CVE-2021-1368: Cisco NX-OS UDLD RCE Vulnerability

CVE-2021-1368 Overview

A vulnerability exists in the Unidirectional Link Detection (UDLD) feature of Cisco FXOS Software and Cisco NX-OS Software that could allow an unauthenticated, adjacent attacker to execute arbitrary code with administrative privileges or cause a denial of service (DoS) condition on affected devices. The vulnerability stems from insufficient input validation when processing UDLD protocol packets, enabling attackers with physical adjacency to send crafted packets that can crash the UDLD process or achieve code execution.

Critical Impact

Successful exploitation allows attackers to execute arbitrary code with administrative privileges on affected Cisco network infrastructure, potentially compromising entire data center switching fabrics and unified computing environments.

Affected Products

• Cisco NX-OS Software (versions 7.x, 8.x, 9.x)
• Cisco FXOS (Firepower Extensible Operating System)
• 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
• Cisco Firepower 4100 and 9300 Series Appliances

Discovery Timeline

• 2021-02-24 - CVE-2021-1368 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-1368

Vulnerability Analysis

This vulnerability (CWE-787: Out-of-Bounds Write) exists in the UDLD protocol handler within Cisco FXOS and NX-OS software. The UDLD protocol is designed to detect unidirectional links on fiber optic and twisted-pair Ethernet cables, allowing network devices to identify when traffic flows in only one direction. When processing specially crafted UDLD protocol packets, the software fails to properly validate input boundaries before writing data to memory buffers.

The exploitation scenario requires several conditions to be met: the attacker must have full control of a directly connected device, that device must be connected over a port channel with UDLD enabled, and specific system conditions must exist for arbitrary code execution. While the UDLD feature is disabled by default, organizations that enable it for link integrity monitoring become potentially vulnerable.

Root Cause

The root cause is insufficient input validation in the UDLD packet processing code. When the UDLD daemon receives protocol packets on enabled interfaces, it processes various Type-Length-Value (TLV) fields without adequate bounds checking. This allows an attacker to supply malformed packet data that causes an out-of-bounds write condition, corrupting adjacent memory and potentially redirecting program execution flow.

Attack Vector

The attack requires the adversary to be on an adjacent network segment with a directly connected device under their control. The attacker must craft malicious UDLD protocol packets and transmit them over a port channel where UDLD is enabled. The exploitation path differs based on system state:

Code Execution Path: When both UDLD-enabled port channels and specific system memory conditions exist, the out-of-bounds write can be leveraged to overwrite critical process structures, allowing the attacker to inject and execute arbitrary code with administrative privileges.

DoS Path: In the absence of the required memory conditions, the malformed packets cause the UDLD process to crash and restart repeatedly. Multiple process crashes trigger device reloads, resulting in denial of service.

The attacker sends crafted UDLD frames containing malformed TLV structures that exceed expected buffer sizes, triggering the memory corruption condition when parsed by the vulnerable UDLD implementation.

Detection Methods for CVE-2021-1368

Indicators of Compromise

• Unexpected UDLD process crashes or restarts logged in system messages
• Repeated device reloads without apparent hardware cause
• Anomalous UDLD protocol traffic patterns on port channel interfaces
• System log entries indicating memory corruption or segmentation faults in UDLD daemon

Detection Strategies

• Monitor for UDLD process crash events using syslog analysis: look for messages containing UDLD and crash-related keywords
• Implement network traffic analysis to detect malformed UDLD frames with abnormal TLV structures
• Enable enhanced logging on affected switches to capture UDLD protocol debugging information
• Deploy network monitoring to baseline normal UDLD traffic and alert on anomalies

Monitoring Recommendations

• Configure centralized syslog collection from all affected Cisco network devices
• Establish baseline UDLD traffic patterns and alert on statistical deviations
• Monitor for unauthorized physical connections to port channel interfaces
• Review device stability metrics for unexpected reloads or process restarts

How to Mitigate CVE-2021-1368

Immediate Actions Required

• Identify all Cisco FXOS and NX-OS devices in your environment running affected software versions
• Audit UDLD configuration across the network using show udld commands to identify enabled interfaces
• Evaluate operational need for UDLD and disable the feature where not strictly required
• Implement network segmentation to limit adjacent network access to critical switches
• Apply vendor patches according to the Cisco security advisory

Patch Information

Cisco has released software updates that address this vulnerability. Administrators should consult the Cisco Security Advisory for version-specific fixed release information. The advisory provides detailed guidance on determining affected versions and obtaining appropriate software updates for each product family (Nexus, MDS, UCS, and Firepower).

Workarounds

• Disable UDLD on port channels where the feature is not operationally required using interface-level configuration
• Implement physical security controls to prevent unauthorized device connections to port channel interfaces
• Use access control lists where possible to restrict Layer 2 protocol traffic
• Segment network management traffic from untrusted adjacent networks

# Disable UDLD on specific interfaces (workaround)
configure terminal
interface port-channel 1
  no udld enable
exit
# Verify UDLD status
show udld interface