#StopRansomware: LockBit 3.0 Ransomware Affiliates Exploit CVE 2023-4966 Citrix Bleed Vulnerability

SUMMARY

Note: This joint Cybersecurity Advisory (CSA) is part of an ongoing #StopRansomware effort to publish advisories for network defenders that detail various ransomware variants and ransomware threat actors. These #StopRansomware advisories include recently and historically observed tactics, techniques, and procedures (TTPs) and indicators of compromise (IOCs) to help organizations protect against ransomware. Visit stopransomware.gov to see all #StopRansomware advisories and to learn more about other ransomware threats and no-cost resources.

The Cybersecurity and Infrastructure Security Agency (CISA), Federal Bureau of Investigation (FBI), Multi-State Information Sharing & Analysis Center (MS-ISAC), and Australian Signals Directorate’s Australian Cyber Security Centre (ASD’s ACSC) are releasing this joint Cybersecurity Advisory (CSA) to disseminate IOCs, TTPs, and detection methods associated with LockBit 3.0 ransomware exploiting CVE-2023-4966, labeled Citrix Bleed, affecting Citrix NetScaler web application delivery control (ADC) and NetScaler Gateway appliances.

This CSA provides TTPs and IOCs obtained from FBI, ACSC, and voluntarily shared by Boeing. Boeing observed LockBit 3.0 affiliates exploiting CVE-2023-4966, to obtain initial access to Boeing Distribution Inc., its parts and distribution business that maintains a separate environment. Other trusted third parties have observed similar activity impacting their organization.

Historically, LockBit 3.0 affiliates have conducted attacks against organizations of varying sizes across multiple critical infrastructure sectors, including education, energy, financial services, food and agriculture, government and emergency services, healthcare, manufacturing, and transportation. Observed TTPs for LockBit ransomware attacks can vary significantly in observed TTPs.

Citrix Bleed, known to be leveraged by LockBit 3.0 affiliates, allows threat actors to bypass password requirements and multifactor authentication (MFA), leading to successful session hijacking of legitimate user sessions on Citrix NetScaler web application delivery control (ADC) and Gateway appliances. Through the takeover of legitimate user sessions, malicious actors acquire elevated permissions to harvest credentials, move laterally, and access data and resources.

CISA and the authoring organizations strongly encourage network administrators to apply the mitigations found in this CSA, which include isolating NetScaler ADC and Gateway appliances and applying necessary software updates through the Citrix Knowledge Center.

The authoring organizations encourage network defenders to hunt for malicious activity on their networks using the detection methods and IOCs within this CSA. If a potential compromise is detected, organizations should apply the incident response recommendations. If no compromise is detected, organizations should immediately apply patches made publicly available.

For the associated Malware Analysis Report (MAR), see: MAR-10478915-1.v1 Citrix Bleed

Download the PDF version of this report:

For a downloadable copy of IOCs, see:

TECHNICAL DETAILS

Note: This advisory uses the MITRE ATT&CK® for Enterprise framework, version 14. See the MITRE ATT&CK Tactics and Techniques section for a table of the threat actors’ activity mapped to MITRE ATT&CK tactics and techniques. For assistance with mapping malicious cyber activity to the MITRE ATT&CK framework, see CISA and MITRE ATT&CK’s Best Practices for MITRE ATT&CK Mapping and CISA’s Decider Tool.

CVE-2023-4966

CVE-2023-4966 is a software vulnerability found in Citrix NetScaler ADC and NetScaler Gateway appliances with exploitation activity identified as early as August 2023. This vulnerability provides threat actors, including LockBit 3.0 ransomware affiliates, the capability to bypass MFA [T1556.006] and hijack legitimate user sessions [T1563].

After acquiring access to valid cookies, LockBit 3.0 affiliates establish an authenticated session within the NetScaler appliance without a username, password, or access to MFA tokens [T1539]. Affiliates acquire this by sending an HTTP GET request with a crafted HTTP Host header, leading to a vulnerable appliance returning system memory information [T1082]. The information obtained through this exploit contains a valid NetScaler AAA session cookie.

