One can think of many attacks aimed at certifying authorities, all of which can be defended against.

For instance, an attacker may attempt to discover the private key of a certifying authority by reverse engineering the device in which it is stored. For this reason, a certifying authority must take extreme precautions to prevent illegitimate access to its private key;

The certifying authority's key pair might be the target of an extensive cryptanalytic attack. For this reason, CAs should use long keys, and should also change keys regularly. Top-level certifying authorities need especially long keys, as it may not be practical for them to change keys frequently because the public key may be written into software used by a large number of verifiers.