LinkedIn has been hit with a potential class-action lawsuit alleging it failed to meet "industry standard" security practices in connection with a massive data breach earlier this month, according to court documents.

On June 6, users learned that hackers had gained access to LinkedIn's databases when 6.5 million LinkedIn passwords were posted to an underground forum.

The lawsuit was filed Monday on behalf of a single subscriber to LinkedIn's premium services, Illinois resident Katie Szpyrka. It's seeking certification as a class-action lawsuit on behalf of all LinkedIn users.

The suit claims LinkedIn failed to use "long standing industry standard encryption protocols," exposing its users' personally identifiable information. LinkedIn engaged in deceptive practices, the suit says, by claiming to use industry standard protocols to safeguard users' information.

LinkedIn called the suit "without merit" and said it would defend itself "vigorously."

"No member account has been breached as a result of the incident, and we have no reason to believe that any LinkedIn member has been injured. Therefore, it appears that these threats are driven by lawyers looking to take advantage of the situation," LinkedIn spokeswoman Erin O'Harra said in an email.

Lawyers for Szpyrka could not immediately be reached for comment.

LinkedIn stored passwords in "hashed," or encrypted format, but did not "salt" them as many websites do, meaning it did not add additional random characters to make the encryption more difficult to break. After being posted in their hashed format, some of the passwords were decrypted. LinkedIn has since begun salting passwords.

According to the lawsuit, LinkedIn also relied on an outmoded hashing format to store passwords and did not adhere to "basic security checklists" supplied by the U.S. National Institute of Standards and Technology to prevent the type of attack, called a SQL injection attack, that allowed hackers to gain access.

Cameron Scott covers search, web services and privacy for The IDG News Service. Follow Cameron on Twitter at CScott_IDG.