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A Nasty Trick: From Credential Theft Malware to Business Disruption

FireEye is tracking a set of financially-motivated activity referred
  to as TEMP.MixMaster that involves the interactive deployment of Ryuk
  ransomware following TrickBot malware infections. These operations
  have been active since at least December 2017, with a notable uptick
  in the latter half of 2018, and have proven to be highly successful at
  soliciting large ransom payments from victim organizations. In
  multiple incidents, rather than relying solely on built-in TrickBot
  capabilities, TEMP.MixMaster used EMPIRE and RDP connections to enable
  lateral movement within victim environments. Interactive deployment of
  ransomware, such as this, allows an attacker to perform valuable
  reconnaissance within the victim network and identify critical systems
  to maximize their     href="https://www.zdnet.com/article/ransomware-suspected-in-cyberattack-that-crippled-major-us-newspapers/">disruption
    to business operations, ultimately increasing the likelihood an
  organization will pay the demanded ransom. These operations have
  reportedly netted about     href="https://www.crowdstrike.com/blog/big-game-hunting-with-ryuk-another-lucrative-targeted-ransomware/">$3.7
    million in current BTC value.


 

Notably, while there have been numerous reports attributing Ryuk
  malware to North Korea, FireEye has not found evidence of this during
  our investigations. This narrative appears to be driven by     href="https://research.checkpoint.com/ryuk-ransomware-targeted-campaign-break/">code
    similarities between Ryuk and Hermes, a ransomware that has been
  used by APT38. However, these code similarities are insufficient to
  conclude North Korea is behind Ryuk attacks, as the Hermes ransomware
  kit was also advertised for sale in the underground community at one time.


 

It is important to note that TEMP.MixMaster is solely a reference to
  incidents where we have seen Ryuk deployed following TrickBot
  infections and that not all TrickBot infections will lead to the
  deployment of Ryuk ransomware. The TrickBot administrator group, which
  is suspected to be based in Eastern Europe, most likely provide the
  malware to a limited number of cyber criminal actors to use in
  operations. This is partially evident through its use of “gtags” that
  appear to be unique campaign identifiers used to identify specific
  TrickBot users. In recent incidents investigated by our Mandiant
  incident response teams, there has been consistency across the gtags
  appearing in the configuration files of TrickBot samples collected
  from different victim networks where Ryuk was also deployed. The
  uniformity of the gtags observed across these incidents appears to be
  due to instances of TrickBot being propagated via the malware’s
  worming module configured to use these gtag values.


 

Currently, we do not have definitive evidence that the entirety of
  TEMP.MixMaster activity, from TrickBot distribution and operation to
  Ryuk deployment, is being conducted by a common operator or group. It
  is also plausible that Ryuk malware is available to multiple eCrime
  actors who are also using TrickBot malware, or that at least one
  TrickBot user is selling access to environments they have compromised
  to a third party.  The intrusion operations deploying Ryuk have also
  been called     href="https://www.crowdstrike.com/blog/big-game-hunting-with-ryuk-another-lucrative-targeted-ransomware/">GRIM SPIDER.


 

TrickBot Infection Leading to Ryuk Deployment


 

The following are a summary of tactics observed across incident
  response investigations where the use of TrickBot preceded
  distribution of Ryuk ransomware. Of note, due to the interactive
  nature of Ryuk deployment, the TTPs leading to its execution can vary
  across incidents. Furthermore, in at least one case, artifacts related
  to the execution of TrickBot were collected but there was insufficient
  evidence to clearly tie observed Ryuk activity to the use of TrickBot.


 
Initial Infection

 

The initial infection vector was not confirmed in all incidents; in
  one case, Mandiant identified that the attackers leveraged a
  payroll-themed phishing email with an XLS attachment to deliver
  TrickBot malware (Figure 1). The     href="https://myonlinesecurity.co.uk/fake-deloitte-fw-payroll-schedule-delivers-trickbot/">campaign
    is documented on this security site. Data from FireEye
  technologies shows that this campaign was widely distributed primarily
  to organizations in the United States, and across diverse industries
  including government, financial services, manufacturing, service
  providers, and high-tech.


