Original release date: November 05, 2013 | Last revised: June 05, 2014
Systems Affected
Microsoft Windows systems running Windows 8, Windows 7, Vista, and XP operating systems
Overview
US-CERT is aware of a malware campaign that surfaced in 2013 and is associated with an increasing number of ransomware infections. CryptoLocker is a new variant of ransomware that restricts access to infected computers and demands the victim provide a payment to the attackers in order to decrypt and recover their files. As of this time, the primary means of infection appears to be phishing emails containing malicious attachments.
Description
CryptoLocker appears to have been spreading through fake emails designed to mimic the look of legitimate businesses and through phony FedEx and UPS tracking notices.  In addition, there have been reports that some victims saw the malware appear following after a previous infection from one of several botnets frequently leveraged in the cyber-criminal underground.
Impact
The malware has the ability to find and encrypt files located within shared network drives, USB drives, external hard drives, network file shares and even some cloud storage drives.  If one computer on a network becomes infected, mapped network drives could also become infected. CryptoLocker then connects to the attackers’ command and control (C2) server to deposit the asymmetric private encryption key out of the victim’s reach.Victim files are encrypted using asymmetric encryption. Asymmetric encryption uses two different keys for encrypting and decrypting messages. Asymmetric encryption is a more secure form of encryption as only one party is aware of the private key, while both sides know the public key.While victims are told they have three days to pay the attacker through a third-party payment method (MoneyPak, Bitcoin), some victims have claimed online that they paid the attackers and did not receive the promised decryption key.  US-CERT and DHS encourage users and administrators experiencing a ransomware infection to report the incident to the FBI at the Internet Crime Complaint Center (IC3).
Solution
PreventionUS-CERT recommends users and administrators take the following preventative measures to protect their computer networks from a CryptoLocker infection:Conduct routine backups of important files, keeping the backups stored offline.Maintain up-to-date anti-virus software.Keep your operating system and software up-to-date with the latest patches.Do not follow unsolicited web links in email. Refer to the Security Tip Avoiding Social Engineering and Phishing Attacks for more information on social engineering attacks.Use caution when opening email attachments. For more information on safely handling email attachments read Recognizing and Avoiding Email Scams (pdf), and Refer to the Security Tip Using Caution with Email Attachments.Follow safe practices when browsing the web. For further reading on Safe Browsing habits, see Good Security Habits and Safeguarding Your Data. MitigationUS-CERT suggests the following possible mitigation steps that users and administrators can implement, if you believe your computer has been infected with CryptoLocker malware:Users who are infected with the malware should consult with a reputable security expert to assist in removing the malware.If possible, change all online account passwords and network passwords after removing the system from the network. Change all system passwords once the malware is removed from the system.If your computer has not yet been encrypted with the CryptoLocker malware, the tools listed in TA14-150A may be able to remove this malware from your machine.
References

CryptoLocker Virus: New Malware Holds Computers For Ransom, Demands $300 Within 100 Hours And Threatens To Encrypt Hard Drive
CryptoLocker Wants Your Money!
CryptoLocker ransomware – see how it works, learn about prevention, cleanup and recovery
Microsoft Support – Description of the Software Restriction Policies in Windows XP
Microsoft Software Restriction Policies Technical Reference – How Software Restriction Policies Work
CryptoLocker Ransomware Information Guide and FAQ

Revision History

November 5, 2013: Initial Release
November 13, 2013: Update to Systems Affected (inclusion of Windows 8)
November 15, 2013: Updates to Impact and Prevention sections.
November 18, 2013: Updated Prevention and Mitigation Sections
June 2, 2014: Update to include GameOver Zeus Alert (TA14-150A) reference in Mitigation Section

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