Key Behaviours of CryptoWall v4
This document details some initial research undertaken by Hivint into the newly released CryptoWall version 4 series of ransomware. A number of organisations we have worked with have experienced infections by CryptoWall and its variants, in some cases leading to severe consequences.
This research paper outlines more information about the latest version of CryptoWall, as well as providing guidance on possible methods for creating custom security controls within your IT environment to mitigate the threat of CryptoWall infections, as well as how to detect and respond to these infections if they do occur. Some lists of known payload sources, e-mail domains and payment pages associated with CryptoWall are also provided at the end of this paper for use in firewall rulesets and/or intrusion detection systems.
CryptoWall version exhibits the following new behaviours:
- It now encrypts not only the data in your files, but the file names as well;
- It still includes malware dropper mechanisms to avoid anti-virus detection — but this new version also possesses vastly improved communication capabilities. It still uses TOR, which it may be possible to block with packet-inspection functions on some firewalls. However, it has a modified version of the protocol that attempts to avoid being detected by 2nd generation enterprise firewall solutions.
- It appears to inject itself into or migrate to svchost.exe and iexplore.exe. It also calls bcdedit.exe to disable the start-up restore feature of Windows. This means the system restore functions that were able to recover data in previous versions of the ransomware no longer work.
Antivirus detection for this variant is generally very low, but there’s some work on detection taking place. ESET’s anti-virus solution, for example, detects the .js files used by CryptoWall in emails as JS/TrojanDownloader.Agent;
The most reliable method to detect Cryptowall v4 infections when creating rules in intrusion detection systems, firewalls, antivirus systems, or centralised log management servers is to create a rule to alert on creation of the following filenames, which are static within CryptoWall v4:
It’s also worth noting that having in place a comprehensive, regular and consistent backup process for key organisational data is extremely important to combat the threat posed by ransomware such as CryptoWall v4. This will facilitate the prompt restoration of important files, limiting impacts of productivity.
Limiting the risk of Infection
CryptoWall v4 connects to a series of compromised web pages to download the payload. Some of the domain names hosting compromised pages are listed below — a useful step would be to create a regular expression on firewalls and other systems to block access to these domains:
Note that the list of compromised web pages is constantly evolving and so the implemented regular expression will require ongoing maintenance within corporate networks. See the lists at the end for more domains.
In the new version of CryptoWall, infected files have their file names appended with pseudorandom strings. As a result, filename encryption is harder to identify through pure examination of file extension names, unlike past versions of CryptoWall (in which ‘.encrypted’ was appended to the end of encrypted files). Thus, implementing an alert or blocking mechanism becomes more challenging.
However, it is possible to implement regular expression-based rules by considering the executable file names which are downloaded as part of an attempt to infect a system with CryptoWallv4. These are two known to be associated with CryptoWall v4 infections:
It may also be possible to write detection rules to find a static registry key indicating the presence of a CryptoWall infection. This can then be used to search over an entire corporate domain to locate infected machines, or possibly used in anti-virus / IDS signatures. An example is:
- HKEY_USERSSoftwareMicrosoftWindowsCurrentVersionRun a6c784cb “C:UsersadminAppDataRoaminga6c784cb4ae38306a6.exe
Another step to consider is writing a custom list for corporate firewalls containing the domains that phishing e-mails associated with CryptoWall v4 infections are known to come from, as well as a list of known command-and-control servers. For example, one of the first e-mail domains to be reported was 163.com. In addition, some of the known command and control hosts that the ransomware makes calls to include:
CryptoWall v4 also makes use of Google’s 126.96.36.199 service for DNS — this behaviour can be taken into account as part of determining whether there are additional security controls that can be implemented to mitigate the risk of infection. In addition, it appears that CryptoWall v4 makes outgoing calls to the following URLs (among others). These may also be useful in developing infection detection controls:
The initial controls we have worked with most customers to implement on their corporate networks included adding a rule to anti-virus detection systems to identify the ransom note file when it is created (i.e.: HELP_MY_FILES.txt). This enables network administrators to be promptly alerted to infections on the network. This is a valuable strategy in conjunction with maintaining lists of known bad domains related to the malware’s infection sources and infrastructure.
Lists of known payload sources, e-mail domains and payment pages associated with CryptoWall
We’ve included the following lists of payload sources, domains and pages associated with Cryptowall v4 infections — which some of our clients have used — to identify activity potentially associated with the ransomware. These can be used in addition to blacklists created and maintained by firewall and IDS vendors:
- Decrypt Service contains a small list of the IP addresses for the decryption service. This is the page victims are directed to in order to pay the authors of Cryptowall for the decryption keys. These servers are located on the TOR Network but use servers on the regular web as proxies.
- Email Origin IPs — contains IP addresses of known sources of CryptoWall v4 phishing e-mail origin servers — can be used in developing black lists on e-mail gateways and filtering services.
- Outgoing DNS Requests — contains a list of IP addresses that CryptoWall v4 attempts to contact.
- Payload Hosts — contains known sources of infection — including compromised web pages and other infection sources.
Article by John McColl, Principal Advisor, Hivint