In a blog post, Talos said it had notified Avast, the owners of CCleaner, about the issue on 13 September. The malware can download other components like ransomware.
The attackers were using version 5.33 of CCleaner to spread a multi-stage malware payload. CCleaner had more than two billion downloads as of November last year. This version of CCleaner had a valid digital signature issued to Piriform, the company that makes CCleaner and which was recently acquired by Avast.
The presence of malware in CCleaner was first spotted by security firm Morphisec; however, they were late with a full write-up which has led to them being somewhat sidelined in the media.
"Users of CCleaner Cloud version 1.07.3191 have received an automatic update. In other words, to the best of our knowledge, we were able to disarm the threat before it was able to do any harm," he claimed.
Why does the CCleaner website have absolutely no mention of the trojan issue at all? It doesn't even mention it in the version history. pic.twitter.com/v0uhtJp4lU— Kevin Beaumont (@GossiTheDog) 18 September 2017
An Avast spokesperson told iTWire that about 2.27 million people downloaded the affected version of CCleaner. But Talos said the impact of the attack could be more severe, given that CCleaner was reported to be adding five million new users a week.
"We estimate that 2.27 million users had the v5.33.6162 software, and 5010 users had the v1.07.3191 of CCleaner Cloud installed on 32-bit Windows machines. We believe that these users are safe now as our investigation indicates we were able to disarm the threat before it was able to do any harm," the spokesperson said.
The malicious payload included a domain generation algorithm as well as hard-coded command and control server functionality and was being hosted on the official CCleaner download servers as recently as 11 September.
The Talos team said it appeared that the affected version had been released on 15 August and was signed using a valid certificate issued to Piriform by Symantec.
When a second sample associated with the same threat was discovered, it was also found to be signed using a valid digital certificate. But the signing timestamp was 15 minutes after the initial sample was signed.
The Talos team said they had found a compilation artifact within the binary that they analysed. In view of these two factors, they said it was likely that an external attacker had compromised a part of the CCleaner development or build environment and inserted malware into the CCleaner build.
It was also possible that an insider had included the malware in the build or had an account compromised.
As far as the malware was concerned, once it had gained access to a Windows system, it ascertained whether the user in question was running as admin or not; if the latter, then the malware terminated its activity.
If the user had admin privileges, then the malware established a connection to a command and control server.
The domain generation algorithm in the malware generated a domain for each month and Talos detected an increase of activity to the domain which was generated for August, and later for that generated for September.
The team said it had registered these domains and sinkholed them to avoid their being used for any nefarious purpose.
Talos said: "In many organisations 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."