The following was posted by Linden Lab yesterday.
It has come to our attention that some Residents are sending messages - which may appear as popup windows in some viewers - informing other Residents that their accounts have been compromised and encouraging them to contact Support, using a phone number that is not associated with Linden Lab.
These messages are phishing attempts to gain access to your Second Life account. Neither Linden Lab, nor Second Life Customer Support, would attempt to contact you in this manner. You can always find Linden Lab’s official customer support contact methods within the following links:
As always, please be wary of suspicious messages and contact from other users. If you believe your account has been compromised, please contact us via support case at https://support.secondlife.com/
Hat Tip: Fuzzball Ortega