[Yaehob]’s parents have a security system in their house, and when they wanted to make a few changes to their alarm rules – not arming the bathroom at night – an installer would come out, plug a box into the main panel, press a few buttons, and charge 150 €. Horrified at the aspect of spending that much money to flip a few bits, [yaehob] set out to get around the homeowner lockout on the alarm system, and found security where he wasn’t expecting.

Opening the main panel for the alarm system, [yaehob] was greeted with a screeching noise. This was the obvious in retrospect tamper-evident seal on the alarm box, easily silenced by entering a code on the keypad. The alarm, however, would not arm anymore, making the task of getting ‘installer-level’ access on the alarm system a top priority.

After finding a DE-9 serial port on the main board, [yaehob] went to the manufacturer’s website thinking he could download some software. The website does have the software available, but only for authorized distributors, installers, and resellers. You can register as one, though, and no, there is no verification the person filling out a web form is actually a distributor, installer, or reseller.

Looking at the installer and accompanying documentation, [yaehob] could see everything, but could not modify anything. To do that would require the installer password, which, according to the documentation was between four and six characters. The system also responded quickly, so brute force was obviously the answer here.

After writing up a quick script to go through all the possible passwords, [yaehob] started plugging numbers into the controller board. Coming back a bit later, he noticed something familiar about what was returned when the system finally let him in. A quick peek at where his brute force app confirmed his suspicions; the installer’s code was his postal code.

From the installer’s point of view, this somewhat makes sense. Any tech driving out to punch a few numbers into a computer and charge $200 will always know the postal code of where he’s driving to. From a security standpoint, holy crap this is bad.

Now that [yaehob]’s parents are out from under the thumb of the alarm installer, he’s also tacked on a little bit of security of his own; the installer’s code won’t work anymore. It’s now changed to the house number.