This new vulnerability in Citrix Virtual Apps and Desktop, as revealed by cybersecurity experts at watchTowr, targets the Session Recording component, which is meant for capturing user sessions for auditing and troubleshooting. The flaw lies in an improperly exposed Microsoft Message Queuing (MSMQ) service, which, when combined with incorrectly set permissions, creates an opportunity for exploitation. This exposure can be accessed via HTTP and leverages the BinaryFormatter class in .NET, which is known for its deserialization issues, to potentially allow unauthenticated remote code execution (RCE).

This vulnerability poses significant risks because an attacker can exploit it remotely, without authentication, to execute arbitrary code on the targeted system. Administrators should take immediate steps to review and secure MSMQ configurations and ensure robust permission settings are in place to mitigate this risk. It is also advisable to apply any available patches or security updates provided by Citrix to address this critical flaw.

The disclosed vulnerabilities in Citrix Virtual Apps and Desktops highlight significant security issues that could allow for privilege escalation and limited remote code execution:

Details of the Vulnerabilities:

1. CVE-2024-8068 (CVSS score: 5.1) – This flaw permits privilege escalation, enabling an attacker to gain access with the permissions of the NetworkService account.
2. CVE-2024-8069 (CVSS score: 5.1) – This issue allows for limited remote code execution but with the same NetworkService account’s privileges.

Requirements for Exploitation:

• Authentication: An attacker must be an authenticated user within the same Windows Active Directory (AD) domain as the session recording server.
• Network Location: The attacker must be on the same intranet as the session recording server, limiting the scope to internal threats but still posing a risk in compromised environments.

Mitigation and Fixed Versions:

Citrix has addressed these vulnerabilities in updated releases:

• Citrix Virtual Apps and Desktops versions prior to 2407: Update to hotfix 24.5.200.8.
• Citrix Virtual Apps and Desktops 1912 LTSR: Update to CU9 hotfix 19.12.9100.6.
• Citrix Virtual Apps and Desktops 2203 LTSR: Update to CU5 hotfix 22.03.5100.11.
• Citrix Virtual Apps and Desktops 2402 LTSR: Update to CU1 hotfix 24.02.1200.16.

Underlying Issue: Unsafe Use of BinaryFormatter

The vulnerabilities are exacerbated by the unsafe use of .NET’s BinaryFormatter for deserialization, a practice that Microsoft has strongly advised against due to its inherent security risks. As of .NET 9 (August 2024), BinaryFormatter has been removed because it does not conform to modern security standards and poses risks of remote code execution and information disclosure.

TECHNICAL DETAILS:

Core Component Affected:

• Session Recording Storage Manager: A Windows service used to handle recorded session data, receiving input through Microsoft Message Queuing (MSMQ).

Vulnerability Mechanism:

1. MSMQ and Permissions:
   • Excessive Privileges: The MSMQ instance handling incoming session recording data has misconfigured, excessive permissions. This broad access allows unauthorized users to send and manipulate messages in the queue.
   • Accessibility: The MSMQ service can be reached locally via the TCP port but can also be accessed externally over HTTP, increasing the exposure risk.
2. Deserialization with Binary Formatter:
   • Serialization Process: Session data received from MSMQ is processed using a serialization format. The deserialization process employs the Binary Formatter class, which is notoriously vulnerable when deserializing untrusted data.
   • Attack Surface: The use of Binary Formatter allows attackers to embed serialized objects within MSMQ messages. When deserialized, these objects can execute code if crafted maliciously.

 

Exploitation Details:

• Steps for Exploitation:
  1. Craft Malicious MSMQ Messages: An attacker crafts specially formatted MSMQ messages containing payloads that take advantage of the deserialization process.
  2. Send Over HTTP: These messages are transmitted to the vulnerable MSMQ instance via HTTP, leveraging the misconfigured permissions to insert them into the queue.
  3. Deserialization and RCE: The BinaryFormatter.Deserialize method processes the incoming message data. If the deserialized object includes executable code, it triggers remote code execution on the session recording server with the privileges of the NetworkService account.
• Privileges and Impact:

• • CVE-2024-8068: Allows privilege escalation by enabling attackers to gain NetworkService account-level access.
  • CVE-2024-8069: Permits remote code execution within the limits of the NetworkService account’s permissions.

Security Flaws Explained:

• MSMQ Misconfiguration: The service’s permissions allow unauthorized users to interact with the MSMQ instance without proper checks.
• BinaryFormatter Vulnerability: This deserialization method has long been identified as a weak point, as it executes code during the deserialization process without sufficient safeguards against malicious input.

Why BinaryFormatter Is Risky:

• Insecure by Design: BinaryFormatter was designed before deserialization vulnerabilities were widely understood. It does not conform to modern security standards, and deserialization can lead to remote code execution or data exposure.
• .NET Deprecation: Microsoft deprecated BinaryFormatter in newer versions, including removing it from .NET 9 (August 2024), emphasizing the importance of using safer serialization methods.

