Ransomware is a common method of cyber extortion for financial gain that typically involves users being unable to interact with their files, applications or systems until a ransom is paid. Accessibility of cryptocurrency such as Bitcoin has directly contributed to this ransomware model. Based on data from FireEye Dynamic Threat Intelligence (DTI), ransomware activities have been rising fairly steadily since mid-2015.

On June 10, 2016, FireEye’s HX detected a Cerber ransomware campaign involving the distribution of emails with a malicious Microsoft Word document attached. If a recipient were to open the document a malicious macro would contact an attacker-controlled website to download and install the Cerber family of ransomware.

Exploit Guard, a major new feature of FireEye Endpoint Security (HX), detected the threat and alerted HX customers on infections in the field so that organizations could inhibit the deployment of Cerber ransomware. After investigating further, the FireEye research team worked with security agency CERT-Netherlands, as well as web hosting providers who unknowingly hosted the Cerber installer, and were able to shut down that instance of the Cerber command and control (C2) within hours of detecting the activity. With the attacker-controlled servers offline, macros and other malicious payloads configured to download are incapable of infecting users with ransomware.

FireEye hasn’t seen any additional infections from this attacker since shutting down the C2 server, although the attacker could configure one or more additional C2 servers and resume the campaign at any time. This particular campaign was observed on six unique endpoints from three different FireEye endpoint security customers. HX has proven effective at detecting and inhibiting the success of Cerber malware.

Attack Process

The Cerber ransomware attack cycle we observed can be broadly broken down into eight steps:

1. Target receives and opens a Word document.
2. Macro in document is invoked to run PowerShell in hidden mode.
3. Control is passed to PowerShell, which connects to a malicious site to download the ransomware.
4. On successful connection, the ransomware is written to the disk of the victim.
5. PowerShell executes the ransomware.
6. The malware configures multiple concurrent persistence mechanisms by creating command processor, screensaver, startup.run and runonce registry entries.
7. The executable uses native Windows utilities such as WMIC and/or VSSAdmin to delete backups and shadow copies.
8. Files are encrypted and messages are presented to the user requesting payment.

Rather than waiting for the payload to be downloaded or started around stage four or five of the aforementioned attack cycle, Exploit Guard provides coverage for most steps of the attack cycle – beginning in this case at the second step.

The most common way to deliver ransomware is via Word documents with embedded macros or a Microsoft Office exploit. FireEye Exploit Guard detects both of these attacks at the initial stage of the attack cycle.

PowerShell Abuse

When the victim opens the attached Word document, the malicious macro writes a small piece of VBScript into memory and executes it. This VBScript executes PowerShell to connect to an attacker-controlled server and download the ransomware (profilest.exe), as seen in Figure 1.

Figure 1. Launch sequence of Cerber – the macro is responsible for invoking PowerShell and PowerShell downloads and runs the malware

It has been increasingly common for threat actors to use malicious macros to infect users because the majority of organizations permit macros to run from Internet-sourced office documents.

In this case we observed the macrocode calling PowerShell to bypass execution policies – and run in hidden as well as encrypted mode – with the intention that PowerShell would download the ransomware and execute it without the knowledge of the victim.

Further investigation of the link and executable showed that every few seconds the malware hash changed with a more current compilation timestamp and different appended data bytes – a technique often used to evade hash-based detection.

Cerber in Action

Initial payload behavior

Upon execution, the Cerber malware will check to see where it is being launched from. Unless it is being launched from a specific location (%APPDATA%\&#60GUID&#62), it creates a copy of itself in the victim's %APPDATA% folder under a filename chosen randomly and obtained from the %WINDIR%\system32 folder.

If the malware is launched from the specific aforementioned folder and after eliminating any blacklisted filenames from an internal list, then the malware creates a renamed copy of itself to “%APPDATA%\&#60GUID&#62” using a pseudo-randomly selected name from the “system32” directory. The malware executes the malware from the new location and then cleans up after itself.

Shadow deletion

As with many other ransomware families, Cerber will bypass UAC checks, delete any volume shadow copies and disable safe boot options. Cerber accomplished this by launching the following processes using respective arguments:

Vssadmin.exe "delete shadows /all /quiet"

WMIC.exe "shadowcopy delete"

Bcdedit.exe "/set {default} recoveryenabled no"

Bcdedit.exe "/set {default} bootstatuspolicy ignoreallfailures

Coercion

People may wonder why victims pay the ransom to the threat actors. In some cases it is as simple as needing to get files back, but in other instances a victim may feel coerced or even intimidated. We noticed these tactics being used in this campaign, where the victim is shown the message in Figure 2 upon being infected with Cerber.

Figure 2. A message to the victim after encryption

The ransomware authors attempt to incentivize the victim into paying quickly by providing a 50 percent discount if the ransom is paid within a certain timeframe, as seen in Figure 3.

 

 

Figure 3. Ransom offered to victim, which is discounted for five days

Multilingual Support

As seen in Figure 4, the Cerber ransomware presented its message and instructions in 12 different languages, indicating this attack was on a global scale.

