Many users have reported their email addresses are being used to send out spam.
The extent of the breach is unknown.
"AOL takes the safety and security of consumers very seriously, and we are actively addressing consumer complaints," the company said in a statement. "We are working to resolve the issue of account spoofing to keep users and their respective accounts running smoothly and securely."
Users' email addresses are sending messages that contain a link in them which could lead to malware, viruses or phishing attacks.
ThreatWatch is a regularly updated catalog of data breaches successfully striking every sector of the globe, as reported by journalists, researchers and the victims themselves.