSK skipper with brains and a set of brass balls that need their own wheelbarrow for transportation would go deep in the path of an oncoming CBG, guess right, and wind up in torpedo distance of a carrier as the CBG ran him over. This is of course a suicide mission as the second he opens his outer doors the entire CBG goes apeshit and once the active sonar starts going he's a dead man, but he could get off his torps before he dies. A few things worth noting though, first this requires him to be in front of the CBG and accurately guess to within a few miles where they'll be in hours. Once he goes under he has to stay absolutely silent. If he makes more than a few knots he'll be found out and killed. He can't make an approach or run the ships down. Also, this assumes the CBG is running passive sonar only. If the escorts say, "Fuck it" and run active the SSK will be found and killed. Finally, it's is a one way, one shot trip. Surrounded by the CBG they're going to kill him after he fires. The only hope he's got is that he'll get hits as the remains of his sub are imploding on their way to the bottom of the ocean.