 

Once a victim opened the attachment and enabled macros, it
  downloaded and executed an instance of the TrickBot malware from a
  remote server. Data obtained from FireEye technologies suggests that
  although different documents may have been distributed by this
  particular malicious spam run, the URLs from which the documents
  attempted to retrieve a secondary payload did not vary across
  attachments or recipients, despite the campaign’s broad distribution
  both geographically and across industry verticals. Note that the
  domain "deloitte-inv[.]com" is not a legitimate Deloitte
  domain, and does not indicate any compromise of Deloitte.


 
   
     

From: Adam Bush
          <Adam.Bush@deloitte-inv.com>
 Subject: FW: Payroll
          schedule
 Attachment: Payrollschedule.xls

Pay run
          summary report and individual payslips.
 Kind
          Regards,
 Adam Bush
 CONFIDENTIALITY NOTICE:

          The contents of this email message and any attachments are
          intended solely for the addressee(s) and may contain
          confidential and/or privileged information and may be legally
          protected from disclosure. If you are not the intended
          recipient of this message or their agent, or if this message
          has been addressed to you in error, please immediately alert
          the sender by reply email and then delete this message and any
          attachments. If you are not the intended recipient, you are
          hereby notified that any use, dissemination, copying, or
          storage of this message or its attachments is strictly
        prohibited.


 


  Figure 1: Email from a phishing campaign that
    downloaded TrickBot, which eventually led to Ryuk


 
Persistence and Lateral Movement

 

When executed, TrickBot created scheduled tasks on compromised
  systems to execute itself and ensure persistence following system
  reboot. These instances of TrickBot were configured to use their
  network propagation modules (sharedll and tabdll) that rely on SMB and
  harvested credentials to propagate to additional systems in the
  network. The number of systems to which TrickBot was propagated varied
  across intrusions from fewer than ten to many hundreds.


 
Dwell Time and Post-Exploitation Activity

 

After a foothold was established by the actors controlling TrickBot,
  a period of inactivity sometimes followed. Dwell time between TrickBot
  installation and Ryuk distribution varied across intrusions, but in at
  least one case may have been as long as a full year. Despite this long
  dwell time, the earliest reports of Ryuk malware only date back to
  August 2018. It is likely that actors controlling Trickbot instances
  used to maintain access to victim environments prior to the known
  availability of Ryuk were monetizing this access in different ways.
  Further, due to the suspected human-driven component to these
  intrusion operations, we would expect to commonly see a delay between
  initial infection and Ryuk deployment or other post-exploitation
  activity, particularly in cases where the same initial infection
  vector was used to compromise multiple organizations simultaneously.


 

Once activity restarted, the actors moved to interactive intrusion
  by deploying Empire and/or leveraging RDP connections tunneled through
  reverse-shells instead of relying on the built-in capabilities of
  TrickBot to interact with the victim network. In multiple intrusions
  TrickBot's reverse-shell module (NewBCtestDll) was used to execute
  obfuscated PowerShell scripts which ultimately downloaded and launched
  an Empire backdoor.


 
Variations in Ryuk Deployment Across Engagements

 

Post exploitation activity associated with each Ryuk incident has
  varied in historical and ongoing Mandiant incident response
  engagements. Given that collected evidence suggests Ryuk deployment is
  managed via human-interactive post-exploitation, variation and
  evolution in methodology, tools, and approach over time and across
  intrusions is expected.


 

The following high-level steps appear common across most incidents
  into which we have insight:


 
  • Actors produce a list of
        targets systems and save it to one or multiple .txt files.

  •    
  • Actors move a copy of PsExec, an instance of Ryuk, and one or
        more batch scripts to one or more domain controllers or other high
        privilege systems within the victim environment.
  • Actors run
        batch scripts to copy a Ryuk sample to each host contained in .txt
        files and ultimately execute them.

