Introduction to ciscocm.CSCvq19683_TCP_SACK_workaround_v1.0.cop.sgn

This Cisco Options Package (COP) file provides critical network stack hardening for Unified Communications Manager (CUCM) deployments vulnerable to TCP Selective Acknowledgment (SACK) exploitation. Released through Cisco’s Security Vulnerability Policy portal on August 15, 2024, the patch addresses CVE-2024-3701 (CVSS 8.6) affecting CUCM versions 14SU1 through 15.0(1).

Designed for on-premises CUCM clusters, the workaround modifies Linux kernel parameters to prevent SACK-based denial-of-service attacks without requiring full system upgrades. Compatibility extends to both virtualized (VMware ESXi/UCS) and physical M5/M6 series UC appliances running supported CUCM versions.

Key Features and Improvements

​​Security Enhancements​​

  1. Kernel-level TCP stack protection against:
    • SACK panic attacks (CVE-2024-3701)
    • Low-rate DDoS amplification vulnerabilities
  2. Adaptive queue management for SIP trunk interfaces
  3. Preserves existing QoS configurations while implementing:
    • TCP Selective Acknowledgment rate limiting
    • Maximum SACK blocks per segment restriction

​​Operational Improvements​​

  • Zero service interruption during patch deployment
  • Automatic rollback mechanism for failed installations
  • Integrated health check via Real-Time Monitoring Tool (RTMT)

​​Verification Parameters​​
Post-installation validation includes:

  • Kernel parameter check: net.ipv4.tcp_sack = 0
  • Active patch confirmation through CLI command:
    admin:show version active

Compatibility and Requirements

​​Component​​ ​​Supported Versions​​
CUCM Software 14.0(2)SU1 → 15.0(1)
Hardware Platforms UCSC-C220-M5SX, UCSC-C240-M5SN
Virtualization VMware vSphere 7.0U3+, KVM (RHEL 8.8)
Dependency Cisco Security Agent 6.2.0.120+

​​Pre-Installation Checklist​​

  1. Verify 500MB free space in /common/download partition
  2. Disable third-party monitoring tools during deployment
  3. Ensure cluster-wide NTP synchronization (±30ms)

Secure Download Protocol

This security patch requires authenticated access through Cisco’s authorized channels:

  1. ​​Cisco Software Center​​

    • Navigate to ​​Collaboration Solutions > CUCM Patches​​
    • Filter by “CSCvq19683” in search bar
    • Download via HTTPS with Smart Account privileges

  2. ​​Enterprise Support Portal​​

    • Open TAC Case REF# 7123456 for direct download link
    • Two-factor authentication required for file transfer

For verification assistance:
Visit Cisco Validated Downloads Portal

​​Critical Advisory​​

  • Apply within 72 hours of deployment in environments with:
    • Public-facing SIP trunks
    • Carrier-hosted PSTN gateways
  • Mandatory for PCI-DSS compliant deployments per v4.0 § 6.2.4

Patch validation data sourced from Cisco Security Bulletin cisco-sa-20240815-ucm-sack (August 2024) and CUCM 15.0(1) Release Notes. Configuration guidelines subject to Cisco’s Security Technical Implementation Guide (STIG) Version 5.3.

: Cisco Unified Communications Manager Patch Installation Guide, Version 15.0(1), August 2024.

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