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Implementing Network Security Controls for CryptoLocker

What is CryptoLocker?

CryptoLocker is a Trojan horse in terms of mechanism and a ransomware in terms of objective. Being a Trojan horse, it comes in disguised forms and once unlocked it starts searching and encrypting the files present on your local Hard Disks, shared networks or Cloud networks. This means that your computer and software keep on working, but your personal files, such as documents, spreadsheets and images, are encrypted.


Often, CryptoLocker arrives as a file with a double extension, such as .pdf.exe. Since Windows doesn’t display file extensions by default, this file may look like a PDF file rather than an executable.

Targets of CryptoLocker

CryptoLocker targets files with following extensions:

*.odt, *.ods, *.odp, *.odm, *.odc, *.odb, *.doc, *.docx, *.docm, *.wps, *.xls, *.xlsx, *.xlsm, *.xlsb, *.xlk, *.ppt, *.pptx, *.pptm, *.mdb, *.accdb, *.pst, *.dwg, *.dxf, *.dxg, *.wpd, *.rtf, *.wb2, *.mdf, *.dbf, *.psd, *.pdd, *.pdf, *.eps, *.ai, *.indd, *.cdr, *.jpg, *.jpe, *.jpg, *.dng, *.3fr, *.arw, *.srf, *.sr2, *.bay, *.crw, *.cr2, *.dcr, *.kdc, *.erf, *.mef, *.mrw, *.nef, *.nrw, *.orf, *.raf, *.raw, *.rwl, *.rw2, *.r3d, *.ptx, *.pef, *.srw, *.x3f, *.der, *.cer, *.crt, *.pem, *.pfx, *.p12, *.p7b, *.p7c.
CryptoLocker then finds files that match with one of these types and encrypt the file using the pubic encryption key.

History of CryptoLocker

CryptoLocker was first discovered in the fall of 2013 and targeted computers running on Microsoft Windows. It displayed all the characteristics of a ransomware, i.e., the ability to target victims through phishing and malicious email links, encryption of user files and a notification box demanding a ransom for their return.

How it Works

CryptoLocker infects like normal malware, placing its files in Windows directories, and creating registry entries that allow it to restart when you reboot. It then tries to contact its command and control (C&C) server. The malware uses a random domain name generation algorithm to try and find the current C&C server. Some sample Crytpolocker domains might look like this:


Once CryptoLocker contacts its C&C, it generates a public/private cryptographic key for your specific computer, using very strong and standard RSA and AES 2048-bit encryption. The private key is only stored on the attacker’s C&C servers, but the public key is saved in a registry entry on your computer. CryptoLocker then uses that key pair to encrypt many different types of files on your computer.

After encrypting your files, following screen will be displayed to you:
So the screen will have a warning message with the time left to decrypt your data and asks for money.
The payment mechanism includes receiving a key on payment which unlocks the encrypted files. And those who deny paying before the timer expires, the corresponding private decryption key on a remote server is claimed to be deleted and hold a fair risk of losing access to their files forever.
The malware sends out system information and creates registry entries to get started after system reboots.

Modes of infection

Just like any other malware or Trojan attack, a CryptoLocker has common modes of infection. Some of which include:

–  Spam Emails
  Infected portable data transfer devices
  Click on an infected link
  An infected system in the network

How Cyberoam can protect you?

Cyberoam Implements network based controls which can impact malware such as CryptoLocker and while the Antivirus, Anti Spam, Web Filtering are important, the IPS Engine also has the ability to block malware command & control (C&C) communications.

Following are some control mechanisms that you should follow to protect your network from malwares such as CryptoLocker in Cyberoam:

•   Keep your operating system and software up to date with patches : This lessens the chance of malware sneaking onto your computer 
    unnoticed through security holes. The CryptoLocker authors didn't need to use fancy intrusion techniques in their malware because they 
    used other malware that had already broken in, to open the door for them.

    Cyberoam issues maintenance releases regularly, these should be tested and installed as required

    You can find the information about upgrading your Cyberoam Appliance from Upgrade Firmware of Cyberoam Appliance.

Secure DNS Settings : DNS settings play an important role in protecting your network from malwares. In DNS settings you need to consider
     following points
     –     Reliable DNS settings: Ensure your appliance is configured with reliable, trusted DNS server settings. You can configure DNS settings in 
            Cyberoam from Network > DNS > DNS.

    Secure Firewall Rule
: Malware often spreads by changing DNS settings on devices to redirect users to malware serving sites. To prevent
           this, you should lock down DNS so that devices use only approved internal servers. The advantage of using approved internal server is that only 
           approved forwarders on the Internet are allowed to access that internal server.

    HTTPS Inspection : Malware often uses encrypted sessions and encrypted websites, to provide the fullest coverage, you need to perform both
     HTTP and HTTPS inspection.
     To do this successfully you will need to distribute and install the Self Signed Certificate or set the appliance up as a trusted sub-ordinate authority to
     your existing Enterprise Certificate Authority
Information on SSL CA Certificate Installation Guide can be found at from the given link.
•   Antivirus/Anti Spam Scanning : One of the modes of malware entering into the network is through Emails. This makes it important to not only 
    scan the Internet traffic but also Email communication channels. To prevent this, configure following settings in your network :

    –   Enable SMTP/SMTPS/POP3/IMAP/HTTP/HTTPS Antivirus scanning in your network : You can enable SMTP/ SMTPS/ POP3/ HTTP/ HTTPS 
         scanning in Cyberoam to scan Internet/Web traffic, as shown in the following screen shot:

      –   Secure SMTP Email Communication : You can define rules for SMTP/S scanning in Cyberoam from Antivirus > Email > SMTP/s Scanning Rules.
           For more details refer
Secure SMTP Email Configuration.

