CVE-2020-3517: Cisco FXOS DoS Vulnerability

CVE-2020-3517 Overview

A vulnerability in the Cisco Fabric Services component of Cisco FXOS Software and Cisco NX-OS Software could allow an unauthenticated attacker to cause process crashes, resulting in a denial of service (DoS) condition on affected devices. The vulnerability stems from insufficient error handling when the affected software parses Cisco Fabric Services messages. An attacker could exploit this vulnerability by sending malicious Cisco Fabric Services messages to an affected device, potentially causing the device to reload.

Critical Impact

Successful exploitation allows unauthenticated attackers to trigger device reloads, causing service disruption across critical network infrastructure including data center switches, fabric interconnects, and firewall appliances.

Affected Products

• Cisco Firepower Extensible Operating System (FXOS)
• Cisco NX-OS Software (multiple versions from 6.0 through 9.3)
• Cisco Firepower 4100/9300 Series Appliances
• Cisco Nexus 3000, 5000, 6000, 7000, and 9000 Series Switches
• Cisco MDS 9000 Series Multilayer Switches
• Cisco UCS 6200, 6300, and 6400 Series Fabric Interconnects

Discovery Timeline

• August 27, 2020 - CVE-2020-3517 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2020-3517

Vulnerability Analysis

This vulnerability is classified as CWE-476 (Null Pointer Dereference), indicating that the Cisco Fabric Services component fails to properly validate or handle certain message inputs before dereferencing pointers. When malformed Cisco Fabric Services messages are processed, the software attempts to access memory through a null pointer, causing the process to crash.

The attack vector is configuration dependent and could be either remote (network-based) or adjacent depending on how Cisco Fabric Services is configured on the target device. Devices configured to use Cisco Fabric Services over IP (CFSoIP) would be vulnerable to remote attacks, while devices using Cisco Fabric Services over Fibre Channel would require adjacent network access.

The vulnerability affects a wide range of Cisco networking equipment commonly deployed in enterprise data centers and service provider environments, making it a significant concern for organizations relying on these platforms for critical network infrastructure.

Root Cause

The root cause is insufficient error handling in the Cisco Fabric Services message parsing code. The software does not adequately validate incoming Cisco Fabric Services messages before processing them, leading to a null pointer dereference condition when encountering malformed or malicious message content. This lack of defensive programming allows attackers to craft messages that trigger the vulnerable code path.

Attack Vector

An unauthenticated attacker can exploit this vulnerability by sending specially crafted Cisco Fabric Services messages to a vulnerable device. The attack does not require any authentication or user interaction, making it particularly dangerous in exposed network environments.

The vulnerability can be exploited remotely over the network when CFSoIP is enabled, or from an adjacent network segment when using Cisco Fabric Services over Fibre Channel. A successful exploit causes the affected device to reload, resulting in service interruption for all traffic passing through the device.

The Cisco Fabric Services protocol is used for distribution of configuration information across Cisco switches and fabric interconnects, meaning the attack surface is present on devices configured for fabric-wide configuration synchronization.

Detection Methods for CVE-2020-3517

Indicators of Compromise

• Unexpected device reloads or process crashes on affected Cisco FXOS or NX-OS devices
• Syslog messages indicating Cisco Fabric Services process failures or abnormal terminations
• Core dump files generated on affected devices following crash events
• Network monitoring alerts indicating sudden loss of connectivity to Cisco switches or fabric interconnects

Detection Strategies

• Monitor syslog and system logs for Cisco Fabric Services process crash messages and unexpected device reloads
• Implement network-based intrusion detection to identify anomalous Cisco Fabric Services traffic patterns
• Configure SNMP traps to alert on device reload events across affected Cisco infrastructure
• Deploy network traffic analysis to detect malformed Cisco Fabric Services messages targeting vulnerable devices

Monitoring Recommendations

• Enable detailed logging for Cisco Fabric Services on all affected devices to capture potential exploitation attempts
• Implement centralized log collection and analysis for all Cisco FXOS and NX-OS devices in the environment
• Configure real-time alerting for device reload events and process crash conditions
• Establish baseline behavior for Cisco Fabric Services traffic to identify anomalous activity

How to Mitigate CVE-2020-3517

Immediate Actions Required

• Apply the security patches provided by Cisco for affected FXOS and NX-OS software versions immediately
• Review Cisco Fabric Services configuration and disable CFSoIP if not required for operations
• Implement network segmentation to limit access to Cisco Fabric Services ports from untrusted networks
• Monitor affected devices for unusual behavior or unexpected reloads while planning patch deployment

Patch Information

Cisco has released security patches to address this vulnerability. Organizations should consult the Cisco Security Advisory for specific fixed software versions and upgrade guidance for their affected platforms.

The advisory provides detailed information about affected software releases and the corresponding fixed versions for each platform, including Cisco Firepower 4100/9300 Series, Nexus switches across multiple series, MDS 9000 Series, and UCS 6000 Series Fabric Interconnects.

Workarounds

• Disable Cisco Fabric Services over IP (CFSoIP) if the feature is not operationally required
• Implement access control lists (ACLs) to restrict access to Cisco Fabric Services ports from authorized management networks only
• Use control plane policing (CoPP) to rate-limit Cisco Fabric Services traffic and mitigate potential denial of service impacts
• Segment Cisco Fabric Services traffic to isolated management VLANs to reduce attack surface exposure

# Example: Disable CFSoIP on Cisco NX-OS devices
configure terminal
no cfs ipv4 distribute
no cfs ipv6 distribute
end
copy running-config startup-config