Companies in Hull and East Riding risk being held to ransom by scam tax emails using an online storage service.
The emails seek to get employees to click on a link to a Dropbox file containing malicious software. They are then prompted to download an "invoice payment" or "tax return form".
Once this is done, files are encrypted to make them unreadable and a ransom demanded to unlock them.
The warning comes from the Business Crime Reduction Centre (BCRC) has warned
Earlier this year, Humberside Police's head of crime management revealed one of the Humber's biggest companies had fallen victim to a similar attack.
Detective Chief Superintendent Phil Walker said: "It affected a multi-million-pound business.
"They had basic, but not very good, security and, as a result of responding to this email they received, they effectively made themselves a target.
"These people send out thousands of emails."
In this case, the firm's website was targeted and hackers demanded payment.
The use of Dropbox, which is a legitimate file-sharing service, is a relatively new development. Its owners have acted quickly to delete dangerous files.
BCRC cyber security specialist Mark Connell said "Dropbox is being used to host malware as its name provides credibility to the scam emails. While many users are suspicious of links in emails, Dropbox is widely used in legitimate business communications.
"Tell-tale signs these emails are not genuine include grammatical errors, spelling mistakes and generic greetings such as 'Dear customer'."
BCRC is warning businesspeople not to click on suspicious links in unsolicited emails, reply or forward them or contact the senders in anyway.
Businesses using Dropbox are encouraged to make staff aware of the threat.
Anyone who receives a suspect email should report it to Action Fraud online or by calling 0300 123 2040.
• BCRC is delivered in partnership with Yorkshire-based charity, People United Against Crime and South Yorkshire, North Yorkshire, West Yorkshire and Humberside Police forces. BCRC is part financed by the European Union through the Yorkshire and Humber European Regional Development Fund Programme 2007-2013. Contact them on 0114 275 1283 or email info@bcrcuk.org for advice.
