It's not a strong electric shock else you'd be shouting that from the rooftops so, I am presuming you mean something more like an irritating tingle. Based on that assumption I would suggest that the power supply is leaking a little AC current from the power side through to the DC side and this side is also connected to the metal casing around the cameras. This leakage is controlled and is due to the component marked inside the red box below: -
As you can see it bridges the isolation transformer and thus can inject a small current into anyone touching the DC side. That component is present to reduce EMI on the output and thus allow the power supply to meet EMC requirements. But the down side is that about 1 mA can be pushed through anyone touching the DC output if it hasn't been grounded.
If the AC power adaptor used a proper three pin plug it could meet the EMC requirements by adding a capacitor on the RTN pin to ground. Unfortunately this is a cost issue in a savage cost-cutting market hence the slight inconvenience of a tingle that is below the level where it can harm you. As you can see the normal conducting middle prong on the adaptor is plastic so, for EMC compliance it relies on the filter capacitor to connect to the un-isolated DC side of the appliance: -
The circuit diagram is from this question and answer.
Do some investigation if you can and measure the current with your multimeter just to ascertain the current is below a couple of mA. If it is more than this then there may be something unsafe with your installation.