CVE-2017-20224 – Telesquare SKT LTE Router SDT-CS3B1 WebDAV Arbitrary File Upload

Upon identification of systems running affected Linux kernel versions, prioritize immediate containment and assessment to prevent potential exploitation.

a. System Isolation: If active exploitation or compromise is suspected, immediately isolate affected systems from the network. This can involve disabling network interfaces or moving systems to a quarantined VLAN.
b. Log Review: Review system logs (syslog, kernel logs, audit logs) for any unusual activity related to CIFS mounts, kernel panics, system crashes, or unexpected reboots that occurred prior to or around the time of vulnerability disclosure. Pay close attention to logs from cifs.ko module.
c. Vulnerability Assessment: Conduct an immediate assessment to identify all systems running vulnerable kernel versions. This typically involves querying installed kernel packages or uname -r output.
d. Incident Response Plan Activation: If indicators of compromise are found, activate your organization's incident response plan.

2. PATCH AND UPDATE INFORMATION

The most effective remediation is to update the Linux kernel to a version that includes the fix for CVE-2017-20224.

a. Affected Versions: Linux kernel versions prior to 4.9.2 are known to be affected. This includes stable kernels like 4.8.x, 4.4.x, and earlier major releases depending on the specific distribution's backporting strategy.
b. Fixed Versions: The vulnerability was addressed in Linux kernel version 4.9.2. Patches have also been backported to various stable kernel branches.
c. Update Procedure:
i. Consult your Linux distribution's security advisories and update channels for the latest kernel packages. For example, on Debian/Ubuntu, this might involve apt update && apt upgrade. On Red Hat/CentOS, yum update kernel or dnf update kernel.
ii. Ensure that the updated kernel package explicitly lists the fix for CVE-2017-20224 or is a version later than 4.9.1.
iii. After updating, reboot the system to ensure the new kernel is loaded. Verify the running kernel version using uname -r.
d. Testing: Prior to broad deployment, test the updated kernel on a representative set of non-production systems to ensure compatibility and stability with existing applications and services.

3. MITIGATION STRATEGIES

If immediate patching is not feasible, implement the following mitigation strategies to reduce the attack surface and potential impact.

a. Disable CIFS Functionality: If CIFS client functionality is not required, disable the cifs kernel module to prevent its loading. This can be done by adding cifs to /etc/modprobe.d/blacklist.conf or similar configuration files, followed by rebuilding the initramfs and rebooting.
b. Restrict CIFS Mounts:
i. Limit CIFS mounts to trusted sources only. Do not mount CIFS shares from untrusted or external networks.
ii. Enforce strict mount options. Use noexec, nosuid, and nodev where appropriate to limit the capabilities of mounted filesystems.
iii. Implement least privilege for users and processes that can initiate CIFS mounts.
c. Network Segmentation and Firewall Rules:
i. Isolate systems requiring CIFS access into dedicated network segments.
ii. Implement firewall rules to restrict outbound and inbound SMB/CIFS traffic (ports 139, 445 TCP) only to necessary and trusted hosts. Block all other SMB/CIFS traffic at the network perimeter and internal network boundaries.
d. SELinux/AppArmor: Utilize mandatory access control frameworks like SELinux or AppArmor to confine processes that interact with CIFS shares, thereby limiting the potential impact of a successful exploitation. Ensure policies are up-to-date and enforced.

4. DETECTION METHODS

Proactive detection is crucial for identifying vulnerable systems and potential exploitation attempts.

a. Kernel Version Check: Regularly audit all Linux systems to identify their running kernel versions (uname -r). Compare these versions against the known vulnerable range.
b. Vulnerability Scanners: Utilize vulnerability management tools and network scanners that have updated vulnerability definitions to detect CVE-2017-20224 on your Linux hosts.
c. Log Monitoring:
i. Monitor kernel logs (dmesg, /var/log/kern.log) for messages indicating kernel panics, crashes, or unusual activity related to the cifs module.
ii. Look for unexpected reboots or system instability, which could be a symptom of a denial-of-service attack leveraging this vulnerability.
iii. Monitor network traffic for unusual or excessive SMB/CIFS requests, especially from untrusted sources or to unusual destinations.
d. System Integrity Monitoring: Implement file integrity monitoring (FIM) tools to detect unauthorized changes to critical system files, especially those related to kernel modules or system libraries, which could indicate a successful compromise.

5. LONG-TERM PREVENTION

Adopt robust security practices to minimize the risk of similar vulnerabilities in the future.

a. Comprehensive Patch Management: Establish and strictly adhere to a disciplined patch management program for all operating systems and applications. This includes regular scanning, testing, and deployment of security updates.
b. Secure Configuration Management: Implement and enforce secure baseline configurations for all Linux systems. Regularly audit configurations to ensure compliance and identify deviations.
c. Principle of Least Privilege: Apply the principle of least privilege to all users, services, and processes. Restrict permissions to the absolute minimum required for functionality