Spammers are telling bank business customers that their SSL certificates had expired in efforts to exploit the blacklisting of certificate authority DigiNotar.
DigitNotar was blacklisted by major browsers after it was hacked and issued fraudulent certificates.
Barracuda Networks security researchers Dave Michmerhuizen and Luis Chapetti said the spam carried a dangerous message.
"The spammers try to create a sense of urgency with the hope that you will click one of the links to see what happens; which in this case is a particularly bad idea because the second link in the message directs the browser to a server hosting an exploit kit," they said.
“Once the browser visits that site a series of attacks begin which can result in the download of Trojan.Buzus."
That malware payload stole login credentials and created a backdoor that allowed remote control of compromised machines.
Barracuda said that it is seeing more overtly malicious spam directing users to malicious sites since the Blackhole exploit kit became widely available earlier this year.
Websense Security Labs security research manager Carl Leonard said it was a low volume campaign of less than 100 messages.
“It took the user to a .scr file that delivered the exploits. But this shows that scammers are tuned into the hot topics."
“This is not a targeted attack in an advanced persistent threat style, but it looks like a phishing email but this is much more sinister as it delivers an exploit kit and not a standard phish."
He also said the Blackhole exploit kit was one of the most popular kits in the wild.
Blackhole was based on PHP and a MySQL backend and targeted Windows operating systems and applications.
It also allowed a malicious payload file's name to be changed to make it undetectable by anti-virus, while exploits were encrypted with custom algorithms.