No, it's not an attack on the PRNG in the usual sense. One way to think about it though is that CBC mode "sort of" uses the previous ciphertext block as the ou

No, it's not an attack on the PRNG in the usual sense. One way to think about it though is that CBC mode "sort of" uses the previous ciphertext block as the output of a PRNG which is used to randomize the plaintext before it's encrypted with the block cipher. However, the way TLS 1.0 uses it, the attacker is able to learn the output of that PRNG well in advance of when it can be used. If he can choose some plaintext during that time, this adaptive chosen plaintext attack is made possible.