UEFI is designed to scale up - servers that can boot from network, which brings a certain baseline complexity. It doesn't help that the designers didn't want to

UEFI is designed to scale up - servers that can boot from network, which brings a certain baseline complexity. It doesn't help that the designers didn't want to set any rules and run everything through a central, extensible function call dispatch. The effect is that UEFI can load and run executables (from flash, disk, network), has a network stack and things like openssl (when did you last update your firmware's SSL implementation? :-) ), all of which process lots of ingress data - while maintaining a larger degree of control over the system than the OS that comes after it. So, it's not UEFI-specific per-se, but UEFI's design was optimized for the large scale (it pretty much started on Itanium, so there) and at a time when security wasn't much of a priority. Now they sit in that corner and look for ways out. Such as signature checks on executables (Secure Boot).