The owners of those compromised accounts are still probably unaware that they were compromised because the hackers did not change any settings or passwords. The hackers are utilizing these accounts to send out massive amounts of spam to the addressbooks of those compromised accounts. This means that there is a higher probability of the spam message being opened, and any links or attachments opened because the sender is trusted by the receiver.
While this problem occurs all the time on infected computers with email programs, this is the first widespread attack to utilize web-based email accounts.
Many of these emails that contain links go to websites that are either advertising some medical drug or a video that asks you to upgrade Flash or other video software on your computer. The attachments are many times PDF files. In all cases the intent is to infect your computer with some type of malware - many times some variation of a scareware antivirus/antimalware program telling you your computer has been infected and requesting payment to clean the
infection.Adobe has admitted and computer security analysis of the attacks over the past 18 months have shown that over 50% have targeted vulnerabilities in Adobe's Acrobat PDF creation and reader software as well as it's Flash product line.
Adobe has recently patched a number of major security flaws in both products and EVERYONE should update to Acrobat Reader 10.1 (link to adobe download site) and Flash 10.3 (link to Adobe download site).
These new versions fix some critical flaws that can allow malicious attackers install software on your computer without your knowledge as well as potentially take over your computer entirely.
Adobe has included in these updates a much more reliable software updater that will alert you to updates and allow them to be installed fairly quickly after they are released to keep your computer safe.
Please do not ignore these alerts as doing so will at some point cause your computer to become infected. Please note that this alert does not only pertain to Windows users but Mac users as well. While OSX has a built in PDF viewer, some users do still install the Adobe Acrobat Reader product. Flash 10.3 on the otherhand should absolutely be installed on your Mac and security firm INTEGO agrees