Threat Explorer

The Threat Explorer is a comprehensive resource consumers can turn to for daily, accurate, up-to-date information on the latest threats, risks and vulnerabilities.



19 June 2014
02 September 2016
Systems Affected:
Ransom.Cryptowall is a Trojan horse that encrypts files on the compromised computer. It then asks the user to pay to have the files decrypted.

The threat typically arrives on the affected computer through spam emails, exploit kits hosted through malicious ads or compromised sites, or other malware.

Once the Trojan is executed on the compromised computer, it creates a number of registry entries to store the path of the encrypted files and run every time the computer restarts. It encrypts files with particular extensions on the computer and creates additional files with instructions on how to obtain the decryption key.

This threat family attempts to convince the user to pay money in order to get the key to unlock their files. It uses a variety of different techniques in order to encourage the user to pay the ransom.

  • Definitions prior to September, 2016 may detect this threat as Trojan.Cryptowall.
  • Ransom.Cryptodefense is a variant of Ransom.Cryptowall.

The Trojan is mainly distributed through spam campaigns, compromised websites, malicious ads, or other malware.

In Cryptowall spam campaigns, the emails usually contain a malicious attachment and include a message attempting to convince the user to download the file. The email could claim that the attachment is an invoice, an undelivered package notice, or an incoming fax report. If the user opens the attachment, then their computer will be infected with Ransom.Cryptowall.

The Trojan may also be distributed through exploit kits hosted on compromised websites or malicious ads. Some of the exploit kits that have been used to compromise users’ computers with the threat include the Rig exploit kit and the Nuclear exploit kit. Symantec has extensive IPS protections in place against these kits.

The Trojan may also arrive through other threats that have already compromised the computer, such as Downloader.Upatre or Trojan.Zbot .


The Trojan was designed to prevent the user from accessing their files and force them to pay the attacker in order to regain access. It does this by encrypting a wide variety of files on the compromised computer using public/private key encryption with a strong 2048-bit RSA key .

Once the files are encrypted, the Trojan displays a text document or HTML page with a message. The message informs the user that their files have been encrypted and gives instructions on how to obtain the decryption key needed to unlock the files. It may also warn users that the decryption key will be deleted after a certain time period to pressure the user into paying sooner. The attacker may demand hundreds of US dollars in payment and the amount may increase after a specified time period.

The message also contains a link to a website where the user can make the payment. These sites are typically hosted on the anonymous Tor network, which helps the attacker hide their identity. The threat may ask the user to download a Tor network browser in order to view the site, though newer versions of the threat do not require the user to do this. The user may have to pay using cryptocurrencies such as bitcoin to further prevent the attacker’s identity from being traced.

Even if the user pays the ransom, there’s no guarantee that the attacker will provide the decryption key needed to unlock their files.

Geographical distribution
Symantec has observed the following geographic distribution of this threat:

Symantec has observed the following global infection trends between April and October 2014:

Symantec protection
The following Symantec detections protect against this threat family.

Heuristic detections
Reputation detections

Antivirus Protection Dates

  • Initial Rapid Release version 19 June 2014 revision 034
  • Latest Rapid Release version 22 September 2016 revision 024
  • Initial Daily Certified version 20 June 2014 revision 002
  • Latest Daily Certified version 22 September 2016 revision 025
  • Initial Weekly Certified release date 25 June 2014
Click here for a more detailed description of Rapid Release and Daily Certified virus definitions.
1. Prevention and avoidance
1.1 User behavior and precautions
1.2 Patch operating system and software
2. Infection methods
2.1 Spam campaigns
2.2 Exploit kits
2.3 Malware
3.1 File encryption
3.2 Ransom notice
3.3 Ransom payment web page
4. Additional information

The following actions can be taken to avoid or minimize the risk from this threat.

1.1 User behavior and precautions
Be aware of attackers’ social engineering techniques and avoid opening attachments or links in emails that were sent by unknown recipients. Attackers will attempt to entice users into opening attachments or links in their messages in many different ways, such as claiming the attachment is a bill, a fax notification, a special offer, or a delivery notice.

Do not download archive files (.zip, .jar, .tar, .7z, .msi, etc.) or executable/script files (.com, .exe, .scr, .bat, .js, .jse, .vb, .vbe, .wsf, .wsh, .cmd). Companies should not need to use these file types when distributing a document.

Do not click on any links or advertisements if it’s unclear if they come from trusted sources. The web browser will normally show where the link leads to when the user hovers over the link with the mouse. Users can also check online website rating services such as to see if the site is deemed safe to visit.

