Starting in mid-December 2020, malicious actors that Mandiant tracks
as UNC2546 exploited multiple zero-day vulnerabilities in Accellion’s
legacy File Transfer Appliance (FTA) to install a newly discovered web
shell named DEWMODE. The motivation of UNC2546 was not immediately
apparent, but starting in late January 2021, several organizations
that had been impacted by UNC2546 in the prior month began receiving
extortion emails from actors threatening to publish stolen data on the
“CL0P^_- LEAKS” .onion website. Some of the published victim data
appears to have been stolen using the DEWMODE web shell.

Notably, the number of victims on the “CL0P^_- LEAKS” shaming
website has increased in February 2021 with organizations in the
United States, Singapore, Canada, and the Netherlands recently outed
by these threat actors. Mandiant has previously reported that FIN11
has threatened to post stolen victim data on this same .onion
site as an additional tactic to pressure victims into paying extortion
demands following the deployment of CLOP ransomware. However, in
recent CLOP extortion incidents, no ransomware was deployed nor were
the other hallmarks of FIN11 present.

We are currently tracking the exploitation of the zero-day Accellion
FTA vulnerabilities and data theft from companies running the legacy
FTA product as UNC2546, and the subsequent extortion activity as
UNC2582. We have identified overlaps between UNC2582, UNC2546, and
prior FIN11 operations, and we will continue to evaluate the
relationships between these clusters of activity. For more information
on our use of ‘UNC’ designations, see our blog post, “DebUNCing
Attribution: How Mandiant Tracks Uncategorized Threat Actors.”

Mandiant has been working closely with Accellion in response to
these matters and will be producing a complete security assessment
report in the coming weeks. At this time, Accellion
has patched all FTA vulnerabilities known to be exploited by the
threat actors and has added new monitoring and alerting capabilities
to flag anomalies associated with these attack vectors. Mandiant has
validated these patches. Mandiant is currently performing penetration
testing and code review of the current version of the Accellion FTA
product and has not found any other critical vulnerabilities in the
FTA product based on our analysis to date. Accellion customers using
the FTA legacy product were the targets of the attack.

Accellion FTA is a 20-year-old product nearing end of life.
Accellion strongly recommends that FTA customers migrate
to kiteworks, Accellion’s enterprise content firewall
platform. Per Accellion, Kiteworks is built on an entirely different
code base.

The following CVEs have since been reserved for tracking the
recently patched Accellion FTA vulnerabilities:

• CVE-2021-27101
  – SQL injection via a crafted Host header
• CVE-2021-27102
  – OS command execution via a local web service call
• CVE-2021-27103
  – SSRF via a crafted POST request
• CVE-2021-27104
  – OS command execution via a crafted POST request

UNC2546 and DEWMODE

In mid-December 2020, Mandiant responded to multiple incidents in
which a web shell we call DEWMODE was used to exfiltrate data from
Accellion FTA devices. The Accellion FTA device is a purpose-built
application designed to allow an enterprise to securely transfer large
files. The exfiltration activity has affected entities in a wide range
of sectors and countries.

Across these incidents, Mandiant observed common infrastructure
usage and TTPs, including exploitation of FTA devices to deploy the
DEWMODE web shell. Mandiant determined that a common threat actor we
now track as UNC2546 was responsible for this activity. While complete
details of the vulnerabilities leveraged to install DEWMODE are still
being analyzed, evidence from multiple client investigations has shown
multiple commonalities in UNC2546’s activities.

Evidence of Exploitation and DEWMODE Installation

Mandiant has been able reconstruct many of the details about how
Accellion FTAs have been compromised through examination of Apache and
system logs from impacted devices—from initial compromise, to
deployment of DEWMODE, and follow-on interaction.

The earliest identification of activity associated with this
campaign occurred in mid-December 2020. At this time, Mandiant
identified UNC2546 leveraging an SQL injection vulnerability in the
Accellion FTA. This SQL injection served as the primary intrusion vector.

Mandiant observed evidence of SQL injection followed by subsequent
requests to additional resources, as shown in Figure 1.

Figure 1: SQL injection log

UNC2546 has leveraged this SQL injection vulnerability to retrieve a
key which appears to be used in conjunction with a request to the file
sftp_account_edit.php. Immediately after
this request, the built-in Accellion utility admin.pl was executed, resulting in an eval web
shell being written to oauth.api.

