3D Printed Protection Against “Under-Door” Attacks

“Under-door” style attacks are when an attacker slides a tool through the gap underneath a door, hooks the interior handle from below, and opens the door by pulling the handle downward. This kind of attack works on the sort of doors and locks commonly found in hotels, where turning the handle from the inside always results in an open door. [Michal Jirků] found himself in a hotel room with a particularly large gap underneath the door, and decided to quickly design and print a door guard to protect against just such an attack.

It’s a simple object, and twenty minutes of printing and a little double-sided tape is all it takes to deploy. Because an attacker performs an under-door attack with a sizable mechanical disadvantage, it doesn’t take much to frustrate the attempt, and that’s exactly what the object does. Physical security in hotels is especially important, after all, and crooks have been known to exploit known flaws like the face-palmingly bad Onity key card lock exploit.

If you’re having trouble picturing how it all works, this video demonstrates an under-door attack in action, so you can see how blocking the space by the handle would easily prevent the tool from getting where it needs to go.

44 thoughts on “3D Printed Protection Against “Under-Door” Attacks

  1. A similar technique is how I accessed the network closets at a former workplace.
    The IT group figured they knew our infrastructure needs better than we did.
    They soon gave us the key.

  2. Same attack but through the letterbox has gotten me indoors several times. The last time there was an embarrassing naked incident and I changed the mechanism to not auto lock on close immediately after.

    1. Possibly, though where in the situation where the handle is not locked in place by the locking mechanism, or disengaged between internal and external. you could jiggle it out by using the external handle… however you could also deflect the handle past the end of the block for this thing.

  3. I was reading recently that building codes are specifying an inch gap under doors now for HVAC air circulation and air quality reasons. Guess the fire code bods haven’t pushed back yet.

    Anyway, I know of a couple of methods of that under the door attack which it does not look like that would defeat, just make it a bit more fiddly and another where it grabs the handle from hooking rather than looping over it, which it would not hinder in the slightest. Also just thought of another attack which would get around it. Basically you’re fending off opportunists, not professional MOE dudes.

    1. I’m thinking that if you really, really, really want to stop someone getting in without scamming a key from the desk or lifting it off housekeeping, or electronically poaching it, then you need a 6″ wall around the arc of the door handle. Possibly achieved in a life and death emergency by hacking the bottom out of one of the supplied garbage cans or ice buckets in the room with your Leatherman etc.

      1. Placing a chair under the door handle should also help. Either you can jam the backrest it under the handle or the seating surface of the chair will make it much more difficult to reach the handle from under the door.

      2. I seem to recall watching a video of a penetration expert saying he once came up against a door that he could not for love or money open with the usual under / over door tools. Turns out it was fitted with a bump guard to stop trolleys getting hung up the handle. Which looked not to dissimilar to what you are describing.

    1. Yes it would be much improved in a two piece design where the piece there is a pillar, and you have a sleeve with a spring printed into the bottom to provide detente action and the top of it cupped around the end of the handle, preventing it being moved until you pushed the sleeve down the pillar, compressing the spring and releasing the handle. You’d need sheer sides on the “cup” part or it would possibly pop out with sufficient force…. would probably break anyway from leaning on it from inside, but for most tools used it would exceed the force that they were able to exert if done halfway right.

  4. Why not just use a round door knob instead of a handle? I use round door knobs to prevent my pets from being able to open doors in the house.

    It does make it harder for patrons to open the door after a night of heavy drinking and/or drugs, but that also prevents a lot of potential sexual assaults from occurring behind closed doors.

    Your room number is private information, so if someone is stalking you and manages to trick the hotel staff into divulging your room number, then I would say a lock that can be breached with a coat hanger is the least of your worries.

    1. Well there’s the old fashion door under the knob trick, or the modern equivalent that jams the door. Coat hangers might have a problem, as well as the noise of one trying.

  5. Just spit-balling here… A piece of stiff card (e.g. laminated menu, folder of info pages) taped to be perpendicular to the door, under and parallel to the handle. Would make it hard to get a hook or a loop onto the handle, but leave it able to be opened by hand.

      1. Sure…… You didn’t have a 3D printer with you. ;) Anyway.. asking for a friend. How hard was it getting your 3D printer on the plane? I mean how hard was it for your “friend” to get “his” 3D printer in that plane?

        My concern would be getting it off the door. The double sided tape I buy has epic holding power. I’ve messed a few things up removing it.

  6. I’am not completely sure:But if the attacker manages to slide his tool onto the door handle, the small plastic part even protects it from sliding off the handle.

    1. There isn’t a huge gap under the door for any functional reason. It’s poor workmanship they aren’t going to fix. That said there are purpose built device meant for weather sealing that would so a better job. Then we’ll just use an over the door tool.

  7. This could really be helpful especially for travellers who often stay in hotels. I love how 3D printing has so much uses. The possibilities are really endless. Do you think that this 3d printed protection could possible be out in the market soon? Is it something necessary?

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