CVE-2023-20078: Cisco IP Phone 6871 Firmware RCE Flaw

CVE-2023-20078 Overview

CVE-2023-20078 is a critical stack-based buffer overflow vulnerability affecting the web-based management interface of multiple Cisco IP Phone models. This vulnerability allows an unauthenticated, remote attacker to execute arbitrary code with root privileges or cause a denial of service (DoS) condition on affected devices. The flaw exists in how the web management interface processes user-supplied input, enabling attackers to send specially crafted requests that overflow stack buffers and achieve code execution.

Critical Impact

Unauthenticated remote attackers can achieve complete device compromise with root-level code execution on vulnerable Cisco IP Phones, potentially enabling persistent network access, call interception, and lateral movement within enterprise environments.

Affected Products

• Cisco IP Phone 6800 Series (Models 6825, 6841, 6851, 6861, 6871)
• Cisco IP Phone 7800 Series (Models 7811, 7821, 7832, 7841, 7861)
• Cisco IP Phone 8800 Series (Models 8811, 8832, 8841, 8845, 8851, 8861, 8865)

Discovery Timeline

• March 3, 2023 - CVE-2023-20078 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2023-20078

Vulnerability Analysis

This vulnerability stems from improper bounds checking in the web-based management interface of affected Cisco IP Phones. The web interface fails to properly validate the length of user-supplied input before copying it into fixed-size stack buffers. When an attacker submits a crafted HTTP request containing oversized input data, the buffer overflow overwrites adjacent stack memory, including saved return addresses and function pointers.

The vulnerability is classified under CWE-121 (Stack-based Buffer Overflow) and CWE-787 (Out-of-bounds Write). Stack-based buffer overflows are particularly dangerous because they allow attackers to hijack program execution flow by overwriting the return address on the stack, redirecting code execution to attacker-controlled payloads.

Since the vulnerability requires no authentication and is exploitable over the network, any IP phone with its web management interface accessible represents a potential entry point for attackers.

Root Cause

The root cause is insufficient input validation in the web management interface's request handling code. The vulnerable code copies user-controlled data into stack-allocated buffers without verifying that the input length does not exceed the buffer's capacity. This lack of bounds checking allows attackers to write beyond the allocated buffer boundaries, corrupting adjacent stack memory structures.

Attack Vector

The attack is executed remotely over the network by sending malicious HTTP requests to the web-based management interface of vulnerable Cisco IP Phones. The attacker does not require any credentials or prior authentication to exploit this vulnerability.

An attacker would craft an HTTP request containing an oversized payload targeting the vulnerable input parameter. When the web server processes this request, the oversized data overflows the stack buffer, allowing the attacker to:

1. Overwrite the saved return address to redirect execution
2. Inject and execute shellcode for arbitrary command execution
3. Crash the device causing denial of service

The attack can be performed from any network location that can reach the phone's web management interface, making network segmentation and access controls critical defensive measures.

Detection Methods for CVE-2023-20078

Indicators of Compromise

• Unexpected reboots or crashes of Cisco IP Phones without administrative action
• Unusual outbound network connections originating from IP phone devices
• Modified firmware or configuration files on affected phone models
• Anomalous HTTP traffic patterns to IP phone web management interfaces
• Evidence of shell access or unauthorized processes running on phone devices

Detection Strategies

• Monitor network traffic for unusually large HTTP requests targeting Cisco IP Phone web interfaces
• Implement intrusion detection rules to identify buffer overflow exploitation patterns against IP phone management ports
• Deploy network-based anomaly detection to identify phones exhibiting unexpected behavior such as outbound connections to unknown hosts
• Conduct regular firmware integrity checks on all Cisco IP Phone devices

Monitoring Recommendations

• Enable logging on network firewalls and proxies for all traffic to/from IP phone subnets
• Configure SIEM alerting for patterns consistent with web-based exploitation attempts against IoT/VoIP devices
• Establish baseline behavior profiles for IP phones and alert on deviations such as new listening ports or DNS queries to unusual domains
• Review Cisco CUCM (Call Manager) logs for unauthorized configuration changes to phone devices

How to Mitigate CVE-2023-20078

Immediate Actions Required

• Apply the latest firmware updates from Cisco immediately to all affected IP Phone models
• Disable web-based management interfaces on IP phones if not operationally required
• Implement network segmentation to isolate voice networks from general user and internet-facing networks
• Configure access control lists (ACLs) to restrict web management interface access to authorized administrative hosts only

Patch Information

Cisco has released firmware updates that address this vulnerability. Organizations should consult the Cisco Security Advisory for specific fixed firmware versions for each affected IP Phone model. The advisory provides detailed information on which firmware releases contain the security fix.

To obtain updated firmware, administrators should:

1. Access the Cisco Software Download Center
2. Navigate to the appropriate IP Phone product page
3. Download the latest firmware version that addresses cisco-sa-ip-phone-cmd-inj-KMFynVcP
4. Deploy updates using Cisco Unified Communications Manager (CUCM) or through manual upgrade procedures

Workarounds

• Disable the web-based management interface on affected phones using Cisco Unified Communications Manager (CUCM) configuration
• Implement strict network access controls to limit management interface exposure to trusted administrator workstations only
• Deploy IP phones on dedicated voice VLANs with firewall rules preventing unauthorized access to management ports
• Consider using CUCM-based management exclusively and blocking HTTP/HTTPS access to phones at the network level

# Example ACL configuration to restrict IP phone management access
# Apply on network infrastructure devices protecting voice VLANs
ip access-list extended RESTRICT-PHONE-MGMT
 permit tcp host 10.1.1.100 10.10.10.0 0.0.0.255 eq 443
 permit tcp host 10.1.1.100 10.10.10.0 0.0.0.255 eq 80
 deny tcp any 10.10.10.0 0.0.0.255 eq 443
 deny tcp any 10.10.10.0 0.0.0.255 eq 80
 permit ip any any