Figure 2: Excerpt from log showing creation of
eval web shell

The decoded contents are shown in Figure 3.

Figure 3: Decoded eval web shell

Almost immediately following this sequence, the DEWMODE web shell is
written to the system. The timing of these requests suggests that
DEWMODE was delivered via the oauth.api web
shell; however, the available evidence does not indicate the exact
mechanism used to write DEWMODE to disk.

Mandiant has identified the DEWMODE web shell in one of the
following two locations:

• /home/seos/courier/about.html
• /home/httpd/html/about.html

The DEWMODE web shell (Figure 4) extracts a list of available files
from a MySQL database on the FTA and lists those files and
corresponding metadata—file ID, path, filename, uploader, and
recipient—on an HTML page. UNC2546 then uses the presented list to
download files through the DEWMODE web shell. Download requests are
captured in the FTA’s web logs, which will contain requests to the
DEWMODE web shell with encrypted and encoded URL parameters, where dwn
is the file path and fn is the requested file name (Figure 5). The
encrypted file path and name values visible in web logs can be
decrypted using key material obtained from the database used by the
targeted FTA. Given the complex nature of this process, if your
organization needs assistance reviewing relevant logs, please contact
Mandiant or Accellion.

Figure 4: DEWMODE web shell screenshot

Figure 5: DEWMODE File Download URL parameters

Following file downloads, UNC2546 initiates a cleanup routine by
passing a specific query parameter named csrftoken with the value 11454bd782bb41db213d415e10a0fb3c to DEWMODE. The
following actions are performed:

• A shell script is written to /tmp/.scr, which will:
  • Remove all
    references to about.html from log
    files located in /var/opt/apache/
  • Write the modified log file to /tmp/x then replace the original log file at
    /var/opt/apache/
  • Delete the
    contents of the /home/seos/log/adminpl.log log file.
  • Remove /home/seos/courier/about.html (DEWMODE) and
    /home/seos/courier/oauth.api (eval
    web shell), and redirect command output to the file /tmp/.out
  • Change the permissions of
    the output file to be readable, writeable and executable by all
    users, and set the owner to “nobody”
• Delete
  the script file /tmp/.scr and other
  temporarily created files to assist in cleanup
• Display
  cleanup output to the requesting user

An example of a cleanup request and subsequent execution of the
cleanup script can be seen in Figure 6.

Figure 6: DEWMODE cleanup request

Mandiant also identified a variant of DEWMODE (bdfd11b1b092b7c61ce5f02ffc5ad55a) which contained
minor changes to the cleanup operation, including wiping of /var/log/secure and removing about.html and oauth.api
from the directories /home/httpd/html/
instead of /home/seos/courier/.

In a subset of incidents, Mandiant observed UNC2546 requesting a
file named cache.js.gz (Figure 7). Based on
temporal file access to the mysqldump
utility and mysql data directories, the
archive likely contained a dump of the database. With the exception of
cache.js.gz, Mandiant has not observed
UNC2546 acquiring files from Accellion appliances through any method
besides DEWMODE.

Figure 7: cache.js.gz file request

UNC2582 Data Theft Extortion

Shortly after installation of the web shell, in multiple cases
within hours, UNC2546 leveraged DEWMODE to download files from
compromised FTA instances. While the actors’ motivations were not
immediately clear, several weeks after delivery of the DEWMODE web
shell, victims began to receive extortion emails from an actor
claiming association with the CLOP ransomware team (Figure 8 and
Figure 9). The actors threatened to publish data on the “CL0P^_-
LEAKS” .onion shaming website, unless the victim paid an
extortion fee. We are tracking the subsequent extortion activity under
a separate threat cluster, UNC2582. Despite tracking the exploitation
and extortion activity in separate threat clusters we have observed at
least one case where an actor interacted with a DEWMODE web shell from
a host that was used to send UNC2582-attributed extortion email.