Citrix publicly disclosed CVE-2023-4966 on Oct. 10, 2023, within their Citrix Security Bulletin, which issued guidance, and detailed the affected products, IOCs, and recommendations. Based on widely available public exploits and evidence of active exploitation, CISA added this vulnerability to the Known Exploited Vulnerabilities (KEVs) Catalog. This critical vulnerability exploit impacts the following software versions [1]:

• NetScaler ADC and NetScaler Gateway 14.1 before 14.1-8.50
• NetScaler ADC and NetScaler Gateway 13.1 before 13.1-49.15
• NetScaler ADC and NetScaler Gateway 13.0 before 13.0-92.19
• NetScaler ADC and NetScaler Gateway version 12.1 (EOL)
• NetScaler ADC 13.1FIPS before 13.1-37.163
• NetScaler ADC 12.1-FIPS before 12.1-55.300
• NetScaler ADC 12.1-NDcPP before 12.1-55.300

Due to the ease of exploitation, CISA and the authoring organizations expect to see widespread exploitation of the Citrix vulnerability in unpatched software services throughout both private and public networks.

Threat Actor Activity

Malware identified in this campaign is generated beginning with the execution of a PowerShell script (123.ps1) which concatenates two base64 strings together, converts them to bytes, and writes them to the designated file path.

The resulting file (adobelib.dll) is then executed by the PowerShell script using rundll32.

rundll32 C:\Users\Public\adobelib.dll,main <104 hex char key>

The Dynamic Link Library (DLL) will not execute correctly without the 104 hex character key. Following execution, the DLL attempts to send a POST request to https://adobe-us-updatefiles[.]digital/index.php which resolves to IP addresses 172.67.129[.]176 and 104.21.1[.]180 as of November 16, 2023. Although adobelib.dll and the adobe-us-updatefiles[.]digital have the appearance of legitimacy, the file and domain have no association with legitimate Adobe software and no identified interaction with the software.

Other observed activities include the use of a variety of TTPs commonly associated with ransomware activity. For example, LockBit 3.0 affiliates have been observed using AnyDesk and Splashtop remote management and monitoring (RMM), Batch and PowerShell scripts, the execution of HTA files using the Windows native utility mshta.exe and other common software tools typically associated with ransomware incidents.

INDICATORS OF COMPROMISE (IOCS)

See Table 1–Table 5 for IOCs related to Lockbit 3.0 affiliate exploitation of CVE-2023-4966.

[Fidelity] Legend:

• High = Indicator is unique or highly indicates LockBit in an environment.
• Medium = Indicator was used by LockBit but is used outside of LockBit activity, albeit rarely.
• Low = Indicates tools that are commonly used but were used by LockBit.

Low confidence indicators may not be related to ransomware.

Disclaimer: Some IP addresses in this CSA may be associated with legitimate activity. Organizations are encouraged to investigate the activity around these IP addresses prior to taking action, such as blocking. Activity should not be attributed as malicious without analytical evidence to support they are used at the direction of, or controlled by, threat actors.

The authoring organizations recommended monitoring/reviewing traffic to the 81.19.135[.]* class C network and review for MSHTA being called with HTTP arguments [2].

MITRE ATT&CK TACTICS AND TECHNIQUES

See Table 6 and Table 7 for all referenced threat actor tactics and techniques in this advisory.

DETECTION METHODS

Hunting Guidance

Network defenders should prioritize observing users in session when hunting for network anomalies. This will aid the hunt for suspicious activity such as installing tools on the system (e.g., putty, rClone ), new account creation, log item failure, or running commands such as hostname, quser, whoami, net, and taskkill. Rotating credentials for identities provisioned for accessing resources via a vulnerable NetScaler ADC or Gateway appliance can also aid in detection.

For IP addresses:

• Identify if NetScaler logs the change in IP.
• Identify if users are logging in from geolocations uncommon for your organization’s user base.
• If logging VPN authentication, identify if users are associated with two or more public IP addresses while in a different subnet or geographically dispersed.

Note: MFA to NetScaler will not operate as intended due to the attacker bypassing authentication by providing a token/session for an already authenticated user.

