Here's the password you should never use for an RDP system

Rapid7, an IT security company, set up honeypots that were designed to mimic systems running RDP (remote desktop protocol). The protocol is used to remotely login into a system and is often run by POS (point-of-sale) devices, kiosks and other Windows systems.

Attackers often scan the Internet to find RDP systems and then try to log in. Rapid7 logged more than 221,000 login attempts and then studied the credentials that attackers used.

Rapid7 wondered whether attackers were using some of periodic lists released of the most common weak passwords, said Tod Beardsley, a security expert with Rapid7.

"We see some correlation in there, but the ordering is all wrong," he said in a phone interview. "Our number one password we see is 'x.'"

X is one of three single-character passwords that Rapid7 noticed. Of course, it's a terrible password, and security experts recommend passwords that are long and random, with odd characters to lessen the chance of someone guessing it. Two other common guesses were "Zz" and "St@rt123."

Since password guesses are often limited by RDP systems, attackers have only so many chances to try different ones before they're locked out. Beardsley thinks the weak passwords the attackers used -- even though there are many out there -- have in some cases been carefully selected.

"These are clearly dictionary attacks," he said. "They've correlated and they're cultivating small lists of passwords."

Over the year the honeypots were active, Rapid7 collected about 4,000 passwords, about 20 percent of which just showed up once and were never used again.

Beardsley said he'd expected that the attackers would use the same credentials repeatedly. But it appears when they discover a new potential credential for a possible POS system running RDP, they try it, find all the vulnerable systems, then move on.

"They never look again," he said. "It's not like a weekly scan."

Using default credentials or weak ones is particularly dangerous for companies running a fleet of POS devices. A large number of credit card numbers could be collected if a network was breached or a device compromised.

Although it's advised that RDP should not be left running open to the Internet on a POS device, it still happens. Rapid7 scanned the Internet and found 11 million systems running RDP. Beardsley said it wasn't possible to known how many of those are POS systems, but it's likely many are.

It is illegal to attempt to log into those systems, so Beardsley said Rapid7 couldn't take its research further. But it shows that one scan by a group of cybercriminals could provide many targets for possible exploitation.

"I'm willing to bet when you sweep through the 11 million or so endpoints, you will have hundreds and hundreds of hits on these things," Beardsley said. "So you've got plenty of targets to work with at that point, particularly if they're point-of-sale systems."