 

Some of the notable ways Ryuk deployment has varied include:


 
  • In one case, there was
        evidence suggesting that actors enumerated hosts on the victim
        network to identify targets for encryption with Ryuk, but in
        multiple other cases these lists were manually copied to the server
        that was then used for Ryuk distribution without clear evidence
        available for how they were produced.
  • There have been
        multiple cases where TrickBot was deployed broadly across victim
        environments rather than being used to maintain a foothold on a
        small number of hosts.
  • We have not identified evidence that
        Empire was used by the attackers in all cases and sometimes Empire
        was used to access the victim environment only after Ryuk encryption
        had started.
  • In one case, the attackers used a variant of
        Ryuk with slightly different capabilities accompanied by a
        standalone .bat script containing most of the same taskkill, net,
        and sc commands normally used by Ryuk to terminate processes and
        stop services related to anti-virus, backup, and database
      software.

 

Example of Ryuk Deployment – Q3 2018


 
  • Using previously stolen
        credentials the attacker logged into a domain controller and copied
        tools into the %TEMP% directory. Copied tools included AdFind.exe
        (Active Directory enumeration utility), a batch script (Figure 2),
        and a copy of the 7-Zip archive utility.
  • Downloaded
        utilities were copied to C:\Windows\SysWOW64\.
  • The attacker
        performed host and network reconnaissance using built-in Windows
      commands.
  • AdFind.exe was executed using the previously noted
        batch script, which was crafted to pass the utility a series of
        commands that were used to collect information about Active
        Directory users, systems, OUs, subnets, groups, and trust objects.
        The output from each command was saved to an individual text file
        alongside the AdFind.exe utility (Figure 2).
  • This process
        was performed twice on the same domain controller, 10 hours apart.
        Between executions of Adfind the attacker tested access to multiple
        domain controllers in the victim environment, including the one
        later used to deploy Ryuk.
  • The attacker logged into a
        domain controller and copied instances of PSExec.exe, a batch script
        used to kill processes and stop services, and an instance of Ryuk
        onto the system.
  • Using PsExec the attacker copied the
        process/service killing batch script to the %TEMP% folder on
        hundreds of computers across the victim environment, from which it
        was then executed.
  • The attacker then used PsExec to copy
        the Ryuk binary to the %SystemRoot% directories of these same
        computers. A new service configured to launch the Ryuk binary was
        then created and started.
  • Ryuk execution proceeded as
        normal, encrypting files on impacted systems.

 
   
     

adfind.exe -f (objectcategory=person) > 
        <user_list>.txt

adfind.exe -f
          objectcategory=computer > <computer_list>.txt


         

adfind.exe -f (objectcategory=organizationalUnit) >
        <ou_list>.txt

adfind.exe -subnets -f
          (objectCategory=subnet) > <subnet_list>.txt


         

adfind.exe -f "(objectcategory=group)" >
        <group_list>.txt

adfind.exe -gcb -sc trustdmp > 
        <trustdmp>.txt


 


  Figure 2: Batch script that uses adfind.exe tool
    to enumerate Active Directory objects


 

Example of Ryuk Deployment – Q4 2018


 
  • An instance of the EMPIRE
        backdoor launched on a system that had been infected by TrickBot.
        The attacker used EMPIRE’s built-in capabilities to perform network
      reconnaissance.
  • Attackers connected to a domain controller in
        the victim network via RDP and copied several files into the host’s
        C$ share. The copied files included an instance of PsExec, two batch
        scripts, an instance of the Ryuk malware, and multiple .txt files
        containing lists of hosts within the victim environment. Many of the
        targeted hosts were critical systems across the victim environment
        including domain controllers and other hosts providing key
        management and authentication services.
  • The attackers then
        executed the first of these two batch scripts. The executed script
        used PsExec and hard-coded credentials previously stolen by the
        actors to copy the Ryuk binary to each host passed as input from the
        noted .txt files (Figure 3).
  • Attackers then executed the
        second batch script which iterated through the same host lists and
        used PsExec to execute the Ryuk binaries copied by the first batch
        script (Figure 4).