The following procedures can help identify potential exploitation of CVE-2023-4966 and LockBit 3.0 activity:

• Search for filenames that contain tf0gYx2YI for identifying LockBit encrypted files.
• LockBit 3.0 actors were seen using the C:\Temp directory for loading and the execution of files.
• Investigate requests to the HTTP/S endpoint from WAF.
• Hunt for suspicious login patterns from NetScaler logs
• Hunt for suspicious virtual desktop agent Windows Registry keys
• Analyze memory core dump files.

Below, are CISA developed YARA rules and an open-source rule that may be used to detect malicious activity in the Citrix NetScaler ADC and Gateway software environment. For more information on detecting suspicious activity within NetScaler logs or additional resources, visit CISA’s Malware Analysis Report (MAR) MAR-10478915-1.v1 Citrix Bleed or the resource section of this CSA [3]:

YARA Rules

CISA received four files for analysis that show files being used to save registry hives, dump the Local Security Authority Subsystem Service (LSASS) process memory to disk, and attempt to establish sessions via Windows Remote Management (WinRM). The files include:

• Windows Batch file (.bat)
• Windows Executable (.exe)
• Windows Dynamic Link Library (.dll)
• Python Script (.py)

This file is a Windows batch file called a.bat that is used to execute the file called a.exe with the file called a.dll as an argument. The output is printed to a file named 'z.txt' located in the path C:\Windows\Tasks. Next, a.bat pings the loop back internet protocol (IP) address 127.0.0[.]1 three times.

The next command it runs is reg save to save the HKLM\SYSTEM registry hive into the C:\Windows\tasks\em directory. Again, a.bat pings the loop back address 127.0.0[.]1 one time before executing another reg save command and saves the HKLM\SAM registry hive into the C:\Windows\Task\am directory. Next, a.bat runs three makecab commands to create three cabinet (.cab) files from the previously mentioned saved registry hives and one file named C:\Users\Public\a.png. The names of the .cab files are as follows:

• c:\windows\tasks\em.cab
• c:\windows\tasks\am.cab
• c:\windows\tasks\a.cab

This file is a 64-bit Windows command-line executable called a.exe that is executed by a.bat. This file issues the remote procedure call (RPC) ncalrpc:[lsasspirpc] to the RPC end point to provide a file path to the LSASS on the infected machine. Once the file path is returned, the malware loads the accompanying DLL file called a.dll into the running LSASS process. If the DLL is correctly loaded, then the malware outputs the message "[*]success" in the console.

This file is a 64-bit Windows DLL called a.dll that is executed by a.bat as a parameter for the file a.exe. The file a.exe loads this file into the running LSASS process on the infected machine. The file a.dll calls the Windows API CreateFileW to create a file called a.png in the path C:\Users\Public.

Next, a.dll loads DbgCore.dll then utilizes MiniDumpWriteDump function to dump LSASS process memory to disk. If successful, the dumped process memory is written to a.png. Once this is complete, the file a.bat specifies that the file a.png is used to create the cabinet file called a.cab in the path C:\Windows\Tasks.

This file is a Python script called a.py that attempts to leverage WinRM to establish a session. The script attempts to authenticate to the remote machine using NT LAN Manager (NTLM) if the keyword "hashpasswd" is present. If the keyword "hashpasswd" is not present, then the script attempts to authenticate using basic authentication. Once a WinRM session is established with the remote machine, the script has the ability to execute command line arguments on the remote machine. If there is no command specified, then a default command of “whoami” is run.

Open Source YARA Rule

INCIDENT RESPONSE

Organizations are encouraged to assess Citrix software and your systems for evidence of compromise, and to hunt for malicious activity (see Additional Resources section).If compromise is suspected or detected, organizations should assume that threat actors hold full administrative access and can perform all tasks associated with the web management software as well as installing malicious code.

If a potential compromise is detected, organizations should:

1. Quarantine or take offline potentially affected hosts.
2. Reimage compromised hosts.
3. Create new account credentials.
4. Collect and review artifacts such as running processes/services, unusual authentications, and recent network connections.
   • Note: Removing malicious administrator accounts may not fully mitigate risk considering threat actors may have established additional persistence mechanisms.
5. Report the compromise to FBI Internet Crime Complaint Center (IC3) at IC3.gov, local FBI Field Office, or CISA via the agency’s Incident Reporting System or its 24/7 Operations Center (report@cisa.gov or 888-282-0870). State, local, tribal, or territorial government (SLTT) entities can also report to MS-ISAC (SOC@cisecurity.org or 866-787-4722). If outside of the US, please contact your national cyber center.

