Guest blog: Nemucod ransomware analysis
Posted by on Sep 2, 2016
[Original Post HERE]
In the run up to VB2016, we invited the sponsors of the conference to write guest posts for our blog. In the third of this series, Webroot’s Jesse Lopez writes about the Nemucod ransomware.
Note: some security vendors refer to the downloader component alone (which has been seen to download other kinds of malware as well) as ‘Nemucod’. This analysis focuses on both the downloader and the ransomware.
Nemucod is a piece of ransomware that changes file names to *.crypted. While it’s not a brand new variant, a lot has changed in the last few months, and different methods have been used, but one thing has remained constant: it is deployed via bogus shipping invoice spam messages.
- January 2016: Nemucod changes file names to ‘.crypted’, but does not actually encrypt them.
- March 2016: Adds XOR encryption using a 255-byte key contained in a downloaded executable. This downloaded executable encrypts the first 2048 bytes of a file.
- April 2016: 7-Zip is used instead, which creates an archive to password-protect files.
- May 2016: A small change is added to the previous build, which encrypts 2048 bytes instead of 1024 bytes.
- June – August 2016: A PHP script is used along with a PHP interpreter to encrypt the first 1024 bytes of a file.
The following is an example of an email used to distribute Nemucod:
When the sample is first opened, it is heavily obfuscated; this is done by design as a means to thwart AV analysis and static detection:
After de-obfuscating the script, I found that several compromised domains are used to store multiple files for use later on in the execution routine. Of the downloaded files, we can see that the first two (a1.exe and a2.exe) are designed as backdoors to the system. a1.exe is usually W32.Kovter and a2.exe is usually W32.Boaxxe. Since PHP is not installed natively on the Windows OS, the third and fourth files to be downloaded (a.exe and php4ts.dll) are part of a portable PHP interpreter, which allows the ransomware (a.php – the fifth file to be downloaded) to run.
Analysis of a.php
At first, we saw several samples of a.php written in plain text without obfuscation, but the developers soon changed this to thwart static detection techniques. The obfuscation techniques below use chr() to encode each as a number specified in ASCII, while also using array() to store the php script in a list of array values.
Examples of obfuscated ransomware variants
To de-obfuscate this, I converted all of the chr values to ASCII characters and finally decoded base64 stored to get the original script.
To de-obfuscate this, I echoed the output of implode for all of the arrays (and removed eval), using the following at the end of the script:
;echo implode($f,"); ?>
The PHP script first uses ‘set_time_limit(0);’ to keep the interpreter running.
A recursive TREE function is then used with preg_match to match folders:
If a match is found, the script opens the directory and checks for more directories using is_dir; if a directory is found, it runs TREE again, which continues the loop to check whether the object is a folder or a file.
Once a file is found, it uses preg_match again to match its file extension:
Once a file matching the file extensions above is found, it stores that file name and path as the variable ‘$fp’ and a new variable, ‘$x’, is made, which uses the function fread.
fread() reads up to length bytes from the file pointer referenced by handle.
After reading the first 1024 bytes of a file, a for loop is used with strlen and the variable $k (a base64 string) to encrypt the files.