Accounts on messaging service Snapchat have been hijacked to send spam - but the site has said it was not the victim of a hack.
User profiles were used to send out advertisements for a weightloss site.
It's not clear how many people have been affected, but Snapchat users in several countries took to Twitter to complain about the problem.
Snapchat has suffered breaches in the past, including the leak of 4.6m usernames and phone numbers in January.
In that instance, a website called SnapchatDB released the personal data but censored the last two digits of phone numbers. Those behind it said they wanted to raise awareness of a vulnerability that Snapchat had not fixed.
In a statement, Snapchat said the latest incident was due to user credentials being found on other sites.
"We have seen evidence that hackers who have access to a trove of credentials leaked from other websites, have started using them to gain access to Snapchat accounts," the service told the BBC.
"In many instances, our defences have notified the user that their account has been compromised.
"We recommend using a unique and complex password to access your Snapchat account."
The breach takes control of a person's account to send an image to everyone in a user's contact list.
Users who log back into their accounts are then emailed by Snapchat, warning that they should change their password.
The image sent out does not appear to cause any harm to the sender or recipient, and instead promotes a website offering suspect weightloss products.
The web address is registered under the name of Stanislaw Wojciechowski - the former president of Poland.
Snapchat, which launched in 2011, has been hit by a number of security set-backs.
The app is designed to send pictures and messages that self-destruct after a short time. However, an app called Snapchat Hack allowed users to save messages permanently.
Security researcher Brian Honan said the service's track record was not encouraging - and that it could soon put users off.
"One thing that moves people off apps is if it's not cool anymore, or it's been replaced by another fantastic service. Or that people have lost confidence in it.
"For apps like Snapchat, they're competing with other applications and newcomers - if people lose confidence in them, they're going to stop using them."
Follow Dave Lee on Twitter @DaveLeeBBC