MITIGATIONS

These mitigations apply to all critical infrastructure organizations and network defenders using Citrix NetScaler ADC and Gateway software. CISA and authoring organizations recommend that software manufacturers incorporate secure-by-design and -default principles and tactics into their software development practices to limit the impact of exploitation such as threat actors leveraging unpatched vulnerabilities within Citrix NetScaler appliances, which strengthens the security posture of their customers.

For more information on secure by design, see CISA’s Secure by Design and Default webpage and joint guide.

The authoring organizations of this CSA recommend organizations implement the mitigations below to improve your cybersecurity posture on the basis of the threat actor activity and to reduce the risk of compromise associated with Citrix CVE 2023-4966 and LockBit 3.0 ransomware & ransomware affiliates. These mitigations align with the Cross-Sector Cybersecurity performance goals (CPGs) developed by CISA and the National Institute of Standards and Technology (NIST). The CPGs provide a minimum set of practices and protections that CISA and NIST recommend all organizations implement. CISA and NIST based the CPGs on existing cybersecurity frameworks and guidance to protect against the most common and impactful threats, tactics, techniques, and procedures. Visit CISA’s Cross-Sector Cybersecurity Performance Goals for more information on the CPGs, including additional recommended baseline protections.

• Isolate NetScaler ADC and Gateway appliances for testing until patching is ready and deployable.
• Secure remote access tools by:
  • Implement application controls to manage and control the execution of software, including allowlisting remote access programs. Application controls should prevent the installation and execution of portable versions of unauthorized remote access and other software. A properly configured application allowlisting solution will block any unlisted application execution. Allowlisting is important because antivirus solutions may fail to detect the execution of malicious portable executables when the files use any combination of compression, encryption, or obfuscation.
• Strictly limit the use of RDP and other remote desktop services. If RDP is necessary, rigorously apply best practices, for example [CPG 2.W]:
  • Audit the network for systems using RDP.
  • Close unused RDP ports.
  • Enforce account lockouts after a specified number of attempts.
  • Apply phishing-resistant multifactor authentication (MFA).
  • Log RDP login attempts.
• Restrict the use of PowerShell, using Group Policy, and only grant access to specific users on a case-by-case basis. Typically, only those users or administrators who manage the network or Windows operating systems (OSs) should be permitted to use PowerShell [CPG 2.E].
• Update Windows PowerShell or PowerShell Core to the latest version and uninstall all earlier PowerShell versions. Logs from Windows PowerShell prior to version 5.0 are either non-existent or do not record enough detail to aid in enterprise monitoring and incident response activities [CPG 1.E, 2.S, 2.T].
• Enable enhanced PowerShell logging [CPG 2.T, 2.U].
  • PowerShell logs contain valuable data, including historical OS and registry interaction and possible TTPs of a threat actor’s PowerShell use.
  • Ensure PowerShell instances, using the latest version, have module, script block, and transcription logging enabled (enhanced logging).
  • The two logs that record PowerShell activity are the PowerShell Windows Event Log and the PowerShell Operational Log. FBI and CISA recommend turning on these two Windows Event Logs with a retention period of at least 180 days. These logs should be checked on a regular basis to confirm whether the log data has been deleted or logging has been turned off. Set the storage size permitted for both logs to as large as possible.
• Configure the Windows Registry to require User Account Control (UAC) approval for any PsExec operations requiring administrator privileges to reduce the risk of lateral movement by PsExec.
• Implement a recovery plan to maintain and retain multiple copies of sensitive or proprietary data and servers in a physically separate, segmented, and secure location (e.g., hard drive, storage device, or the cloud).
• Require all accounts with password logins (e.g., service account, admin accounts, and domain admin accounts) to comply with NIST's standards for developing and managing password policies.
  • Use longer passwords consisting of at least 15 characters [CPG 2.B].
  • Store passwords in hashed format using industry-recognized password managers.
  • Add password user “salts” to shared login credentials.
  • Avoid reusing passwords [CPG 2.C].
  • Implement multiple failed login attempt account lockouts [CPG 2.G].
  • Disable password “hints."
  • Require administrator credentials to install software.
• Keep all operating systems, software, and firmware up to date. Timely patching is one of the most efficient and cost-effective steps an organization can take to minimize its exposure to cybersecurity threats. Organizations should patch vulnerable software and hardware systems within 24 to 48 hours of vulnerability disclosure. Prioritize patching known exploited vulnerabilities in internet-facing systems [CPG 1.E].
  • Upgrade vulnerable NetScaler ADC and Gateway appliances to the latest version available to lower the risk of compromise.

