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Gamarue, Nemucod, and JavaScript

May 9th, 2016 No comments

JavaScript is now being used largely to download malware because it’s easy to obfuscate the code and it has a small size. Most recently, one of the most predominant JavaScript malware that has been spreading other malware is Nemucod.

This JavaScript trojan downloads additional malware (such as Win32/Tescrypt and Win32/Crowti – two pervasive ransomware trojans that have been doing the rounds for a few years[1] – and Win32/Fareit) and installs it on a victim’s system through spam email.

Recently, however, we’ve seen another version of Nemucod distributing Gamarue malware to users.

Gamarue, also known as “Andromeda bot”, has been known to arrive through exploit kits, other executable malware downloaders (including Win32/Dofoil and Win32/Beebone), removable drives, and through that old stand-by: spam campaigns.

The shift to a JavaScript-obfuscated downloader might be an attempt by the malware authors to evade the increasing detection capabilities and sophistication in antimalware products.

A quick look into the obfuscated JavaScript code shows us that, aside from the encrypted strings, it uses variables with random names to hide its real code.

Sample of an obfuscated JavaScript code

Figure 1: Obfuscated code

 

The decrypted code is shown in the following image:

Sample of a decrypted JavaScript previously-obfuscated code

Figure 2: De-obfuscated code

 

Nemucod is known to have different hashes for each variant. For this one particular hash, since the detection was written in early April, 2016, it reached in total of 982 distinct machines with 4,192 reports – which indicates the number of Gamarue installations that could have occurred if it was not detected.

Nemucod detection rate

Figure 3:  Nemucod detection rate

 

Gamarue has been observed stealing vital information from your PC. It can also accept commands from a command and control (C&C) server. Depending on the commands received, a malicious hacker can perform various actions on the machine. See our family description of Win32/Gamarue for more information.

 

 

Nemucod impact

Since the start of 2016, Nemucod has risen in prevalence.

Rising Nemucod prevalence trend

Figure 4:  Rising Nemucod prevalence trend shows that it peaked on April

 

For the top 10 countries for Nemucod detections, the US takes a third, followed by Italy and Japan. The spread of infections is quite widespread across the globe.

Nemucod geoloc distribution from January to April 2016

Figure 5: Majority of the Nemucod infections are seen in the United States

Overall, however, it still remains relatively low, especially when compared to Gamarue.

 

Gamarue impact

Unlike Nemucod, Gamarue detections started high and have remained high since late last year. Overall, numbers have dropped a small amount since the start of 2016. Interestingly, there are large troughs during every weekend, with a return to higher numbers on Monday. This can indicate that Gamarue is especially pervasive either in enterprises, or in spam email campaigns.

Gamarue prevalence chart shows steady pattern from January to April 2016

Figure 6: The Gamarue infection trend shows a steady pattern

 

For Gamarue, the top 10 countries see distribution largely through India, Asia, Mexico, and Pakistan.

Gamarue geoloc distribution from January to April 2016

Figure 7: Majority of the Gamarue infection hits third world countries

 

Mitigation and prevention

To help stay protected from Nemucod, Gamarue, and other threats, use Windows Defender for Windows 10, or other up-to-date real-time product as your antimalware scanner.

Use advanced threat and cloud protection

You can boost your protection by using Office 365 Advanced Threat Protection and enabling Microsoft Active Protection Service (MAPS).

Office 365 helps by blocking dangerous email threats; see Overview of Advanced Threat Protection in Exchange: new tools to stop unknown attacks, for details.

MAPS uses cloud protection to help guard against the latest malware threats. You should check if MAPS is enabled on your PC.

