Behind those somewhat convoluted words, economy of mechanism simply means that the more complex a system is, the more likely security vulnerabilities will be present, and the more likely it will be missed during a review.
This is because the time to review and find vulnerabilities increases (sometimes exponentially) the more mechanisms, or the more code there is.
According to Bishop 1 in Chapter 13, “Design Principles,” in “Principle of Economy of Mechanism” from pages 344-345:
This principle simplifies the design and implementation of security mechanisms.
Definition 13-3. The principle of economy of mechanism states that security mechanisms should be as simple as possible.
If a design and implementation are simple, fewer possibilities exist for errors. The checking and testing process is less complex, because fewer components and cases need to be tested. Complex mechanisms often make assumptions about the system and environment in which they run. If these assumptions are incorrect, security problems may result.
Interfaces to other modules are particularly suspect, because modules often make implicit assumptions about input or output parameters or the current system state; should any of these assumptions be wrong, the module’s actions may produce unexpected, and erroneous, results. Interaction with external entities, such as other programs, systems, or humans, amplified this problem.
The ident protocol sends the user name associated with a process that has a TCP connection to a remote host. A mechanism on host A that allows access based on the results of an ident protocol result makes the assumption that the originating host is trustworthy. If host B decides to attack host A, it can connect and then send any identity it chooses in response to the ident request. This is an example of a mechanism making an incorrect assumption about the environment (specifically that host B can be trusted).
The finger protocol transmits information about a user or system. Many client implementations assume that the server’s response is well-formed. However, if an attacker were to create a server that generated an infinite stream of characters, and a finger client were to connect to it, the client would print all the characters. As a result, log files and disks could be filled up, resulting in a denial of service attack on the querying host. this is an example of incorrect assumptions about the input to the client.