Cerber: Analyzing a Ransomware Attack Methodology To Enable Protection

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
  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

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


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


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:TOOLSexecute.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.


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.


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.

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