Defensive Measures:

• Apply Citrix Updates: Ensure that all affected Citrix products are updated to versions with the security fixes:
  • Versions prior to 2407 → Update to hotfix 24.5.200.8
  • 1912 LTSR → Update to CU9 hotfix 19.12.9100.6
  • 2203 LTSR → Update to CU5 hotfix 22.03.5100.11
  • 2402 LTSR → Update to CU1 hotfix 24.02.1200.16
• Restrict MSMQ Permissions: Review and limit MSMQ access to trusted users only.
• Replace BinaryFormatter: Use safer alternatives such as System.Text.Json or Newtonsoft.Json.
• Secure Network Configuration: Limit access to the session recording server to trusted intranet sources and isolate critical components.

Indicators of Compromise (IOCs)

Indicators of Compromise (IOCs) for vulnerabilities involving Citrix Virtual Apps and Desktops and MSMQ exploitation could include the following:

1. Network IOCs:

• Unusual HTTP Requests:
  • Monitor for HTTP traffic targeting the MSMQ instance, especially from non-trusted or external sources.
  • Requests to endpoints associated with session recording services that are out of ordinary hours or unusual patterns.
• Suspicious TCP Traffic:
  • Unexpected connections on the MSMQ TCP port (default is 1801).
  • High volume of traffic directed at MSMQ that deviates from normal behavior.

2. File IOCs:

• Payload Artifacts:
  • Malicious files or serialized objects crafted to trigger remote code execution during deserialization.
  • Temporary files or binaries in directories associated with NetworkService account operations.
• Unexpected Session Recordings:
  • Newly generated session recordings that appear unusual, either due to timing or content, indicating unauthorized session access.

3. Process IOCs:

• Unusual Processes Spawned by Session Recording Services:
  • Processes initiated by the Citrix Session Recording Storage Manager or related services that should not normally run.
  • Execution of commands or scripts tied to deserialization payloads, potentially through powershell.exe, cmd.exe, or custom executables.
• Privilege Escalation Attempts:
  • Instances of processes attempting to escalate privileges using the NetworkService account.

4. Log IOCs:

• Authentication Logs:
  • Failed and successful authentication attempts within the same Windows Active Directory domain as the session recording server.
  • Authentication events originating from suspicious or newly seen IP addresses or user accounts.
• Event Logs Related to MSMQ:
  • System or application logs showing new MSMQ messages received with unusual data or headers.
  • Error or warning logs associated with MSMQ services being accessed unexpectedly.

5. Indicators Specific to Deserialization Attacks:

• Suspicious Object Deserialization:
  • Logs or monitoring tools indicating the deserialization of objects using BinaryFormatter in Citrix environments.
• Code Execution Events:
  • Events where BinaryFormatter.Deserialize is triggered followed by commands indicative of code execution.

6. Malicious Traffic Signatures:

• Known Exploit Patterns:
  • Payloads known for exploiting BinaryFormatter deserialization, especially if linked to past campaigns or threat actor methodologies.
• Anomalous HTTP Headers:
  • Custom or malformed HTTP headers that may indicate an attacker probing for access.

7. Behavioral IOCs:

• Lateral Movement:
  • Evidence of attackers moving laterally within the network post-exploitation, accessing other services or accounts related to Citrix infrastructure.
• Unauthorized System Changes:
  • Modifications to MSMQ configurations or the session recording setup not performed by authorized administrators.

IMPACT:

The impact of these Citrix Virtual Apps and Desktops vulnerabilities (CVE-2024-8068 and CVE-2024-8069) can be significant, particularly for organizations that rely heavily on Citrix for remote access and session management. Here’s a breakdown of the potential impact:

1. Remote Code Execution (RCE):

• Severity: The ability for attackers to execute arbitrary code remotely without authentication is highly severe. Once exploited, attackers can run malicious commands or deploy malware on affected servers.
• Implication: An attacker gaining RCE could fully compromise the session recording server, potentially using it as a pivot point for further network infiltration, exfiltration of sensitive data, or the deployment of ransomware.

2. Privilege Escalation:

• Access to Network Service Account: With CVE-2024-8068, attackers can escalate privileges to the Network Service account. Although this account has limited rights compared to administrative privileges, it still has substantial access to network resources and can be a stepping stone for further attacks.
• Impact on Internal Network: Compromising the Network Service account could enable attackers to map out network structures, access shared resources, or attempt lateral movement to more privileged accounts.

3. Data Breach and Exposure:

• Session Recordings: The session recording component captures user activities, potentially including sensitive information such as credentials, confidential communications, and data entry. If attackers gain unauthorized access, they could harvest this information for espionage or financial gain.
• MSMQ Exploitation: Data moving through the MSMQ could be intercepted, modified, or injected with malicious payloads, affecting the integrity and confidentiality of communications.

4. Operational Disruption:

• Service Downtime: Exploiting these vulnerabilities could lead to service disruptions if the attacker’s actions cause the session recording service or related applications to crash or become unstable.
• Impact on IT Operations: Remediation efforts may require taking affected servers offline for patching and security enhancements, leading to potential operational downtime.