Figure 4.   Interface provided to the victim to pay ransom supports 12 languages

Encryption

Cerber targets 294 different file extensions for encryption, including .doc (typically Microsoft Word documents), .ppt (generally Microsoft PowerPoint slideshows), .jpg and other images. It also targets financial file formats such as. ibank (used with certain personal finance management software) and .wallet (used for Bitcoin).

Selective Targeting

Selective targeting was used in this campaign. The attackers were observed checking the country code of a host machine’s public IP address against a list of blacklisted countries in the JSON configuration, utilizing online services such as ipinfo.io to verify the information. Blacklisted (protected) countries include: Armenia, Azerbaijan, Belarus, Georgia, Kyrgyzstan, Kazakhstan, Moldova, Russia, Turkmenistan, Tajikistan, Ukraine, and Uzbekistan.

The attack also checked a system's keyboard layout to further ensure it avoided infecting machines in the attackers geography: 1049—Russian, ¨ 1058—Ukrainian, 1059—Belarusian, 1064—Tajik, 1067—Armenian, 1068—Azeri, (Latin), 1079—Georgian, 1087—Kazakh, 1088—Kyrgyz (Cyrillic), 1090—Turkmen, 1091—Uzbek (Latin), 2072—Romanian (Moldova), 2073—Russian (Moldova), 2092—Azeri (Cyrillic), 2115—Uzbek (Cyrillic).

Selective targeting has historically been used to keep malware from infecting endpoints within the author’s geographical region, thus protecting them from the wrath of local authorities. The actor also controls their exposure using this technique. In this case, there is reason to suspect the attackers are based in Russia or the surrounding region.

Anti VM Checks

The malware searches for a series of hooked modules, specific filenames and paths, and known sandbox volume serial numbers, including: sbiedll.dll, dir_watch.dll, api_log.dll, dbghelp.dll, Frz_State, C:\popupkiller.exe, C:\stimulator.exe, C:\TOOLS\execute.exe, \sand-box\, \cwsandbox\, \sandbox\, 0CD1A40, 6CBBC508, 774E1682, 837F873E, 8B6F64BC.

Aside from the aforementioned checks and blacklisting, there is also a wait option built in where the payload will delay execution on an infected machine before it launches an encryption routine. This technique was likely implemented to further avoid detection within sandbox environments.

Persistence

Once executed, Cerber deploys the following persistence techniques to make sure a system remains infected:

• A registry key is added to launch the malware instead of the screensaver when the system becomes idle.
• The “CommandProcessor” Autorun keyvalue is changed to point to the Cerber payload so that the malware will be launched each time the Windows terminal, “cmd.exe”, is launched.
• A shortcut (.lnk) file is added to the startup folder. This file references the ransomware and Windows will execute the file immediately after the infected user logs in.
• Common persistence methods such as run and runonce key are also used.

A Solid Defense

Mitigating ransomware malware has become a high priority for affected organizations because passive security technologies such as signature-based containment have proven ineffective.

Malware authors have demonstrated an ability to outpace most endpoint controls by compiling multiple variations of their malware with minor binary differences. By using alternative packers and compilers, authors are increasing the level of effort for researchers and reverse-engineers. Unfortunately, those efforts don’t scale.

Disabling support for macros in documents from the Internet and increasing user awareness are two ways to reduce the likelihood of infection. If you can, consider blocking connections to websites you haven’t explicitly whitelisted. However, these controls may not be sufficient to prevent all infections or they may not be possible based on your organization.

FireEye Endpoint Security with Exploit Guard helps to detect exploits and techniques used by ransomware attacks (and other threat activity) during execution and provides analysts with greater visibility. This helps your security team conduct more detailed investigations of broader categories of threats. This information enables your organization to quickly stop threats and adapt defenses as needed.

Conclusion

Ransomware has become an increasingly common and effective attack affecting enterprises, impacting productivity and preventing users from accessing files and data.

Mitigating the threat of ransomware requires strong endpoint controls, and may include technologies that allow security personnel to quickly analyze multiple systems and correlate events to identify and respond to threats.

HX with Exploit Guard uses behavioral intelligence to accelerate this process, quickly analyzing endpoints within your enterprise and alerting your team so they can conduct an investigation and scope the compromise in real-time.

Traditional defenses don’t have the granular view required to do this, nor can they connect the dots of discreet individual processes that may be steps in an attack. This takes behavioral intelligence that is able to quickly analyze a wide array of processes and alert on them so analysts and security teams can conduct a complete investigation into what has, or is, transpiring. This can only be done if those professionals have the right tools and the visibility into all endpoint activity to effectively find every aspect of a threat and deal with it, all in real-time. Also, at FireEye, we go one step ahead and contact relevant authorities to bring down these types of campaigns.

Click here for more information about Exploit Guard technology.

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 URLs.
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.

Conclusion

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.

Email Hashes

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