CVE-2020-3200: Cisco IOS SSH Server DoS Vulnerability

CVE-2020-3200 Overview

A vulnerability in the Secure Shell (SSH) server code of Cisco IOS Software and Cisco IOS XE Software could allow an authenticated, remote attacker to cause an affected device to reload. The vulnerability is due to an internal state not being represented correctly in the SSH state machine, which leads to unexpected behavior. An attacker could exploit this vulnerability by creating an SSH connection to an affected device and using a specific traffic pattern that causes an error condition within that connection. A successful exploit could allow an attacker to cause the device to reload, resulting in a denial of service (DoS) condition.

Critical Impact

Authenticated remote attackers can crash affected Cisco network devices by exploiting a state machine flaw in the SSH server, causing network outages and service disruption.

Affected Products

• Cisco IOS (multiple versions from 12.2 through 15.9)
• Cisco IOS XE (versions 3.2 through 16.12)

Discovery Timeline

• June 3, 2020 - CVE-2020-3200 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2020-3200

Vulnerability Analysis

This vulnerability affects the SSH server implementation in Cisco IOS and IOS XE operating systems. The flaw stems from improper state management within the SSH state machine, where internal states are not correctly represented during SSH session handling. When an authenticated user establishes an SSH connection and sends a specific sequence of traffic that triggers an error condition, the state machine enters an undefined state, causing the device to reload.

The vulnerability requires authentication, meaning an attacker must have valid credentials to access the SSH service. However, once authenticated, the attacker can reliably trigger a device reload, making this a significant denial of service threat for network infrastructure. The scope is changed, meaning the impact extends beyond the vulnerable component to affect the entire device and potentially downstream network services.

Root Cause

The root cause is classified under CWE-371 (State Issues) and CWE-436 (Interpretation Conflict). The SSH server code fails to properly maintain and validate internal state transitions within the SSH protocol state machine. When certain traffic patterns are processed during an active SSH session, the code encounters an error condition that is not properly handled, leading to state corruption. This corruption causes the SSH service to behave unexpectedly, ultimately triggering a system reload to recover from the invalid state.

Attack Vector

The attack vector is network-based, requiring the attacker to have remote network access to the SSH service on the vulnerable device. The attack requires low complexity to execute and requires low privileges (valid authentication credentials). No user interaction is required beyond the initial attack execution.

An attacker with valid SSH credentials can:

1. Establish an SSH connection to the target Cisco IOS or IOS XE device
2. Send a crafted sequence of traffic that triggers an error condition in the SSH state machine
3. Cause the device to reload, disrupting all network services passing through the device
4. Repeat the attack to maintain a persistent denial of service condition

The vulnerability does not affect confidentiality or integrity but has a high impact on availability due to the device reload. Since the scope is changed, the impact extends to dependent systems and services.

Detection Methods for CVE-2020-3200

Indicators of Compromise

• Unexpected device reloads with crash dumps indicating SSH-related errors
• Repeated SSH connection attempts followed by device restarts in system logs
• Syslog messages indicating SSH state machine errors or exceptions
• Core dumps containing SSH service crash information

Detection Strategies

• Monitor for unusual device reload patterns, especially those coinciding with SSH session activity
• Implement SNMP traps for device reboot notifications to identify potential exploitation attempts
• Review crash dump files and logs for SSH-related exceptions or state machine errors
• Track authentication logs for accounts that precede unexpected device reloads

Monitoring Recommendations

• Configure centralized logging (syslog) to capture device reload events and SSH session information
• Enable NetFlow or similar traffic analysis to identify anomalous SSH traffic patterns
• Set up alerting for multiple device reloads within short time periods
• Implement SSH session auditing to correlate authenticated users with device stability issues

How to Mitigate CVE-2020-3200

Immediate Actions Required

• Apply Cisco security patches as outlined in the Cisco Security Advisory
• Restrict SSH access to trusted IP addresses using access control lists (ACLs)
• Review and audit SSH user accounts to ensure only authorized personnel have access
• Consider implementing additional authentication mechanisms such as TACACS+ or RADIUS

Patch Information

Cisco has released software updates that address this vulnerability. Customers should consult the Cisco Security Advisory cisco-sa-ssh-dos-Un22sd2A for specific fixed software versions and upgrade guidance. The advisory provides detailed information about affected versions and the appropriate remediation path for each product.

Organizations running affected versions of Cisco IOS or IOS XE should prioritize patching network infrastructure devices, as a successful exploit can cause significant network disruptions.

Workarounds

• Implement strict ACLs to limit SSH access to management networks and trusted administrator IP addresses only
• Use VTY access class configurations to restrict which hosts can establish SSH connections
• Consider using console or out-of-band management for critical configuration changes until patches are applied
• Monitor for and investigate any unexpected device reloads to detect potential exploitation attempts

# Example ACL configuration to restrict SSH access
access-list 10 permit 192.168.1.0 0.0.0.255
access-list 10 deny any log

line vty 0 4
 access-class 10 in
 transport input ssh