LastPass is forcing users to change master passwords as a precaution after an unauthorized data transfer. The cloud-based password management service detected larger than normal outbound traffic on May 3. The biggest risk, if indeed this information was stolen, is brute force attacks to recover the original passwords. The company has taken other measures like rebuilding the affected boxes, verifying the integrity of the website and plug-ins code against offline repository snapshots and preparing to roll out PBKDF2 (Password-Based Key Derivation Function)
Source: https://thehackernews.com/2011/05/lastpass-forces-users-to-change-master.html