CVE-2021-1375: Cisco IOS XE Fast Reload RCE Vulnerability

CVE-2021-1375 Overview

Multiple vulnerabilities exist in the fast reload feature of Cisco IOS XE Software running on Cisco Catalyst 3850, Cisco Catalyst 9300, and Cisco Catalyst 9300L Series Switches. These vulnerabilities allow an authenticated, local attacker with privileged CLI access to execute arbitrary code on the underlying operating system, install and boot malicious software images, or execute unsigned binaries on affected devices. The vulnerabilities stem from improper checks performed by system boot routines, effectively bypassing the image verification component of the secure boot process.

Critical Impact

Successful exploitation enables attackers with privileged local access to bypass secure boot protections and execute arbitrary code or install malicious firmware on critical network infrastructure devices.

Affected Products

• Cisco Catalyst 3850 Series Switches running Cisco IOS XE versions 16.5.1 through 17.1.1
• Cisco Catalyst 9300 Series Switches running Cisco IOS XE versions 16.5.1 through 17.1.1
• Cisco Catalyst 9300L Series Switches running Cisco IOS XE versions 16.5.1 through 17.1.1

Discovery Timeline

• 2021-03-24 - CVE-2021-1375 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-1375

Vulnerability Analysis

This vulnerability (CWE-347: Improper Verification of Cryptographic Signature) affects the fast reload feature in Cisco IOS XE Software. The core issue lies in the system boot routines failing to properly validate cryptographic signatures during the fast reload process. When exploited, an attacker with privileged access to the device's command-line interface can leverage these flawed verification checks to execute unsigned code or install malicious software images that would normally be rejected by secure boot protections.

The attack requires local access with administrative privileges, limiting the attack surface to scenarios where an attacker has already gained a foothold with elevated permissions on the device. However, once exploited, the impact is severe as it undermines the fundamental trust model of the device's secure boot architecture.

Root Cause

The root cause is improper verification of cryptographic signatures (CWE-347) within the system boot routines. The fast reload feature does not adequately verify the authenticity and integrity of software images and binaries during the accelerated boot process. This allows an attacker to bypass the secure boot mechanism that normally ensures only Cisco-signed code executes on the device.

Attack Vector

The attack requires local access to the device's CLI with privileged credentials. An attacker must first obtain administrative access to an affected Cisco Catalyst switch. From this position, the attacker can manipulate the fast reload process to load unsigned binaries or malicious software images. The vulnerability can be exploited during the system boot phase when the fast reload feature bypasses normal image verification checks.

The exploitation flow involves:

1. Gaining privileged CLI access to an affected Cisco Catalyst switch
2. Preparing a malicious software image or unsigned binary
3. Leveraging the fast reload feature to boot the device with the malicious code
4. Bypassing secure boot verification to achieve persistent code execution

Detection Methods for CVE-2021-1375

Indicators of Compromise

• Unexpected software images or binaries present on the switch's file system
• Anomalous boot sequences or modified fast reload configurations
• Unauthorized changes to device firmware or boot variables
• Suspicious CLI activity from privileged accounts, particularly involving reload commands

Detection Strategies

• Monitor syslog messages for unexpected reload operations or boot sequence anomalies
• Implement file integrity monitoring on switch file systems to detect unauthorized image modifications
• Review AAA (Authentication, Authorization, and Accounting) logs for suspicious privileged command execution
• Deploy network behavioral analysis to identify unusual administrative access patterns

Monitoring Recommendations

• Enable command logging on all administrative sessions to capture CLI activity
• Configure SNMP traps to alert on device reloads and configuration changes
• Implement centralized log aggregation for Cisco devices to correlate potential exploitation attempts
• Schedule regular integrity verification checks comparing running images against known-good hashes

How to Mitigate CVE-2021-1375

Immediate Actions Required

• Apply the appropriate Cisco IOS XE software update as specified in the Cisco Security Advisory
• Review and restrict privileged CLI access to only essential personnel
• Implement multi-factor authentication for administrative access where supported
• Audit current device configurations and validate running software image integrity

Patch Information

Cisco has released software updates that address these vulnerabilities. Affected versions span Cisco IOS XE releases from 16.5.1 through 17.1.1, including various maintenance releases. Organizations should consult the Cisco Security Advisory for specific fixed release information and upgrade paths for their deployed hardware platforms.

Workarounds

• There are no published workarounds for this vulnerability; patching is the recommended remediation
• Limit privileged CLI access to trusted administrators using role-based access control
• Implement strict network segmentation to isolate management interfaces
• Monitor for and investigate any unexpected device reload operations

# Verify current IOS XE version and check for affected releases
show version | include Software

# Review boot configuration for anomalies
show boot

# Verify software image integrity
verify /md5 flash:packages.conf