Hackers are using PayPal to distribute the Chtonic banking trojan, a variant of Zeus, according to security company Proofpoint.
The emails are ‘legitimate' but low in volume, and are not filtered by anti-spam or antivirus software because they come via genuine PayPal accounts.
"The sender does not appear to be faked. Instead the spam is generated by registering with PayPal (or using stolen accounts) and then using the portal to request money," said Proofpoint in a security advisory.
The attackers take advantage of a feature that allows users to include notes when sending money request messages.
One sample picked up by Proofpoint showed that Gmail failed to block the email since it appeared to be legitimate.
"PayPal's money request feature allows adding a note along with the request [and] the attacker crafted a personalised message and included a malicious URL," said the advisory.
"In a double whammy, the recipient here can fall for the social engineering and lose $100, click on the link and be infected with malware, or both."
"This executable is Chthonic, a variant of the Zeus banking trojan. The command and control for this instance is kingstonevikte[.]com," said Proofpoint.
Intriguingly, perhaps, Chthonic downloads a second-stage payload, a previously undocumented malware known as AZORult which the company is currently investigating.
Fortunately the campaign seems somewhat limited at the moment, according to Proofpoint, perhaps partly owing to the overhead required in opening PayPal accounts or gaining access to compromised accounts.
Google Analytics indicates that the URL of the malicious link, which uses a Goo.gl link, has been clicked only 27 times.
PayPal has responded to V3's request for comment and claims that it has fixed the flaws that were being exploited by the attackers.
"Based on information we received regarding a possible way to use our request money feature to send spoof or phishing emails, we put additional security protocols in place to safeguard our customers," it told V3 in a statement.
It continued: "These protocols recently identified some anomalous usage of this feature and we are aware that attempts were made to use the request money feature to distribute malware to a small number of our customers.
"We have put measures in place in an effort to prevent the misuse of this feature. We are continuing to carefully monitor the situation and will reach out to any impacted customers."
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