The emails warn potential victims of "irregular activities" on their account and say a compulsory security update is required. The emails include an HTML attachment that purports to be a form for performing the update.
The HTML file is actually a copy of LinkedIn's website and login page, wrote Satnam Narang, senior security response manager with Symantec, in a blog post. But the website code in the file has been modified, so if a user logs in, their account credentials are sent to the attackers.
The attached file may be helping the emails to beat spam filters, Narang wrote.
"The most important technique used here is the HTML attachment," he wrote. "This method bypasses browser blacklists that often flag suspicious websites to help prevent users from being phished."
The phishing attempt may be elegant from that perspective but it's still a bit ham-fisted. Many users would be leery by now of opening attachments that purport to be security updates, and web services like as LinkedIn would never distribute fixes that way.
Narang recommended that users enable two-step verification on their account. If that's enabled, LinkedIn sends a one-time passcode over SMS that is required to complete logging into an account. Even if the attackers gain a person's login credentials, they could not take over an account.
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