Tampering with Tamper-Resistant Screws

While their overall security is still moderate at best, it is sufficient for many situations that do not warrant the added expense of locks.

Now let’s examine the spanner-head screw, shown in the middle of Figure 2. It is intended to be installed and removed with a spanner screw-driver or with a spanner bit like the one in Figure 5. Let’s assume for a minute that unauthorized users will not possess any special tools. A screwdriver and a small hammer can be used to exploit the holes, tapping the screw-head counter-clockwise to loosen the screw as shown in Figure 6. Notice, in Figure 7, that this method leaves gouges in the edges of the holes. If one or both holes is gouged like this, then someone perhaps a hoodlum has been screwing with your screw!

The tamper-resistant Allen screw, shown on the right in Figure 2, was installed with a special Allen wrench that has a hole in its end. It was torqued down well. In Figure 8 it is being loosened with the pliers of a common multi-tool. The teeth of the pliers left marks that can be seen upon close examination of the screw. Those marks are shown in Figure 9.

In Figure 10, many tamper-resistant Allen screws secure the playground equipment behind an elementary school. I suppose it is intended to deter hoodlums from stealing or sabotaging the equipment, thereby helping to assure that the children can have a fun and safe recess. The screw, shown up-close in Figure 11, is intended to be removed only with the proper tool. But, once again, a pair of pliers can do the trick as well. This method does leave visible evidence, as seen in Figure 12, but it’s less obvious than on the previous screw. The plainness of the evidence is related to how tightly the pliers are clamped onto the screw-head and to how much torque is required to loosen the screw, whereas this screw had not been fastened as tightly as the other.

What does all of this mean? First, despite some manufacturers’ claims, it proves that these tamper-resistant screws are far from tamper-proof. But, although tamper-resistant screws provide only a moderate level of tamper-resistance, they provide clear evidence when they’re opened with standard hand tools. If the screw-heads are regularly inspected, then this tamper-evidence significantly increases the overall level of security provided by such screws. As such, while their overall security is still moderate at best, it is sufficient for many situations that do not warrant the added expense of locks.


Let’s look at three situations where tamper-resistant screws do not provide sufficient security on their own, even if regularly inspected. The first such situation is shown in Figure 13. It’s a Securitron DK-11 keypad, clearly labeled as such. The security provided by the keypad is circumventable by unscrewing the keypad from the wall and removing any one or more of the four wires that are connected to it. This is because the DK-11 keypad directly controls the power to the electromagnetic lock on the door. (Not everyone knows this weakness, but anyone who ever tries to pull a MacGuyver on this keypad will certainly succeed.) The door is on the exterior of a restaurant that is left unattended for several hours every night.

The DK-11 is not designed for such a scenario, and for anything but low-security applications, Securitron recommends the use of the DK-16 with its separate interior-mounted control unit. We need to pay attention to such recommendations and use security equipment appropriately, as a couple of fancy screws can only do so much for us.

To make matters worse, this keypad has been removed multiple times for combination-changing without the proper tool, and after each removal the same pair of one-way screws has been reused to fasten the keypad to the wall. The limited tamper-evident quality of the screws has thus been defeated, so it may very well be that no one will ever know when this access-control system is bypassed.

Second, consider a double-cylinder deadbolt, fastened to a door separating two business suites that are leased by separate companies that have no reason to trust each other. The two key-operated lock-cylinders are often attached to each other by two one-way screws that traverse the door and lock-bolt. It is common in double-cylinder deadbolt design for both of those screws to be installed from the same side of the door. Inherent in this design is a bias in security. That is, the door is more easily unlocked from the side on which the screw-heads are exposed. When real burglar-resistance is required, the tamper-resistance offered by such screws is insufficient.

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