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CCleaner Incident Investigation Reveals Possible Stage 3 Payload

CANCUN - KASPERSKY SECURITY ANALYST SUMMIT - The investigation into the September 2017 CCleaner incident has revealed what appears to be a stage three payload that attackers supposedly intended to deliver to infected users.

The attack was disclosed on September 18, when security firm Avast revealed that 2.27 million users worldwide had downloaded an infected CCleaner installation file between August 15 and September 12. Hackers had added a backdoor to the 32-bit CCleaner v5.33.6162 and CCleaner Cloud v1.07.3191 releases, Avast revealed.

What led to this was the compromise of the distribution servers of Piriform, the company developing CCleaner, in the months before Avast purchased the software firm. The code in the modified installers could collect non-sensitive information from the infected machines, and could also deliver a second stage binary.

This revealed that the incident was in fact a highly targeted attack, as the second-stage payload was delivered to only 40 computers out of the millions that downloaded stage one. While no stage three binary was found on the affected systems, Avast now says that the attackers, the Chinese hacking group Axiom (also known as APT17 or DeputyDog), apparently had plans to deliver such malware as well.

During its investigation of the Piriform infrastructure, the security firm discovered not only stage one and stage two binaries on the network, but also evidence of a third stage on four computers. Dubbed ShadowPad, this is a specialized tool that provides cybercriminals with remote control capabilities.

In an August 2017 report, Kaspersky revealed that the ShadowPad backdoor was found in NetSarang’s products, which are used by hundreds of companies in the financial, software, media, energy, electronics, insurance, industrial, construction, manufacturing, retail, telecoms, pharmaceutical, and transportation sectors.

“The tool was installed on the four Piriform computers on April 12th, 2017, while the preliminary version of the second stage had been installed on the computers March 12th, 2017,” Avast says.

The command and control (C&C) server the older second stage variant was attempting to connect to was no longer up during the investigation and the researchers don’t know exactly what it was supposed to download. However, given the timeline of events, they assume that it “had downloaded and installed ShadowPad on the four Piriform computers.”

The fact that ShadowPad is believed to have been developed by the Axiom group, the same actor behind the CCleaner attack, is also a strong indicator that this malware was intended to become the third stage payload, Avast says.

The ShadowPad version used in the attack was custom-built, leading investigators to suspect it was explicitly created for Piriform.

The security firm also discovered ShadowPad log files containing encrypted key strokes from a keylogger that became active on the infected machines on April 12, 2017. Other tools were also installed on the four computers, including a password stealer, along with tools that could install more software and plugins on the infected machines.

“While ShadowPad was installed on the Piriform network itself and, as far as we can tell through our investigations today, not on any of the CCleaner customers’ computers, we believe that this tool was the intended third stage for the CCleaner customers,” Avast says.

The second-stage malware deployed to only 40 computers of the millions that downloaded the infected CCleaner versions, but Avast couldn’t determine whether the third stage payload was meant for all of them or only a few, if any.

Related: Researchers Link CCleaner Attack to State-sponsored Chinese Hackers

Related: Backdoors Found in Tools Used by Hundreds of Organizations

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