If I were to be seeking to establish responsibility, I would look to the chief of security to have considered the highly unlikely possibility Geordi's visor could be hacked, since his visor can interface with the ship's computer system.
The Federation does seem slightly more vulnerable to hacking than would seem likely, possibly because their computers are so sophisticated, they may be dependent upon highly specialized crew to be able to determine if changes have been made to the computers. These specialists may or may not be regulars on a starship's crew.
Given the failsafes and capabilities of Federation computers, I can understand how such an oversight could have occurred:
- The computers of the Federation are highly capable, nearly self-aware AI systems. It would be the tiny place between AI and not AI where a virus would have to be inserted.
- Such computers should have multiple levels of security and redundancy to protect them from unauthorized tampering.
- Geordi's visor was perfect for hacking because it is not directly connected to a more secure system and was not routinely security scanned.
- Considering how little access his visor needed from the main computer systems, it was likely overlooked as an unlikely vector for serious hacking.
Considering the rarity of the device in the Federation, and the overall lack of familiarity with Geordi's visor, even by the ship's medical crew, any physical changes in the device or editing of its internal software might not be easily detected.
However, if the bug were a physical device, it had to escape being recognized by the transporter scanner systems, (which should notice any major physical discrepancy between when Geordi left and when he returned) so it makes sense if the "bug" were a software package instead of hardware.