Last month an organised criminal gang unleashed a devastating wave of attacks against more than half a million internet users by attempting to hold their computers hostage.
The attack campaign lasted a little over two weeks, and laced adult advertisements to target users pursuing pornographic websites.
More than 500,000 people clicked on the malicious advertisements and were directed to web sites hosting the infamous Reveton ransomware, a toolkit designed to lock down machines until a ransom is paid.
The attackers were "still very much in operation" and have evaded capture, Symantec security researcher Vikram Thakur told SC.
"They haven't been busted, and we don't know where they are," he said.
The half million victims were saved by Symantec's AV systems which detected attempts by the payload websites to locate application vulnerabilities on victim computers which would be used to deliver the malware.
Reveton, detected as Trojan.Ransomlock.G, could lock down the master boot record to prevent use of the operating system, encrypt data or deny access to the task manager.
Users would be requested to purchase a PIN -- essentially a cash voucher -- to unlock their computers from a variety of likely unfamiliar payment services like Moneypak and Ukash.
The PIN would then need to be sent to the attackers who often masqueraded as law enforcement or government agencies requesting the payment as a fine for the victim's purported breach of computer crime law.
Two months before the attack wave, the Internet Crime Complaint Center was inundated with reports of infection by Reveton.
That scam claimed victim IP address were found by authorities (the malware can set the appropriate law agency based on a victim's region) to have visited child pornography sites.
Symantec did not known how much money the gang made, or indeed how many victims it collected from.
Establishing financial damages was further complicated by the fact that attackers on-sold the PINs on underground markets.
But Symantec's threat researchers fear the damage in the October attack was great.
A smaller and separate attack that compromised some 68,000 computers over a month generated up to US$400,000, Symantec said, based on the average of 2.9 percent of victims paying the ransom.
In a report issued early this morning, the security vendor placed a "conservative estimate" of ransom payments at more than US$5 million a year.
"The real number is, however, likely much higher," threat researchers Gavin O’Gorman and Geoff McDonald said.
They said criminals rarely provided the decryption keys to unlock computers after a ransom was paid and noted only a few varieties of the malware were capable of being uninstalled.
Sydney-based director of security and compliance solutions, Sean Kopelke, said criminals were moving out of fake anti-virus scams and into ransomware, which could increase the low victim count in Australia.
"The Australian market is more educated about fake AV," he said. "There is very little to nothing of ransomware here, but it will be brought here eventually."
In one instance of a local attack, a Northern Territory business paid $3000 to ransomware scammers who had encrypted its vital credit and debitor invoice information with 256bit AES encryption.
The IT manager Matt Cooper told SC at the time that the attackers warned the ransom would increase $1000 for every week the payment was late.
Unlike the huge Reveton scam, targeted ransomware attacks like these demanded more cash from fewer victims, Kopelke said.
Researchers had found the targeted attacks could be launched via brute force password guesses against vulnerable active Remote Desktop Protocol connections.
Targeted attackers could be more likely than operators of mass scams to uninstall the malware following a ransom payment.
Cooper said the attackers were professional and had immediately replied to correspondence with detailed instructions on how to pay the ransom and acted quickly to unlock the encryption.
However both Symantec and Australian police urged victims to report the scams to authorities and not pay ransoms.