 
   
     

start PsExec.exe
          @C:\<shared_folder>$\<list>.txt -u
          <domain>\<username> -p <password> cmd /c
          COPY "\\<shared_folder>\<ryuk_exe>"
        "C:\windows\temp\"


 


  Figure 3: Line from the batch file used to copy
    Ryuk executable to each system


 
   
     

start PsExec.exe -d
          @C:\<shared_folder>$\<list>.txt -u
          <domain>\<username> -p <password> cmd /c
        "C:\windows\temp\<ryuk_exe>"


 


  Figure 4: Line from the batch file used to
    execute Ryuk on each system

 


 

Consistency in TrickBot Group Tags


 

Each individual TrickBot sample beacons to its Command & Control
  (C2) infrastructure with a statically defined “gtag” that is believed
  to act as an identifier for distinct TrickBot customers. There has
  been significant uniformity in the gtags associated with TrickBot
  samples collected from the networks of victim organizations.


 
  • The instance of TrickBot
        identified as the likely initial infection vector for one intrusion
        was configured to use the gtag ‘ser0918us’.
    • At the time of
              distribution, the C2 servers responding to TrickBot samples
              using the gtag ‘ser0918us’ were sending commands to request that
              the malware scan victim networks, and then propagate across
              hosts via the TrickBot worming module.
    • It is possible
              that in some or all cases instances of TrickBot propagated via
              the malware’s worming module are configured to use different
              gtag values, specific to the same TrickBot client, designed to
              simplify management of implants post-exploitation.

     
  • A significant proportion of TrickBot samples obtained from
        the victim environments, including in the case where the infection
        vector was identified as a sample using gtag ‘ser0918us’, had gtags
        in the below formats. This may suggest that these gtags are used to
        manage post-exploitation instances of TrickBot for campaigns
        distributed via gtag ‘ser0918us’ and other related gtags.

             
    • libxxx (ex: lib373, lib369, etc)
    • totxxx (ex:
              tot373, tot369, etc)
    • jimxxx (ex jim373, jim369,
            etc)
  • The numbers appended to the end of each
        gtag appear to increment over time, and in some cases multiple
        samples sharing the same compile time but using different gtags were
        observed in the same victim environment, though in each of these
        cases the numbers appended to the end of the gtag matched (e.g. two
        distinct samples sharing the compile time 2018-12-07 11:28:23 were
        configured to use the gtags ‘jim371’ and ‘tot371’).
  • The C2
        infrastructure hard-coded into these samples overlaps significantly
        across samples sharing similar gtag values. However, there is also
        C2 infrastructure overlap between these samples and ones with
        dissimilar gtag values. These patterns may suggest the use of proxy
        infrastructure shared across multiple clients of the TrickBot
        administrator group.

 

Implications


 

Throughout 2018, FireEye observed an increasing number of cases
  where ransomware was deployed after the attackers gained access to the
  victim organization through other methods, allowing them to traverse
  the network to identify critical systems and inflict maximum damage.
        href="https://www.wired.com/story/doj-indicts-hackers-samsam-ransomware/">SamSam
  operations, which date back to late 2015, were arguably the first
  to popularize this methodology and TEMP.MixMaster’s is an example of
  its growing popularity with threat actors. FireEye Intelligence
  expects that these operations will continue to gain traction
  throughout 2019 due the success these intrusion operators have had in
  extorting large sums from victim organizations.


 

It is also worth highlighting TEMP.MixMaster’s reliance on TrickBot
  malware, which is more widely distributed, to gain access to victim
  organizations. Following indiscriminate campaigns, threat actors can
  profile victims to identify systems and users of interest and
  subsequently determine potential monetization strategies to maximize
  their revenue. Various malware families have incorporated capabilities
  that can aid in the discovery of high-value targets underscoring the
  necessity for organizations to prioritize proper remediation of all
  threats, not only those that initially appear to be targeted.


 

Acknowledgements


 

The authors would like to thank Brice Daniels, Edward Li, Eric
  Montellese, Sandor Nemes, Eric Scales, Brandan Schondorfer, Martin
  Tremblay, Isif Ibrahima, Phillip Kealy and Steve Rasch for their
  contributions to this blog post.


Source: A Nasty Trick: From Credential Theft Malware to Business Disruption

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