Security researchers have uncovered a new DealPly variant which uses a new method to avoid detection by abusing the reputation services provided by Microsoft’s SmartScreen and McAfee’s WebAdvisor.

DealPly is an adware strain that would typically install browser extensions designed to display advertisements in the victim’s browser. It also features “modular code, machine fingerprinting, VM detection techniques and robust C&C infrastructure” according to enSilo researchers who discovered this new variant.

“We suspect that the reason why DealPly is leveraging reputation services is to check which of its variants and download sites are compromised and won’t be effective for future infections,” says enSilo’s research team.

The analyzed adware sample was observed while collecting reputation information on domains received from its operators by querying the services and delivering the responses to its command and control (C2) servers.

Abusing the SmartScreen API

SmartScreen (officially known as Windows Defender SmartScreen) is a service designed to warn Microsoft Windows customers when visiting potentially malicious domains that have been previously used in malware and phishing attacks or when downloading possibly malicious apps. 

If a Windows user tries to access a malicious domain or app, a warning will be shown to recommend proceeding with caution.

To avoid being blacklisted by Microsoft while querying its reputation service, DealPly will take advantage of the machines it manages to infect and will use them as a “distributed network of machines for harvesting this data.”

The adware’s SmartScreen module will automatically deliver an empty request to the C2 server to ask for domain hashes and URLs to query.

DealPly will use JSON-based API requests for querying the SmartScreen reputation server to which it appends “an Authorization header which is responsible for hardening the requests from unwanted alterations.”

The response received from SmartScreen contains a string describing the nature of the tested URL, with the following strings being the ones DealPly will look for in the reply:

The collected data will get sent to DealPly’s C2 server which allows its operators to keep a close eye on what domains or installers they use are already marked as malicious by Microsoft’s reputation service.

DealPly comes with support for multiple versions of the undocumented SmartScreen API which allows it to query the service on several Windows versions.

“It is important to note that the SmartScreen API is undocumented. This means the author has put a lot of effort in reverse engineering the inner workings of the SmartScreen mechanism/feature,” says enSilo.

McAfee SiteAdvisor also abused by DealPly

McAfee’s WebAdvisor reputation service is a free tool which tracks and reports websites’ level of safety using data collected by its web crawler that checks for spam and malicious content.

“The variant starts by checking if WebAdvisor of a specific version is installed. If those conditions are met then the sample will try querying the WebAdvisor reputation service,” found enSilo.

DealPly will send a request to the WebAdvisor service using the https://webadvisorc.rest.gti.mcafee.com/1 URL and will extract the reputation value of the checked domain from the response.

This information is delivered to the C2 server, thus making it possible for the campaign operators to update their database of domains and installers with information on which of them are detected as unsafe.

“With the data from these services, the life-span for the Adware’s installers and components can be prolonged as changes are required only once they are known to be blacklisted,” adds enSilo. “Such techniques are not relevant solely to Adware and may be adopted by malware authors as well.”

The DealPly operators’ approach to implementing this AV evasion technique allows them to be one step ahead of anti-malware solutions and actively updating their adware installers to lower their detection rate.

As enSilo’s research team adds, this method of avoiding detection will most probably be adopted by malware developers seeing that it has already been used by adware peddlers for evasion purposes.

More in-depth info on DealPly’s inner workings, its infection flow, machine fingerprinting features, and modular code are available in enSilo’s adware analysis report, together with a list of indicators of compromise (IOCs) including sample hashes, domains, and URLs.

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