      –   Configure SMTP blocking of compressed attachments and other harmful attachments : Data Leakage through Emails is a serious threat to 
           business operations. Data Leakage Protection is a key necessity of any organization. As a solution, Cyberoam provides means to control the attachments 
           in outgoing Emails. Information of how to do it can be found out at
Blocking Email Attachments over SMTP. 

      –   Configure Anti Spam with Spam, Possible Spam, Virus Outbreak and Probable Virus Outbreak content action: You can configure these settings
           in Cyberoam by identifying the mentioned content in the mail and then deciding the action for such content. You can configure these settings from
           ANTI SPAM > Spam Rules as shown in the following screen shot:

   Enable IPS Scanning
    In case of CryptoLocker, the IPS engine can block the download of the encryption keys, which means CryptoLocker is unable to encrypt the data on the End Point.
    In order to achieve this, appropriate IPS policies should be implemented in Cyberoam.
    Internet traffic (including encrypted HTTPS traffic) which is not monitored by the IPS is a potential exposure which can facilitate C&C communication.

    To achieve this, create an IPS Policy which includes Malware Communication signatures and apply it on the relevant Firewall Rule(s).

•   Web/URL Filtering

    To protect your network from malwares, you should filter Web/URL content as well. For Web or URL filtering related settings, keep following points in mind:

    –   Enable Pharming Protections

        This allows the appliance to protect users against pharming by re-resolving the domain name of the website using the DNS configured on the appliance.
        You can enable pharming in Cyberoam from Web Filter > Settings > Settings, refer the below screen shot for the same.

      –   Filter Websites that can cause security issues
          To protect your network from malwares like CryptoLocker, lock following categories for all users and firewall rules:
          >  HackingSites that provide information about or promote illegal or questionable access to or use of computer or communication 
              equipment, software, or databases.
          >  Illegal/ Unethical: Websites that feature information, methods, or instructions on fraudulent actions or unlawful conduct (non-violent) 
              such as scams, counterfeiting, child abuse, tax evasion, petty theft, blackmail, etc.
          >  Phishing and Fraud: Sites gathering personal information (such as name, address, credit card number, school, or personal schedules) 
              that may be used for malicious intent.
          >  SPAM URL: This category includes URLs that arrive in unsolicited Spam emails. Spam URL content ranges from product marketing to 
              potentially offensive or fraudulent sites.
          >  IP Address: Sites accessed through IP address, this will stop people accessing websites directly via the IP Address. This is not normal/expected 
              user behavior as most people use domain names which means this is often not user based traffic, often it is odd malware serving websites.
          >  Parked Domain: This category includes sites that once served content, but their domains have been sold and are no longer registered. Parked 
              domains do not host their own unique content, but usually redirect users to a generic page that states the domain name is for sale or redirect 
              users to a generic search engine and portal page, some of which provide
valid search engine results. Some of these orphaned domains may 
              redirect users to malware serving sites.
          > Spyware: Sites or pages that download software that without the user's knowledge.
          How the above implementation will look like is showing in the below screen shot:
      In Cyberoam, You can configure above settings from Web Filter. For more information about creating the policy and applying it to user or 
      firewall rule consider Configuring Web Filter Policy article.

•    Application Filtering

     Controls should be implemented to restrict undesired applications in the network, this will normally block “Torrents” and applications that 
     “tunnel other apps”
and “can bypass firewall policy”. It may also include undesired “P2P” Applications.
     Failing to block tunneling applications and other applications that can be used to bypass firewall rules leaves you open to communication 
     channels that are
beyond regular control mechanisms.
     In Cyberoam you can configure these settings from
Application Filter, by considering the Category, Risk, Characteristics and Technology 
     for individual applications.

     For more information about blocking of particular application, refer Block P2P Applications. In the same way you can block other application as well.

     Configuration for application filtering for blocking particular application is shown in the below screen shot:

   GEO Blocking

    Another option in controlling inbound and outbound security issues is GEO Blocking. In many situations malware and attacks can be tracked 
    to specific countries. So, blocking traffic from these countries can be a precautionary measure to minimize the impact of malware.
    If you are able to identify the countries or regions with higher concentration of suspicious traffic you can choose to block them, and you can 
specific bypass rules to minimize exposure.
    In Cyberoam, you can implement GEO blocking by creating Country-based firewall rules. By doing so you can block or manage traffic to/from
    a particular country or group of counties.

    To implement GEO blocking in Cyberoam, first create Country Based Host from
Objects > Hosts > Country Hosts and then create Firewall rule for 
    the country from Firewall > Rule > Rule. For more details about GEO blocking, refer
the article of Creating Country based Firewall Rules.


So to summarize, CryptoLocker is aggressively spreading, and has infected many victims. However, Cyberoam can detect and block it using various security services and control mechanisms mentioned above. CryptoLocker can also spread internally through network shares, which network security solutions can’t prevent. Ultimately, your best defence is awareness and vigilance.






Document Version: 1.0 – 30 June, 2015 

Article ID: 3134