5. Compliance and Regulatory Risks:

• Data Protection Violations: For organizations bound by regulations such as GDPR, HIPAA, or CCPA, unauthorized access to session recordings or data breaches could result in significant fines and legal repercussions.
• Audit and Reporting: Post-exploitation, companies may face extensive audits, potentially uncovering other security gaps and resulting in costly remediation processes.

6. Reputation Damage:

• Trust Erosion: A breach resulting from these vulnerabilities could erode customer trust and damage the reputation of the affected organization. This is especially true for industries that manage sensitive client data, such as finance, healthcare, or legal services.
• Impact on Business Relationships: Partners and clients may reconsider associations if an organization cannot demonstrate robust cybersecurity practices.

7. Wider Implications for Supply Chains:

• Third-Party Risks: If an attacker compromises an organization using Citrix infrastructure, they could potentially leverage that position to infiltrate connected partners or clients, expanding the impact beyond the initial target.
• Critical Infrastructure: Organizations in sectors such as government, utilities, or critical infrastructure using Citrix for remote desktop services could face more severe implications, including national security risks.

Recommendations:

To mitigate the risks associated with CVE-2024-8068 and CVE-2024-8069 in Citrix Virtual Apps and Desktops, organizations should take the following recommendations seriously:

1. Immediate Patch Deployment:

• Apply Updates: Ensure all affected Citrix products are updated to the fixed versions:
  • Citrix Virtual Apps and Desktops prior to 2407: Upgrade to hotfix 24.5.200.8.
  • 1912 LTSR: Update to CU9 hotfix 19.12.9100.6.
  • 2203 LTSR: Update to CU5 hotfix 22.03.5100.11.
  • 2402 LTSR: Update to CU1 hotfix 24.02.1200.16.

2. Secure MSMQ Configurations:

• Review and Restrict Permissions: Audit MSMQ permissions to ensure only trusted and necessary users or systems have access. Remove any excessive permissions that could be exploited.
• Limit Exposure: Restrict access to MSMQ services to internal, trusted IP ranges and block external HTTP traffic that could reach MSMQ.

3. Deserialization Best Practices:

• Avoid BinaryFormatter: Replace or disable BinaryFormatter for deserialization tasks, using safer alternatives such as System.Text.Json or Newtonsoft.Json for handling serialized data.
• Input Validation: Implement strict input validation to ensure that incoming data is sanitized and authenticated before being deserialized.

4. Network Security Enhancements:

• Segment Critical Infrastructure: Ensure that Citrix session recording servers and MSMQ instances are located within segmented network zones with limited access.
• Deploy Firewalls: Use application-level firewalls to monitor and block any suspicious HTTP or TCP traffic aimed at the session recording server.
• Intrusion Detection/Prevention Systems (IDS/IPS): Deploy or update IDS/IPS rules to detect known indicators of exploitation attempts against MSMQ or deserialization vulnerabilities.

5. Access Control and Authentication:

• Strong Authentication Policies: Enforce multi-factor authentication (MFA) and ensure only authenticated users within the same Active Directory domain can access session recording resources.
• User Privilege Review: Regularly review and minimize user privileges, particularly those associated with the Network Service account and other service accounts.

6. Monitoring and Incident Response:

• Set Up Continuous Monitoring: Use security monitoring tools (e.g., SIEM solutions) to detect unusual activity related to MSMQ or deserialization events.
• Log Analysis: Implement comprehensive logging of access attempts and deserialization actions for quick incident response and forensic investigation.
• Prepare an Incident Response Plan: Update response plans to include scenarios involving MSMQ exploitation and remote code execution. Ensure the incident response team is prepared for quick isolation and mitigation of affected systems.

7. Training and Awareness:

• Educate IT Staff: Train your IT and security teams on the risks associated with deserialization and MSMQ exploitation, ensuring they understand how to detect signs of abuse.
• End-User Training: Inform users about safe practices to avoid phishing or other attack vectors that could be used to gain initial access for exploiting these vulnerabilities.

8. Review and Test Backup Procedures:

• Backup Critical Data: Regularly back up data and configurations for Citrix systems. Ensure backups are stored securely and can be restored quickly.
• Test Restorations: Periodically test backup restorations to verify that they are functional and complete, allowing quick recovery in case of compromise.

9. Long-Term Recommendations:

• Migration to Secure Serialization Frameworks: Plan and migrate away from outdated serialization frameworks across all applications, opting for those that follow modern security practices.
• Update Development Practices: Ensure that development teams avoid the use of risky deserialization methods in custom code and adopt best practices outlined by Microsoft and other security authorities.

10. Third-Party Risk Management:

• Assess Vendor Security: Review the security practices of third-party vendors using Citrix solutions or integrating with your systems to ensure they have applied necessary patches and configurations.
• Collaborate with Partners: Share information on securing Citrix environments and vulnerabilities with partners to enhance overall defense posture.