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CCleaner malware spread via supply chain attack

CCleaner malware was spread to users via an infected software update for close to one month, highlighting the dangers of supply chain attacks and the need for code signing.

Researchers discovered a popular system maintenance tool was the victim of a supply chain attack that put potentially millions of users at risk of downloading a malicious update.

CCleaner is a tool designed to help consumers perform basic PC maintenance functions like removing cached files, browsing data and defragmenting hard drives. CCleaner is made by Piriform Ltd., a U.K.-based software maker that was acquired by antivirus company Avast Software in July. The compromised update of the tool was first discovered by Israeli endpoint security firm Morphisec following an investigation that began on Sept. 11, but the company claims it began blocking the CCleaner malware at customer sites on Aug. 20.

"A backdoor transplanted into a security product through its production chain presents a new unseen threat level which poses a great risk and shakes customers' trust," wrote Michael Gorelik, vice president of research and development at Morphisec, in a blog post. "As such, we immediately, as part of our responsible disclosure policy, contacted Avast and shared all the information required for them to resolve the issue promptly. Customers (sic) safety is our top concern."

The CCleaner malware gathered information about systems and transmitted it to a command and control (C&C) server; it was reportedly downloaded by users for close to one month from Aug. 15 to Sept. 12, according to Morphisec. However, Avast noted that the CCleaner malware was limited to running on 32-bit systems and would only run if the affected user profile had administrator privileges.

Avast said CCleaner claims to have more than 2 billion downloads and adds new users at a rate of 5 million per week, but because only the 32-bit and cloud versions of CCleaner were compromised, the company estimated just 2.27 million users were affected.

Impact of the CCleaner malware

A team of researchers at Cisco Talos, which included Edmund Brumaghin, threat researcher; Ross Gibb, senior information security analyst; Warren Mercer, technical leader; Matthew Molyett, research engineer; and Craig Williams, senior technical leader, discovered and analyzed the CCleaner malware soon after Morphisec. According to the Cisco Talos team, Avast unwittingly distributed legitimate signed versions of CCleaner and CCleaner Cloud, which "contained a multi-stage malware payload that rode on top of the installation."

"This is a prime example of the extent that attackers are willing to go through in their attempt to distribute malware to organizations and individuals around the world. By exploiting the trust relationship between software vendors and the users of their software, attackers can benefit from users' inherent trust in the files and web servers used to distribute updates," Talos researchers wrote in their analysis. "In many organizations data received from commonly software vendors rarely receives the same level of scrutiny as that which is applied to what is perceived as untrusted sources. Attackers have shown that they are willing to leverage this trust to distribute malware while remaining undetected."

What makes this attack particularly worrying is the volume of downloads this software receives leaving a huge number of users exposed.
James Maudesenior security engineer, Avecto

James Maude, senior security engineer at Avecto, a privilege management software maker, said it was especially concerning that the CCleaner malware included the official code signature from Avast.

"Given that CCleaner is designed to be installed by a user with admin rights, and the malware was not only embedded within it but also signed by the developers own code signing certificate (giving it a high level of trust); this is pretty dangerous," Maude told SearchSecurity via email. "This means that the malware, and therefore the attacker, would have complete control of the system and the ability to access almost anything they wanted. What makes this attack particularly worrying is the volume of downloads this software receives leaving a huge number of users exposed."

Itsik Mantin, director of security research at security software company Imperva, said the CCleaner malware incident shows "there's not much users can do when the vendor gets infected."

"This hack creates a new reality where users need to assume that their desktops, laptops and smartphones are infected, which has been the reality for security officers at organizations in the last years," Mantin told SearchSecurity. "For organizations, this does not really matter as security officers are accustomed to the reality that they should always assume the attackers are in, are looking for ways to spread the infection within the organization and are searching for business sensitive data to steal or corrupt."

Avast response to the CCleaner malware incident

Vince Steckler, CEO of Avast Software, and Ondřej Vlček, executive vice president and general manager of the consumer business unit, released a statement saying the company remediated the issue within 72 hours of becoming aware of the problem by releasing a clean update without the malware. They also stated Avast is working with law enforcement to shut down the CCleaner malware C&C server on Sept. 15.

The Avast execs downplayed their company's involvement by saying they "strongly suspect that Piriform was being targeted while they were operating as a standalone company, prior to the Avast acquisition," and that the compromise "may have started on July 3rd," two weeks before Avast's acquisition of Piriform was complete. Avast also claimed the compromised update took four weeks to discover due to "the sophistication of the attack."

Avast asserted users "should upgrade even though they are not at risk as the malware has been disabled on the server side," and claimed it was unnecessary to follow the suggestions by Talos and other experts to restore systems to a date before Aug. 15, 2017, to ensure removal of the CCleaner malware.

"Based on the analysis of this data, we believe that the second stage payload never activated, i.e. the only malicious code present on customer machines was the one embedded in the ccleaner.exe binary," Steckler and Vlček wrote. "Therefore, we consider restoring the affected machines to the pre-August 15 state unnecessary. By similar logic, security companies are not usually advising customers to reformat their machines after a remote code execution vulnerability is identified on their computer."

Supply chain attacks

Experts said the CCleaner malware incident should be a reminder of the dangers of supply chain attacks.

Marco Cova, senior security researcher at malware protection vendor Lastline, said the recent NotPetya attacks were another case of a supply chain attack "where an otherwise trusted software vendor gets compromised and the update mechanism of the programs they distribute is leveraged to distribute malware."

"This is sort of a holy grail for malware authors because they can efficiently distribute their malware, hide it in a trusted channel, and reach a potentially large number of users," Cova told SearchSecurity. "It appears that the build process of CCleaner itself was compromised: that is, attackers had access to the infrastructure used to build the software itself. This is very troublesome because it indicates that attackers were able to control a critical piece of the infrastructure used by the vendor."

Jonathan Cran, vice president of product at Bugcrowd, told SearchSecurity the CCleaner malware issue appeared to be "less of a traditional supply chain attack and more of a case of poor vendor security. Given that the affected installer was signed as a verified safe binary by Piriform, this indicates that they didn't realize at the time of release and that the corporate network of Piriform was likely compromised."

Justin Fier, director of cyber intelligence and analytics at threat detection company Darktrace, said this "should come as yet another wake-up call that corporations must have visibility into how their suppliers interact with their systems, as well as a real-time assessment of their suppliers' cyber risk."

"The risk that companies inherit from their suppliers is a pervasive problem for cybersecurity. Quite simply, companies with a supply chain cannot avoid compromises -- supply chain breaches are inevitable," Fier told SearchSecurity. "The assessment of potential supply chain partners is often a rushed process in terms of evaluating their cybersecurity level, and is rarely as in-depth as it should be. While we can't change the security posture of our supply chains, we can have a transparent relationship when it comes to cyber risk."

Next Steps

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