A botnet of at least 75,000 computers at 2,500 companies and government agencies worldwide has been identified by security researchers.
According to NetWitness, the Kneber botnet, named for the username linking the affected machines worldwide, has been used to gather login credentials to online financial systems, social networking sites and email systems for the past 18 months.
A 75GB cache of stolen data discovered by NetWitness included 68,000 corporate login credentials, login data for user accounts at Facebook, Yahoo and Hotmail, 2,000 SSL certificate files and a large amount of highly detailed 'dossier-level' identity information.
In addition, systems compromised by the botnet also give attackers remote access inside the compromised network, the company said.
"Disturbingly, the data was only a one-month snapshot of data from a campaign that has been in operation for more than a year," NetWitness said.
NetWitness did not release the names of the companies compromised in the attacks, which it described as being highly targeted and well co-ordinated.
A story in the Wall Street Journal identified pharmaceutical company Merck & Co, Cardinal Health, Paramount Pictures and Juniper Networks as some of US firms that had been infiltrated.
Systems belonging to 10 government agencies were also penetrated in the attacks.
According to the Journal, the attacks started in late 2008 and appeared to originate in Europe and China.
Computers in as many as 196 countries have been affected, with many systems compromised after users clicked on phishing emails with links to sites containing malicious code.
Most of the compromised systems appeared to be in Egypt, Mexico, Saudi Arabia, Turkey and the US, the Journal reported, quoting an unnamed source with information on the attacks.
NetWitness, which provides a range of network monitoring and forensics services for companies and government agencies, discovered the botnet in January during a routine engagement with one of its clients.
According to the company, the botnet is a variant of the Zeus botnet, which is known primarily for stealing banking credentials.
More than half of the infected systems in the Kneber botnet also contained the competing Waledac Trojan, probably because those behind the attacks wanted to build some redundancy into their attacks, NetWitness said.
"The co-existence of Zeus and Waledac suggests the goals of resilience and survivability and potential deeper cross-crew collaboration in the criminal underground," the company said.
Both incidents underscore what analysts are calling the Advanced Persistent Threat (APT) faced by a growing number of financial, commercial and government entities .
The term has been used for some time in government and military domains to describe targeted cyberattacks carried out by highly organised state-sponsored groups and organised cybergangs with deep technical skills and computing resources.
Such attacks are typically highly targeted, stealthy, customised and persistent. They also often involve intensive surveillance and advanced social engineering.
In many cases, the attacks target highly placed individuals within organisations, who are tricked into visiting malicious sites or downloading malicious software onto their systems.