According to security researcher Brian Krebs, although these green or blue semi-transparent anti-skimming devices are often oddly shaped, the differences between the anti-skimming devices and the fraudulent add-ons are not that easy to spot.
Earlier this year, says Krebs, the authorities in Ireland began dealing with a rash of ATM skimmers and, he says, a source close to the investigation said that identical skimmers have been found attached to ATMs across the country.
"The source said a 33-year-old Moldovan man has been arrested in Limerick in connection with the attacks, which authorities have called part of a global ATM fraud operation", he said in his security blog.
What is interesting, however, is that Krebs reports that Russian fraudsters are developing skimming add-ons for ATMs that quite overt once fitted, but are designed to look like the anti-skimming units.
The security researcher said that, when he first saw the Russian's skimmer photos, he was not too impressed.
"I'd never seen anti-skimming devices that looked even remotely like his in real life. But that changed in December, when the wife and I travelled to Costa Rica for some friends' destination wedding", he said.
"While we were there, we had a chance to stay in and hike through the gorgeous Monteverde Cloud Forest, and at the end of a guided tour through the forest I needed to stop by the ATM to tip our guide. When I got to the town's bank and saw the ATM pictured below, I took a step back. For one thing, the NCR ATM looked like it had one of these fake anti-skimmer devices attached", he added.
Krebs went on to say that he grew more nervous when he noticed that the only other ATM at this bank was out of order - a trick that skimmer thieves use on non-compromised cash machines.
"I yanked pretty hard on the green device affixed to the ATM, and it remained attached. Left with the choice between stiffing our driver and excellent guide without a tip and taking out cash from this machine, I chose the latter", he said.
"I haven't seen any suspicious charges yet, but it just goes to show you how even a little knowledge of these ATM skimmers really can make you paranoid", he added.
The moral of this story, Infosecurity notes, is that if you see a green or blue semi-transparent devices retrofitted to an ATM, you should exercise extreme caution and preferably only use ATMs within bank premises for absolute safety.
There may also be argument for opening a prepaid debit card account and loading with a modest amount for use in suspicious situations.