Regularly back up any data stored on your computer. If your computer has been infected with ransomware, then you can restore the previous backup to regain access to the encrypted files after the malware has been removed.

If your computer has been compromised with Ransom.Cryptowall, do not pay the ransom. There’s no guarantee that the attacker will provide the decryption key needed to unlock the files. By paying, you’ll only encourage the attacker to continue spreading the malware.

Keep your security software up to date to protect yourself from the latest variants of this threat.

1.2 Patch operating system and software
Attackers have been observed spreading the threat through exploit kits hosted on malicious ads or compromised sites. These kits are designed to take advantage of any software bugs on your computer in order to install malware. You can prevent exploit kits from succeeding by keeping your operating system and software up to date. Install the latest updates to your computer’s software and operating system as soon as possible to patch any security vulnerabilities.

You should turn on automatic updates if available so that your computers can receive the latest patches and updates when they are made available.


The Trojan may use the following infection methods to compromise the affected computer.

2.2 Spam campaigns
The Trojan can arrive through spam emails with malicious attachments, usually a zip archive file containing the threat. The emails include a message to convince the user to download and open the attachment.

If the user opens the attachment, then the malware will install itself on the computer.

2.2 Exploit kits
The Trojan may arrive through exploit kits that take advantage of particular vulnerabilities in order to install the malware on the affected computer. The exploit kits that have been observed distributing Ransom.Cryptowall include the Rig exploit kit and the Nuclear exploit kit. These kits exploited the following vulnerabilities to drop the Trojan onto computers:

The attacker may host the exploit kits on legitimate websites that have been compromised . Attackers may compromise these sites by injecting malicious code into them, which redirects the site’s visitors to another web page hosting the exploit kit. The kit then checks the user’s computer for potentially vulnerable programs and attempts to exploit them accordingly. This will allow the kit to drop the Trojan onto the computer.

Along with compromising legitimate websites, the attackers may host the exploit kit on malicious websites. They may email links to these sites to users as part of a spam campaign. The emails will contain a message that attempts to convince users to visit the malicious site.

Additionally, the exploit kits may be linked by malicious ads, which could appear on otherwise unconnected websites. These ads redirect users a number of times before sending them to a web page hosting the exploit kit.

2.3 Malware
The Trojan could arrive on computers through other malware that has already installed on them. Some threats include functionality to allow them to download other threats on the already compromised computer. Downloader.Upatre and Trojan.Zbot have both been used to download Ransom.Cryptowall onto compromised computers.

Once the Trojan has compromised the computer, it may perform the following actions.

3.1 File encryption
When the Trojan is executed, it creates a number of registry keys to store the path of the encrypted files and run every time Windows starts.

The threat then connects to one of a number of remote locations. The Trojan first sends a profile of the compromised computer to the remote server and waits for a reply. Once the Trojan receives the reply, it encrypts all files with the following file extensions:
  • xls
  • wpd
  • wb2
  • txt
  • tex
  • swf
  • sql
  • rtf
  • RAW
  • ppt
  • png
  • pem
  • pdf
  • pdb
  • PAS
  • odt
  • obj
  • msg
  • mpg
  • mp3
  • lua
  • key
  • jpg
  • hpp
  • gif
  • eps
  • DTD
  • doc
  • der
  • crt
  • cpp
  • cer
  • bmp
  • bay
  • avi
  • ava
  • ass
  • asp
  • js
  • py
  • pl
  • db
  • c
  • h
  • ps
  • cs
  • m
  • rm

The Trojan encrypts these files through public-key cryptography using strong 2048-bit RSA encryption . Once the files have been encrypted, the user will not be able to decrypt the files without the necessary private key.

The Trojan then sends the private key back to the remote server. After the remote server confirms that it has received the key, the Trojan will send a screenshot of the computer’s desktop to the remote server.

3.2 Ransom notice
Next, the Trojan creates new files in each folder that contains encrypted files. The new files contain a message describing how the user can get the decryption key. The message is either stored as a text file or a web page. It tells the user that all of their files have been encrypted with a 2048-bit RSA key and if they want to unlock the files, they will need to pay for the private key. The message then points to links to allow the user to pay the ransom demand. It may also contain additional instructions on how to access this page, as the page may be hosted on the anonymous Tor network.

3.3 Ransom payment web page
If the user opens these links, they will be presented with a CAPTCHA page which are designed to determine that the user is human or a bot.

