A key malicious domain name used to control potentially thousands of computer systems compromised via the months-long breach at network monitoring software vendor SolarWinds was commandeered by security experts and used as a “killswitch” designed to turn the sprawling cybercrime operation against itself, KrebsOnSecurity has learned.

The still-unfolding breach at network management software firm SolarWinds may have resulted in malicious code being pushed to nearly 18,000 customers, the company said in a legal filing on Monday. Meanwhile, Microsoft should soon have some idea which and how many SolarWinds customers were affected, as it recently took possession of a key domain name used by the intruders to control infected systems.

Communications at the U.S. Treasury and Commerce Departments were reportedly compromised by a supply chain attack on SolarWinds, a security vendor that helps the federal government and a range of Fortune 500 companies monitor the health of their IT networks. Given the breadth of the company’s customer base, experts say the incident may be just the first of many such disclosures.

On Aug. 13, 2020, someone uploaded a suspected malicious file to VirusTotal, a service that scans submitted files against more than five dozen antivirus and security products. Last month, Microsoft and FireEye identified that file as a newly-discovered fourth malware backdoor used in the sprawling SolarWinds supply chain hack. An analysis of the malicious file and other submissions by the same VirusTotal user suggest the account that initially flagged the backdoor as suspicious belongs to IT personnel at the National Telecommunications and Information Administration (NTIA), a division of the U.S. Commerce Department that handles telecommunications and Internet policy.

At least 30,000 organizations across the United States — including a significant number of small businesses, towns, cities and local governments — have over the past few days been hacked by an unusually aggressive Chinese cyber espionage unit that’s focused on stealing email from victim organizations, multiple sources tell KrebsOnSecurity. The espionage group is exploiting four newly-discovered flaws in Microsoft Exchange Server email software, and has seeded hundreds of thousands of victim organizations worldwide with tools that give the attackers total, remote control over affected systems.

The ongoing breach affecting thousands of organizations that relied on backdoored products by network software firm SolarWinds may have jeopardized the privacy of countless sealed court documents on file with the U.S. federal court system, according to a memo released Wednesday by the Administrative Office (AO) of the U.S. Courts.

U.S. government cybersecurity agencies warned this week that the attackers behind the widespread hacking spree stemming from the compromise at network software firm SolarWinds used weaknesses in other, non-SolarWinds products to attack high-value targets. According to sources, among those was a flaw in software virtualization platform VMware, which the U.S. National Security Agency (NSA) warned on Dec. 7 was being used by Russian hackers to impersonate authorized users on victim networks.