Try as they might, retailers don't seem to be able to get the Backoff malware to actually back off.

According to a new report from the security firm Damballa, detections of the notorious point-of-sale (PoS) malware jumped 57% from August to September. During the month of September alone, Backoff infections increased 27%.

This year, the Secret Service estimated that as many as 1,000 US businesses may be infected by the malware. That list of impacted businesses features some big names, including United Parcel Service (UPS) and Dairy Queen.

According to Damballa, the increase demonstrates that the malware is bypassing network prevention controls, and it underscores the importance of ensuring that PoS traffic is visible.

"In many cases, the PoS systems are free-standing from the corporate network," says Damballa CTO Brian Foster. "They connect to local networks, which have limited security. Without this visibility, it's impossible to discover the device is communicating with criminal command and control."

In addition, many PoS devices are accessible via remote access software for tasks such as software upgrades and patches, providing yet another avenue for compromise, Foster says.

In an advisory issued this summer, US-CERT said that attackers were using remote desktop tools such as Splashtop 2, LogMeIn, and Apple Remote Desktop as a convenient way to deploy PoS malware and steal data.

Curt Wilson, senior research analyst for Arbor Networks' ASERT team says companies that provide for the deployment and ongoing remote support to merchants that run PoS systems should implement strong security, because they are a target.

"If a PoS provider is compromised, the attackers typically obtain access to all their customer deployments via remote access capabilities, leading to complex, distributed compromise," Wilson says. "Strong authentication may provide an extra layer of defense in such a case, unless the strong authentication process is also compromised. Organizations, especially smaller to midsized organizations, should be aware of the potential of remote support being compromised."

All connectivity associated with PoS systems -- even connectivity that appears to be authorized -- should be audited on a regular basis, he says. Merchants purchasing PoS infrastructure should look into the provider's security posture and go elsewhere if they judge the security to be lax or if the appropriate contractual obligations cannot be met.

"Retailers should be implementing best practice security and application controls to prevent this type of malware," says Mike Davis, CTO of CounterTack. "Preventing outbound network connections except to known company owned servers… preventing the saving of data on the PoS except from the PoS software itself, and proper file and disk permissions would have prevented Backoff from working. The problem is, implementing all of this prevention is incredibly difficult, prone to errors, and takes a long time to deploy across the enterprise."

Foster says that, as long as Backoff continues to be effective, organizations should expect it to stick around. "Think of it as the malware du jour. As long as it works, threat actors will keep using it. As soon as its effectiveness diminishes, they will use something else."