Once the user inputs the CAPTCHA code correctly, they will arrive at the ransom payment page. The page tells the user how much they have to pay for the decryption key. It may say that the price will increase if the user does not pay within a certain time period. This is designed to put pressure on the user to pay the ransom as soon as possible. Ransom.Cryptowall typically asks the user to pay with bitcoins and provides instructions on how to purchase bitcoins and use the cryptocurrency to pay for the decryption key.

The page also offers a “My screen” button, which directs users to a screenshot of their desktop to prove that their computer is compromised. The page also has a “Test decrypt” button, which allows the user to decrypt one file to show that decryption is possible. This is like a “try before you buy” service to try and convince users that the decryption works.

However, even if the user pays for the decryption key, there’s no guarantee that the attackers will ever provide a key or remove the malware from the compromised computer.

For more information relating to this threat family, please see the following resource:


Symantec Security Response encourages all users and administrators to adhere to the following basic security "best practices":

  • Use a firewall to block all incoming connections from the Internet to services that should not be publicly available. By default, you should deny all incoming connections and only allow services you explicitly want to offer to the outside world.
  • Enforce a password policy. Complex passwords make it difficult to crack password files on compromised computers. This helps to prevent or limit damage when a computer is compromised.
  • Ensure that programs and users of the computer use the lowest level of privileges necessary to complete a task. When prompted for a root or UAC password, ensure that the program asking for administration-level access is a legitimate application.
  • Disable AutoPlay to prevent the automatic launching of executable files on network and removable drives, and disconnect the drives when not required. If write access is not required, enable read-only mode if the option is available.
  • Turn off file sharing if not needed. If file sharing is required, use ACLs and password protection to limit access. Disable anonymous access to shared folders. Grant access only to user accounts with strong passwords to folders that must be shared.
  • Turn off and remove unnecessary services. By default, many operating systems install auxiliary services that are not critical. These services are avenues of attack. If they are removed, threats have less avenues of attack.
  • If a threat exploits one or more network services, disable, or block access to, those services until a patch is applied.
  • Always keep your patch levels up-to-date, especially on computers that host public services and are accessible through the firewall, such as HTTP, FTP, mail, and DNS services.
  • Configure your email server to block or remove email that contains file attachments that are commonly used to spread threats, such as .vbs, .bat, .exe, .pif and .scr files.
  • Isolate compromised computers quickly to prevent threats from spreading further. Perform a forensic analysis and restore the computers using trusted media.
  • Train employees not to open attachments unless they are expecting them. Also, do not execute software that is downloaded from the Internet unless it has been scanned for viruses. Simply visiting a compromised Web site can cause infection if certain browser vulnerabilities are not patched.
  • If Bluetooth is not required for mobile devices, it should be turned off. If you require its use, ensure that the device's visibility is set to "Hidden" so that it cannot be scanned by other Bluetooth devices. If device pairing must be used, ensure that all devices are set to "Unauthorized", requiring authorization for each connection request. Do not accept applications that are unsigned or sent from unknown sources.
  • For further information on the terms used in this document, please refer to the Security Response glossary.
You may have arrived at this page either because you have been alerted by your Symantec product about this risk, or you are concerned that your computer has been affected by this risk.

Before proceeding further we recommend that you run a full system scan . If that does not resolve the problem you can try one of the options available below.

If you are a Norton product user, we recommend you try the following resources to remove this risk.

Removal Tool

If you have an infected Windows system file, you may need to replace it using the Windows installation CD .

How to reduce the risk of infection
The following resources provide further information and best practices to help reduce the risk of infection.

If you are a Symantec business product user, we recommend you try the following resources to remove this risk.

Identifying and submitting suspect files
Submitting suspicious files to Symantec allows us to ensure that our protection capabilities keep up with the ever-changing threat landscape. Submitted files are analyzed by Symantec Security Response and, where necessary, updated definitions are immediately distributed through LiveUpdate™ to all Symantec end points. This ensures that other computers nearby are protected from attack. The following resources may help in identifying suspicious files for submission to Symantec.

Removal Tool

If you have an infected Windows system file, you may need to replace it using the Windows installation CD .

How to reduce the risk of infection
The following resource provides further information and best practices to help reduce the risk of infection.
Protecting your business network

The following instructions pertain to all current Symantec antivirus products.

1. Performing a full system scan
How to run a full system scan using your Symantec product

2. Restoring settings in the registry
Many risks make modifications to the registry, which could impact the functionality or performance of the compromised computer. While many of these modifications can be restored through various Windows components, it may be necessary to edit the registry. See in the Technical Details of this writeup for information about which registry keys were created or modified. Delete registry subkeys and entries created by the risk and return all modified registry entries to their previous values.
Writeup By: Laura O'Brien, Jeet Morparia