Attack on security model for RSA-FDH
This is 
a) no attack on the security model, but an attack in the security model of EUF-CMA, and
b) a generic attack on any signature scheme that signs the hash of a message instead of the message itself (as done in RSA-FDH).     

The idea is that if you can find a collision for the used hash function $H$, i.e., two messages $m_1, m_2$ such that $H(m_1) = H(m_2)$, then they have the same signature: $\sigma = \textrm{sign}(sk,m_1) = \textrm{sign}(sk,m_2)$.
To see this consider RSA-FDH as example: The signature on $m_1$ is $\textrm{sign}(sk,m_1) = (H(m_1))^d \mod N$ for private exponent $d$ and modulo $N$. The signature on $m_2$ is $\textrm{sign}(sk,m_2) = (H(m_2))^d \mod N = (H(m_1))^d \mod N = \textrm{sign}(sk,m_1),$ where the mid equality follows from $m_1,m_2$ being a collision.

In the EUF-CMA setting you are allowed to ask for signatures on arbitrary messages and have to come up with a signature on a new message. So given collision $m_1,m_2$, you ask for a signature $\sigma$ on $m_1$ and then output the pair $(m_2, \sigma)$ as forgery.