Some of the targets (graphic, below right) that appeared to have been served a second-stage payload by the malware were Cisco, Intel, Microsoft, HTC, Samsung, VMware, Akamai and Sony.
The fact that CCleaner had been infected by malware was spotted by security firm Morphisec on 12 September. This information was conveyed to Avast, the owner of CCleaner, a product made by Piriform which Avast acquired recently.
CCleaner is a popular application that allows Windows users to perform routine maintenance on their systems.
In its initial analysis, the Talos team 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.
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. Then a second-stage payload was downloaded.
In its follow-up, published overnight, the Talos team said a review of the command and control tracking database, covering four days in September, showed that at least 20 victim machines had been served specialised secondary payloads.
Apart from the companies cited above, D-Link, Akamai, O2, Vodafone, German gaming and gambling company Gauselmann, Linksys, Gmail, MSI, Dynamic Network Services, and Epson were listed.
The C2 database had two tables: one describing all machines that had reported to the server and the other detailing all machines that received the second-stage download, both of which had entries were dated between 12 and 16 September.
Talos said during the compromise, "the malware would periodically contact the C2 server and transmit reconnaissance information about infected systems.
"This information included IP addresses, online time, hostname, domain name, process listings, and more. It's quite likely this information was used by the attackers to determine which machines they should target during the final stages of the campaign."
The team added: "These new findings raise our level of concern about these events, as elements of our research point towards a possible unknown, sophisticated actor."
Avast said on Wednesday that the number of infected machines was down to 730,000 from the original estimate of 2.27 million.
However, the Talos team had this to add: "(They) also support and reinforce our previous recommendation that those impacted by this supply chain attack should not simply remove the affected version of CCleaner or update to the latest version, but should restore from back-ups or reimage systems to ensure that they completely remove not only the backdoored version of CCleaner but also any other malware that may be resident on the system."
More than 700,000 machines reported to the C2 server in these four days and more than 20 received the second-stage payload. "It is important to understand that the target list can be and was changed over the period the server was active to target different organisations," the team said.
Graphic: courtesy Cisco Talos Intelligence Group