'Are' makes it clear that it's the personnel; 'internal security' could otherwise be the abstract situation, or its procedures, or its security equipment and measures, all of which would take singular: its internal security is flawed / checked weekly / provided by outside contractors.
In other cases:
The group noun and verb agreement is quite subtle and it is more differentiated in BE than in AE. In AE group nouns usually take the plural (the staff are; the government are; the class are, etc)
If we restrict "security" to be the group name for all the personnel involved in maintaining security, then Security are -> indicates each individual member of "security" -> "Security are unhappy with their pay rise."
in the same meaning, Security is -> (i) the security department; (ii) all of the members of the security department viewed as a whole; (iii) an identifiable section within the security department.
(i) Security is telling me that everyone will have to be searched.
(ii) Security has already been deployed.
(iii) Security (the management of the security department) says their guards are are unhappy with their pay rise.