Locky is Back Asking for Unpaid Debts

On June 21, 2016, FireEye’s Dynamic Threat Intelligence (DTI)
identified an increase in JavaScript contained within spam emails.
FireEye analysts determined the increase was the result of a new Locky
ransomware spam campaign.

As shown in Figure 1, Locky spam activity was uninterrupted until
June 1, 2016, when it stopped for nearly three weeks. During this
period, Locky was the most dominant ransomware distributed in spam
email. Now, Locky distribution has returned to the level seen during
the first half of 2016.

Figure 1. Locky spam activity in 2016

Figure 2 shows that the majority of Locky spam email detections
between June 21 and June 23 of this year were recorded in Japan, the
United States and South Korea.

Figure 2. Locky spam by country from June 21 to
June 23 of this year

The spam email – a sample shown is shown in Figure 3 – purports to
contain an unpaid invoice in an attached ZIP archive. Instead of an
invoice, the ZIP archive contains a Locky downloader written in JavaScript.

Figure 3. Locky spam email

JavaScript based Downloader Updates

In this campaign, few updates were seen in both the JavaScript based
downloader and the Locky payload.

The JavaScript downloader does the following:

  1. Iterates over an array of URLs hosting the Locky payload.
  2. If a connection to one of the URLs fails, the JavaScript sleeps
    for 1,000 ms before continuing to iterate over the array of
  3. Uses a custom XOR-based decryption routine to decrypt
    the Locky payload.
  4. Ensures the decrypted binary is of a
    predefined size. In Figure 4 below, the size of the decrypted binary
    had to be greater than 143,360 bytes and smaller than 153,660 bytes
    to be executed.

Figure 4. Payload download function in JavaScript

5.     Checks (Figure 5) that the first two bytes of the binary
contain the “MZ” header signature.

Figure 5: MZ header check

6.     Executes the decrypted payload by passing it the command line
parameter, “123”.

Locky Payload Updates

The Locky ransomware downloaded in this campaign requires a command
line argument to properly execute. This command line parameter, “123”
in the analyzed sample, is passed to the binary by the first stage
JavaScript-based downloader. This command line parameter value is used
in the code unpacking stage of the ransomware. Legitimate binaries
typically verify the number of arguments passed or compare the command
line parameter with the expected value and gracefully exit if the
check fails. However in the case of this Locky ransomware, the program
does not exit (Figure 6) and the value received as a command line
parameter is added to a constant value defined in the binary. The sum
of the constant and the parameter value is used in the decryption
routine (Figure 7). If no command line parameter is passed, it adds
zero to the constant.

Figure 6. Command line parameter check

Figure 7. Decryption routine

If no command line parameter is passed, then the constant for the
decryption routine is incorrect. This results in program crash as the
decrypted code is invalid. In Figure 8 and Figure 9, we can see the
decrypted code sections with and without the command line parameter, respectively.

Figure 8. Correct decrypted code

Figure 9. Incorrect decrypted code

By using this technique, Locky authors have created a dependency on
the first stage downloader for the second stage to be executed
properly. If a second stage payload such as this is directly analyzed,
it will result in a crash.


As of today, the Locky spam campaign is still ongoing, with an added
anti-analysis / sandbox evasion technique. We expect to see additional
Locky spam campaigns and will remain vigilant in order to protect our customers.

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*** This is a Security Bloggers Network syndicated blog from Threat Research Blog authored by Threat Research Blog. Read the original post at: http://www.fireeye.com/blog/threat-research/2016/06/locky-is-back-and-asking-for-unpaid-debts.html