Some additional preventive measures that you or your administrators can proactively do:

 

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[1] We’ve published a number of blogs about Crowti, including:

It was also featured in the July 2015 version of the Malicious Software Removal Tool (MSRT):

 

Donna Sibangan

MMPC

 

 

Be a real security pro – Keep your private keys private

December 15th, 2013 No comments

One of the many unusual characteristics of the Stuxnet malware that was discovered in 2010 was that its files were distributed with a valid digital signature, created using authentication credentials that belonged to two unrelated legitimate software companies. Normally the signature would verify that the program was issued by the company listed in the signing certificate, and that the contents of the program had not been tampered with since it was signed. By using other companies’ authentication credentials to sign their own files, malware distributors are able to make it appear that their files have come from a more trustworthy source.

Since then, malware signed with poorly secured or stolen credentials has been relatively rare. Most digitally-signed malware uses code-signing certificates that have been paid for and obtained directly from the certification authority (CA) that issued them. These CAs would be unaware the certificates were intended to be used for nefarious purposes. For example, recently the fake antivirus family Rogue:Win32/FakePav reappeared after being inactive for more than a year. Prior to the period of inactivity, FakePav’s executables were not digitally signed, but the new variants have been. After a few days using a single certificate, FakePav switched to a different certificate, issued in the same name as the previous one, but by a different CA.

However, in the past month or so, the use of stolen certificates has become more common. In particular, Rogue:Win32/Winwebsec, another rogue calling itself Antivirus Security Pro, has been distributed signed with credentials stolen from at least twelve different software developers.

Antivirus Security Pro user interface

Figure 1: Antivirus Security Pro user interface

A related family, TrojanSpy:Win32/Ursnif, has also been distributed with files signed using stolen credentials. We have observed Winwebsec downloading Ursnif, a trojan that monitors web traffic, and steals sensitive information, including passwords. Earlier variants of Ursnif were also capable of stealing certificates and private keys, but this functionality does not appear to be present in the latest versions. Instead, it appears to have been added to certain samples of PWS:Win32/Fareit.

Fareit steals certificates

Figure 2: Fareit steals certificates

PWS:Win32/Fareit is a Trojan that mostly steals passwords from a user's FTP client, but sometimes also downloads and installs other malware, such as Winwebsec and Win32/Sirefef.

Fareit infects computers, using stolen signed certificates

Figure 3: Relationship and interactions between Fareit, Sirefef, Winwebsec, and Ursnif families

The stolen certificates were issued by a number of different CAs to software developers in various locations around the world. The table below shows details of some of the certificates used to sign Winwebsec samples. Note that the number of samples column lists only the digitally-signed Winwebsec samples that we have a copy of – there may be many other samples that we have not received. But, it gives an idea of the magnitude of the problem. Interestingly, one of these certificates was issued only three days before we started seeing malware samples signed with it, which suggests that the malware’s distributors are regularly stealing new certificates, rather than using certificates from an older stockpile.

Certificates used to sign Rogue:Win32/Winwebse

Figure 4: Certificates used to sign Rogue:Win32/Winwebsec samples

For those of you who are software developers, Microsoft has a document that describes the best practices for code-signing.  Although that document was written in 2007 and contains a few references to operating system tools that have since changed, all of the recommendations of appropriate security procedures for obtaining and storing code-signing certificates and private keys, and for digitally signing your software, remain as relevant as ever.

Just as it is important to keep your house and car keys secure, securing your code-signing private keys is essential. Not only is it inconvenient, and often expensive, to have the certificate replaced, it can also result in loss of your company’s reputation if it is used to sign malware. The document recommends keeping private keys physically secure by storing them on a securely-stored hardware device such as a smart card, USB token, or hardware security module. Certainly, no system used to store code-signing credentials should ever be used for web browsing, and it is vital that these systems run a regularly updated antivirus solution, and that any file you sign has been scanned for possible virus infection beforehand.

If a system you use for signing has been infected with Win32/Fareit or other malware, and you suspect your private keys have been compromised, you should contact the CA that issued the credentials immediately.

David Wood
MMPC

SHA1s:

d330699f28a295c42b7e3b4a127c79dfed3c34f1 (PWS:Win32/Fareit with certificate stealing capability)
006c4857c6004b0fcbb185660e6510e1feb0a7a3 (Digitally-signed Winwebsec)