Several popular webmail providers are investigating a report that millions of their users' login details are being shared online by a hacker.
Google Gmail, Yahoo Mail, Microsoft Hotmail and Mail.ru are among the services said to have been affected.
The security firm that flagged the issue said that it believed many of the usernames and passwords involved had not been leaked before.
However, it is not clear whether users' accounts have actually been breached.
Hold Security said it had obtained a total of 272 million unique pairs of email addresses and unencrypted passwords from the hacker, 42.5 million of which the company had not seen in earlier leaks.
It said the cybercriminal had initially asked for 50 roubles (75 cents; 52 pence) in exchange for the list, but eventually gave a copy away without charge after Hold's staff posted favourable comments about him in a forum.
Even if many of the credentials are out of date or inaccurate they could still be abused, the company warned.
"There are hacker sites that advertise 'brute forcing' popular services and store fronts by taking a large amount of credentials and running them one-by-one against the site," Alex Holden, the firm's chief information security officer, told the BBC.
"What makes this discovery more significant is the hacker's willingness to share these credentials virtually for free, increasing the number of... malicious people who might have this information."
According to Hold's analysis:
- 57 million credentials were for Mail.ru accounts
- 40 million were for Yahoo accounts
- 33 million were for Hotmail accounts
- 24 million were for Gmail accounts
However, Mail.ru - Russia's most used webmail service - said its initial investigation suggested the problem might not be as bad as the figures indicated.
"A large number of usernames are repeated with different passwords," a spokeswoman said.
"We are now checking whether any combinations of username/password match [active accounts] - and as soon as we have enough information we will warn the users who might have been affected.
"The first check of a sample of data showed that it does not consist of any real live combinations of usernames and passwords."
Microsoft said it had measures in place to identify compromised accounts.
"[We would require] additional information to verify the account owner and help them regain sole access," said a spokesman.
Google said: "We are still investigating, so we don't have a comment at this time."
And Yahoo added: "We've seen the reports and our team is reaching out to Hold Security to obtain the list of accounts now. We'll update going forward."
Hold Security has a track record for bringing significant cyber-breaches to light, including past hacks of Adobe and the US retailer Target.
Independent security consultant Alan Woodward said people should remain alert to the dangers of phishing emails.
Even if the vast majority of the passwords did not work, he explained, cybercriminals could still use the list of email addresses to bulk-send scams.
"Assuming the email addresses are valid, they still give criminals the ability to mount certain types of attack," he said.
However he added there was "no need to panic" or for people to change their passwords at this point.