That link your boss emailed you: make sure it's actually from your boss and not a cabal of suspected Chinese military hackers.

That's one of the biggest takeaways from the cyber-spying indictment unsealed Monday by the U.S. Department of Justice. In it, five Chinese military officers were accused of committing economic espionage by hacking into the computers of U.S. companies involved in nuclear energy, steel manufacturing and solar energy.

One of their most common tactics, according to the 56-page indictment, was "spearphishing" -- a twist on traditional phishing in which the scam email is made to look like it's from someone you know.

The technique isn't particularly sophisticated, but cybersecurity experts warn that it can be tricky. Unlike traditional phishing, in which scammers send out a mass email hoping for someone to bite, the spearphisher "thrives on familiarity" and "knows your name, your email address, and at least a little about you," according to the website for Norton, the malware prevention and removal service. "The salutation on the email message is likely to be personalized: 'Hi Bob' instead of 'Dear Sir.' "

In one instance highlighted in the indictment, a Chinese officer allegedly emailed roughly 20 U.S. Steel employees purporting to be their company's chief executive. The message included a link that installed malware that gave the alleged Chinese conspirators suspects backdoor access to the company's computers, just weeks before the release of a report on an important trade dispute.

Several of the employees took the bait and clicked the link.

In another instance, the same Chinese officer allegedly sent employees at the company a message with the subject line "US Steel Industry Outlook" -- also including a link that surreptitiously installed malware.

"Spearphishing messages were typically designed to resemble e-mails from trustworthy senders, like colleagues, and encouraged the recipients to open attached files or click on hyperlinks in the messages," according to the indictment.

Spearphishers sometimes scan Relevant Products/Services social media sites, such as Facebook, to glean details about users' friends in order to make their messages look more legitimate. These emails can refer to a recent online purchase or a mutual friend, causing users to let their guard down and be more willing to click or link or provide usernames, passwords or banking information.

Jon Heimerl, a strategist for security Relevant Products/Services services provider Solutionary, said he had one client, a CEO at a company, who bought a new BMW every three years. A hacker found out he was looking to buy, sent him an email purporting to be from a local BMW dealer and asking him to fill out a survey in exchange for a discount. Heimel said that after his client did so from his personal email account, a virus opened on his work computer before sending out an email from his work account to everyone in the company.

The subject line, Heimerl said, was something about the company getting acquired, which prompted nearly everyone to open it.

"It pretty much shut them down for the better part of three days," he said.

The best defense, experts say, is to limit your personal information posted online, keep your security software Relevant Products/Services up to date, and most importantly, verify that the people sending you emails are who they claim to be.

The consequences of not being careful can be severe. One of the alleged Chinese phishers, according to the indictment, was able to steal host names and descriptions for more than 1,700 company servers, including those that controlled physical access to the company's facilities and mobile access to the company's networks.