Figure 8: Extortion Note Template 1

Figure 9: Extortion Note Template 2

Based on observations at several engagements, UNC2582 appears to
follow a pattern of escalation to pressure victims into paying
extortion demands. Initial emails are sent from a free email account,
likely unique per victim, to a seemingly limited distribution of
addresses at the victim organization. If the victim does not respond
in a timely manner, additional emails are sent to a much larger number
of recipients from hundreds or thousands of different email accounts
and using varied SMTP infrastructure. In at least one case, UNC2582
also sent emails  to partners of the victim organization that included
links to the stolen data and negotiation chat. Monitoring of the
CL0P^_- LEAKS shaming website has demonstrated that UNC2582 has
followed through on threats to publish stolen data as several new
victims have appeared on the site in recent weeks, including at least
one organization that has publicly confirmed that their Accellion FTA
device had been recently targeted.

Key Overlaps With FIN11

UNC2582 (Extortion) and FIN11

Mandiant identified overlaps between UNC2582’s data theft extortion
activity and prior FIN11
operations, including common email senders and the use of the CL0P^_-
LEAKS shaming site. While FIN11 is known for deploying CLOP
ransomware, we have previously observed the group conduct data theft
extortion without ransomware deployment, similar to these cases.

• Some UNC2582 extortion emails observed in January 2021 were
  sent from IP addresses and/or email accounts used by FIN11 in
  multiple phishing campaigns between August and December 2020,
  including some of the last campaigns that were clearly attributable
  to the group.
• We have not observed FIN11 phishing activity
  in the new year. FIN11 has typically paused their phishing
  operations over the winter holidays and had several extended gaps in
  their operations. However, the timing of this current hiatus is also
  consistent with UNC2582’s data theft extortion activity.
• UNC2582 extortion emails contained a link to the CL0P^_- LEAKS
  website and/or a victim specific negotiation page. The linked
  websites were the same ones used to support historical CLOP
  operations, a series of ransomware and data theft extortion
  campaigns we suspect can be exclusively attributed to FIN11.

UNC2546 (FTA Exploitation and DEWMODE) and FIN11

There are also limited overlaps between FIN11 and UNC2546.

• Many of the organizations compromised by UNC2546 were
  previously targeted by FIN11.
• An IP address that
  communicated with a DEWMODE web shell was in the “Fortunix
  Networks L.P.” netblock, a network frequently used by FIN11 to
  host download and FRIENDSPEAK command and control (C2) domains.

Implications

The overlaps between FIN11, UNC2546, and UNC2582 are compelling, but
we continue to track these clusters separately while we evaluate the
nature of their relationships. One of the specific challenges is that
the scope of the overlaps with FIN11 is limited to the later stages of
the attack life cycle. UNC2546 uses a different infection vector and
foothold, and unlike FIN11, we have not observed the actors expanding
their presence across impacted networks. We therefore have
insufficient evidence to attribute the FTA exploitation, DEWMODE, or
data theft extortion activity to FIN11. Using SQL injection to deploy
DEWMODE or acquiring access to a DEWMODE shell from a separate threat
actor would represent a significant shift in FIN11 TTPs, given the
group has traditionally relied on phishing campaigns as its initial
infection vector and we have not previously observed them use zero-day
vulnerabilities.  

Acknowledgements

David Wong, Brandon Walters, Stephen Eckels and Jon Erickson

Indicators of Compromise (IOCs)

DEWMODE Web Shells

UNC2546 Source IP Addresses

The following source IP addresses were observed in multiple UNC2546 intrusions:

• 45.135.229.179
• 79.141.162.82
• 155.94.160.40
• 192.154.253.120
• 192.52.167.101
• 194.88.104.24

Detections

FireEye Detections

• FE_Webshell_PHP_DEWMODE_1
• FEC_Webshell_PHP_DEWMODE_1
• Webshell.PHP.DEWMODE

Mandiant Security Validation

• A101-515 Malicious File Transfer – DEWMODE Webshell, Upload,
  Variant #1
• A101-516 Malicious File Transfer – DEWMODE
  Webshell, Upload, Variant #2

DEWMODE YARA Rule

The following YARA rule is not intended to be used on production
systems or to inform blocking rules without first being validated
through an organization’s own internal testing processes to ensure
appropriate performance and limit the risk of false positives. This
rule is intended to serve as a starting point for hunting efforts to
identify DEWMODE payloads; however, it may need adjustment over time
if the malware family changes.