VALIDATE SECURITY CONTROLS

In addition to applying mitigations, CISA recommends exercising, testing, and validating your organization's security program against the threat behaviors mapped to the MITRE ATT&CK for Enterprise framework in this advisory. CISA recommends testing your existing security controls inventory to assess how they perform against the ATT&CK techniques described in this advisory.

To get started:

1. Select an ATT&CK technique described in this advisory (see Table 1).
2. Align your security technologies against the technique.
3. Test your technologies against the technique.
4. Analyze your detection and prevention technologies’ performance.
5. Repeat the process for all security technologies to obtain a set of comprehensive performance data.
6. Tune your security program, including people, processes, and technologies, based on the data generated by this process.

CISA and the authoring organizations recommend continually testing your security program, at scale, in a production environment to ensure optimal performance against the MITRE ATT&CK techniques identified in this advisory.

RESOURCES

• Stopransomware.gov is a whole-of-government approach that gives one central location for ransomware resources and alerts.
• The Joint Ransomware Guide provides preparation, prevention, and mitigation best practices as well as a ransomware response checklist.
• Cyber Hygiene Services and Ransomware Readiness Assessment provide no-cost cyber hygiene and ransomware readiness assessment services.
• For more resources to help aid in the mitigation of cyber threats and ransomware attacks visit Strategies to Mitigate Cyber Security Incidents, Protect yourself from Ransomware, and How the ASD’s ACSC can help during a Cyber Security Incident.

REPORTING

The FBI is seeking any information that can be shared, to include boundary logs showing communication to and from foreign IP addresses, a sample ransom note, communications with LockBit 3.0 affiliates, Bitcoin wallet information, decryptor files, and/or a benign sample of an encrypted file. The FBI and CISA do not encourage paying ransom as payment does not guarantee victim files will be recovered. Furthermore, payment may also embolden adversaries to target additional organizations, encourage other criminal actors to engage in the distribution of ransomware, and/or fund illicit activities. Regardless of whether you or your organization have decided to pay the ransom, the FBI and CISA urge you to promptly report ransomware incidents to the FBI Internet Crime Complaint Center (IC3) at ic3.gov, local FBI Field Office, or CISA via the agency’s Incident Reporting System or its 24/7 Operations Center at report@cisa.gov or by calling 1-844-Say-CISA (1-844-729-2472).

Australian organizations that have been impacted or require assistance in regard to a ransomware incident can contact ASD’s ACSC via 1300 CYBER1 (1300 292 371), or by submitting a report to cyber.gov.au.

DISCLAIMER

The information in this report is being provided “as is” for informational purposes only. CISA and authoring organizations do not endorse any commercial entity, product, company, or service, including any entities, products, or services linked within this document. Any reference to specific commercial entities, products, processes, or services by service mark, trademark, manufacturer, or otherwise, does not constitute or imply endorsement, recommendation, or favoring by CISA and the authoring organizations.

ACKNOWLEDGEMENTS

Boeing contributed to this CSA.

REFERENCES

[1] NetScaler ADC and NetScaler Gateway Security Bulletin for CVE-2023-4966
[2] What is Mshta, How Can it Be Used and How to Protect Against it (McAfee)
[3] Investigation of Session Hijacking via Citrix NetScaler ADC and Gateway Vulnerability (CVE-2023-4966)

VERSION HISTORY

November 21, 2023: Initial version.