CVE-2020-3477: Cisco IOS Information Disclosure Flaw

CVE-2020-3477 Overview

A vulnerability in the CLI parser of Cisco IOS Software and Cisco IOS XE Software could allow an authenticated, local attacker to access files from the flash: filesystem. The vulnerability is due to insufficient application of restrictions during the execution of a specific command. An attacker could exploit this vulnerability by using a specific command at the command line. A successful exploit could allow the attacker to obtain read-only access to files that are located on the flash: filesystem that otherwise might not have been accessible.

Critical Impact

Authenticated local attackers can bypass access restrictions to read sensitive files stored on the flash: filesystem, potentially exposing configuration data, credentials, or other protected information.

Affected Products

• Cisco IOS Software (version 16.3.11 and related versions)
• Cisco IOS XE Software
• Cisco 2610XM, 2611XM, 2612, 2620XM, 2621XM, 2650XM, 2651XM, 2691 routers

Discovery Timeline

• September 24, 2020 - CVE-2020-3477 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2020-3477

Vulnerability Analysis

This vulnerability stems from improper input validation (CWE-20) and incorrect authorization (CWE-863) within the CLI parser component of Cisco IOS and IOS XE Software. The flaw allows authenticated users with local access to circumvent intended file system restrictions and gain unauthorized read access to protected files stored on the flash: filesystem.

The attack requires local access and valid authentication credentials, meaning the attacker must already have some level of legitimate access to the device. However, once authenticated, the vulnerability allows bypassing the normal access control mechanisms that would typically prevent users from reading certain files on the flash: filesystem.

Root Cause

The root cause is insufficient application of restrictions during the execution of a specific CLI command. The CLI parser fails to properly validate and enforce access control policies when processing certain commands, allowing authenticated users to access files beyond their authorized scope. This represents a combination of improper input validation (CWE-20) and incorrect authorization checks (CWE-863).

Attack Vector

The attack requires local access to the device through the CLI interface. An authenticated attacker executes a specific command sequence through the command-line interface that bypasses normal file access restrictions. The CLI parser does not properly enforce access controls during command execution, allowing the attacker to read files from the flash: filesystem that should be protected.

The vulnerability exploitation flow involves:

1. Attacker gains authenticated local access to a vulnerable Cisco IOS or IOS XE device
2. Attacker executes a specially crafted command through the CLI
3. The CLI parser fails to properly enforce file system access restrictions
4. Attacker obtains read-only access to protected files on the flash: filesystem

Detection Methods for CVE-2020-3477

Indicators of Compromise

• Unusual CLI command patterns attempting to access flash: filesystem files
• Unexpected file read operations on the flash: filesystem by non-administrative users
• Anomalous access logs showing users accessing files outside their normal operational scope
• Repeated attempts to access configuration files or sensitive data stored on flash:

Detection Strategies

• Monitor CLI command logs for suspicious access patterns targeting the flash: filesystem
• Implement AAA (Authentication, Authorization, and Accounting) logging to track all CLI command execution
• Review privilege level assignments to ensure least privilege access principles
• Deploy network device monitoring solutions that can detect anomalous command patterns

Monitoring Recommendations

• Enable detailed logging of all CLI commands executed on affected devices
• Configure SNMP traps or syslog alerts for file system access events
• Regularly audit user access levels and authentication logs
• Implement behavioral analytics to detect deviations from normal CLI usage patterns

How to Mitigate CVE-2020-3477

Immediate Actions Required

• Review and apply the latest security patches from Cisco for affected IOS and IOS XE versions
• Restrict local CLI access to only essential personnel with legitimate operational needs
• Implement strong authentication mechanisms and enforce principle of least privilege
• Monitor all CLI activity for suspicious file system access attempts

Patch Information

Cisco has released a security advisory addressing this vulnerability. Administrators should consult the Cisco Security Advisory for specific patch information and upgrade guidance. The advisory contains detailed information about affected software versions and the fixed releases available.

Workarounds

• Limit local access to the device CLI to only trusted administrators
• Implement network segmentation to restrict management plane access
• Use role-based access control (RBAC) to limit user privileges on network devices
• Consider implementing terminal access controller access-control system (TACACS+) for enhanced command authorization

# Example: Configure TACACS+ command authorization
aaa new-model
aaa authentication login default group tacacs+ local
aaa authorization exec default group tacacs+ local
aaa authorization commands 15 default group tacacs+ local
aaa accounting commands 15 default start-stop group tacacs+