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Security Research

CCleaner Compromise: Yet another software supply-chain attack

September 22, 2017 - 2 min read

Earlier this week, Avast, a multinational security software vendor, reported a compromise of their Windows system utility CCleaner. CCleaner is a very popular file system and registry clean up utility that optimizes performance by removing unneeded registry entries and files. Attackers managed to compromise the software update infrastructure sometime in August 2017 and inject malicious code in the CCleaner update v5.33 and cloud version v1.07.

The injected malicious code causes the compromised machine to communicate back to a predetermined C&C server (hardcoded IP addresses and DGA domains) to report infection and download a second stage malware payload. Users from a very targeted list of organizations including Microsoft, Cisco, Intel, VMware, Sony, etc., were the only ones to be served a second stage malware payload. Per Avast, 700K users downloaded and installed the compromised version of CCleaner, however, only the 20 users that belonged to the targeted organizations were served with a second stage payload.

It is important to note that the malicious CCleaner installer package was delivered using CCleaner’s software update infrastructure over HTTPS and was signed using a legitimate certificate.

The Zscaler team has been actively monitoring this issue over the past 72 hours and has added multiple protections to block the payloads as well as post-infection activity for the backdoor module.

Avast’s Response

Avast contacted all the impacted customers and revoked the legitimate certificate that was used to sign the compromised version of CCleaner package and issued an updated version of the package.

More details:

How Zscaler Can Help with Preventative Measures

Zscaler added multiple signatures and indicators for blocking the original payloads as well as post-infection activity shortly after the information was disclosed to help any affected organizations in their remediation efforts.

Advanced Threat Signatures:



Win32/CCleaner.A Trojan

Cloud Sandbox provides the best line of defense in a proactive manner against these threats. Zscaler Cloud Sandbox successfully detected the payloads from this compromise. Here is a sample Cloud Sandbox report from one such detonation:



SSL inspection is necessary to protect organizations. Over 60% of Internet traffic is over SSL, yet most advanced threats hide in SSL. Zscaler Cloud Security Platform provides native SSL inspection.


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