University researchers have raised concerns about the security of web-based password managers that free people from the burden of having to remember website credentials.

Scientists at the University of California, Berkeley, studied five password managers and found vulnerabilities in diverse features like one-time passwords, shared passwords and "bookmarklets," which are used to sign into websites on mobile browsers.

[LastPass discloses now-fixed vulnerabilities ahead of security conference]

"The root causes of the vulnerabilities are also diverse: ranging from logic and authorization mistakes to misunderstandings about the web security model," the researchers said in a paper scheduled to be presented in August at the Usenix Security Symposium in San Diego.

"Our study suggests that it remains to be a challenge for the password managers to be secure," the researchers said.

The five password managers studied were LastPass, RoboForm, My1login, PasswordBox and NeedMyPassword, all of which run in a Web browser and represent millions of users.

Password managers are popular because they require the user to remember only one master password.

While all the managers had flaws, the researchers said four of them had vulnerabilities an attacker could exploit to steal users' credentials for arbitrary websites.

Overall, the researchers found such a variety of vulnerabilities that they believed password managers were dropping the ball on security.

"Password managers handle exceptionally sensitive data -- the keys to the kingdom, so to speak," Devdatta Akhawe, co-author of the research, told CSOonline. "As a result, we believe, it behooves them to adopt a higher-than-usual defensive posture while writing these applications and adopt classic principles like least-privilege, defense-in-depth, and open protocols or designs."

The researchers reported their findings to the vendors in August 2013. Four responded in a week and have fixed all the major vulnerabilities. Only one, NeedMyPassword, has not responded.

In a blog post, LastPass acknowledged fixing the bookmarklet flaw, and said the feature was used by less than 1 percent of its user base.

People who used the feature before September 2013 on non-trustworthy sites may consider changing their master password and generating new passwords, but "we don't think it is necessary," the vendor said.

Another vulnerability found in LastPass and some of the other vendors let an attacker create a bogus one-time-password to a user account, if the person visited an attacking site while logged into the password manager.

However, the attacker would have to know the person's username and still would not have the key to decrypt user data, LastPass said. That flaw was also fixed.

The researchers found that some of the password managers were also vulnerable to cross-site request forgery and cross-site scripting attacks.

Other flaws made it possible to share a user's credentials with a bogus account, while others made users of some of the password managers vulnerable to phishing attacks.

[Alert: Your password is probably compromised...again]

"Our work is a wake up call for developers of web-based password managers," the research said. "Since our analysis was manual, it is possible that other vulnerabilities lie undiscovered."

In the future, the researchers plan to create tools to automatically identify such vulnerabilities and develop a "principled, secure-by-construction password manager."