CVE-2023-20159: Cisco Business 250 Switch RCE Vulnerability

CVE-2023-20159 Overview

CVE-2023-20159 is a critical stack-based buffer overflow vulnerability affecting the web-based user interface of multiple Cisco Small Business Series Switches. This vulnerability allows an unauthenticated, remote attacker to cause a denial of service (DoS) condition or execute arbitrary code with root privileges on affected devices. The vulnerability stems from improper validation of requests sent to the web interface, enabling attackers to send specially crafted requests that overflow stack buffers and compromise system integrity.

Critical Impact

Unauthenticated remote attackers can achieve root-level code execution or cause denial of service on network switches, potentially compromising entire network segments and enabling lateral movement within enterprise environments.

Affected Products

• Cisco Business 250 Series Smart Switches (all models)
• Cisco Business 350 Series Managed Switches (all models)
• Cisco Small Business 200 Series Smart Switches (SF200, SG200 models)
• Cisco Small Business 250 Series Smart Switches (SF250, SG250 models)
• Cisco Small Business 300 Series Managed Switches (SF300, SG300 models)
• Cisco Small Business 350 Series Managed Switches (SF350, SG350 models)
• Cisco Small Business 500 Series Stackable Managed Switches (SF500, SG500 models)
• Cisco Small Business 550X Series Stackable Managed Switches (SF550X, SG550X models)

Discovery Timeline

• May 18, 2023 - CVE-2023-20159 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2023-20159

Vulnerability Analysis

This vulnerability is classified as CWE-120: Buffer Copy without Checking Size of Input ('Classic Buffer Overflow'). The web-based management interface on affected Cisco Small Business switches fails to properly validate the size of user-supplied input before copying it to fixed-size stack buffers. When processing certain HTTP requests, the vulnerable code does not perform adequate bounds checking, allowing data to overflow allocated buffer space on the stack.

The vulnerability is particularly dangerous because it can be exploited by unauthenticated attackers over the network. No user interaction is required, and the attack complexity is low. Successful exploitation grants the attacker root privileges on the device, enabling complete control over the switch including the ability to intercept, modify, or redirect network traffic. The impact extends to confidentiality, integrity, and availability of the affected device and potentially the broader network infrastructure it serves.

Root Cause

The root cause is improper input validation within the web interface request handling code. When the switch's web management interface processes incoming HTTP requests, certain parameters are copied into stack-allocated buffers without verifying that the input length does not exceed the buffer size. This classic buffer overflow pattern allows attackers to overwrite adjacent memory on the stack, including return addresses and saved frame pointers, enabling arbitrary code execution.

Attack Vector

An attacker exploits this vulnerability by sending specially crafted HTTP requests to the web-based management interface of a vulnerable switch. The attack can be executed remotely over the network without requiring authentication credentials. The malicious request contains oversized input data that, when processed by the vulnerable code path, overflows the stack buffer. By carefully constructing the overflow payload, an attacker can:

1. Overwrite the return address on the stack to redirect execution flow
2. Inject shellcode that executes with root privileges
3. Crash the device causing a denial of service condition

The web interface typically runs on TCP ports 80 (HTTP) and 443 (HTTPS), making these the primary attack vectors. Since the vulnerability exists in the device firmware's web server component, both encrypted and unencrypted management sessions are vulnerable.

Detection Methods for CVE-2023-20159

Indicators of Compromise

• Unexpected device reboots or crashes of Cisco Small Business switches without administrative action
• Anomalous HTTP/HTTPS traffic patterns to switch management interfaces, particularly requests with unusually large parameters
• Unauthorized configuration changes on affected switches
• New or unknown processes running on switch devices
• Unexpected network traffic originating from switch management interfaces

Detection Strategies

• Monitor network traffic for oversized HTTP requests targeting switch management ports (80/443) with anomalous payload sizes
• Implement IDS/IPS signatures to detect buffer overflow exploitation attempts against Cisco Small Business switches
• Enable syslog logging on all affected switches and monitor for crash events, unexpected reboots, or authentication failures
• Deploy network segmentation monitoring to detect lateral movement from compromised network infrastructure

Monitoring Recommendations

• Configure SNMP traps to alert on device restarts and configuration changes
• Implement netflow or similar traffic analysis to baseline normal management interface traffic patterns
• Review switch logs regularly for signs of exploitation attempts or unauthorized access
• Monitor for firmware changes or unexpected updates to affected devices

How to Mitigate CVE-2023-20159

Immediate Actions Required

• Restrict access to switch web management interfaces using access control lists (ACLs) to trusted management networks only
• Disable HTTP/HTTPS management access if not required and use console or SSH access instead
• Segment network infrastructure management traffic onto dedicated VLANs inaccessible from user networks
• Monitor affected devices for signs of compromise while awaiting firmware updates

Patch Information

Cisco has released security updates to address this vulnerability. Administrators should consult the Cisco Security Advisory for specific firmware versions that remediate CVE-2023-20159. Note that for some older product lines (200 Series, 300 Series, 500 Series), Cisco has indicated that firmware fixes will not be provided as these products have reached end-of-life status. Organizations using end-of-life devices should prioritize migration to supported hardware.

Workarounds

• Disable remote web-based management and use only local console connections for device administration
• Implement strict firewall rules to block external access to switch management interfaces on ports 80 and 443
• Deploy network access control to limit which systems can reach switch management IP addresses
• Consider using out-of-band management networks that are completely isolated from production traffic

# Example ACL configuration to restrict management access (Cisco CLI)
# Apply to VLAN interface or management interface
ip access-list extended MGMT-ACCESS
 permit tcp 10.0.0.0 0.0.0.255 any eq 443
 permit tcp 10.0.0.0 0.0.0.255 any eq 22
 deny tcp any any eq 80
 deny tcp any any eq 443
 permit ip any any