CVE-2023-20259: Cisco Emergency Responder DoS Vulnerability

CVE-2023-20259 Overview

A vulnerability in an API endpoint of multiple Cisco Unified Communications Products could allow an unauthenticated, remote attacker to cause high CPU utilization, which could impact access to the web-based management interface and cause delays with call processing. This API is not used for device management and is unlikely to be used in normal operations of the device.

This vulnerability is due to improper API authentication and incomplete validation of the API request. An attacker could exploit this vulnerability by sending a crafted HTTP request to a specific API on the device. A successful exploit could allow the attacker to cause a denial of service (DoS) condition due to high CPU utilization, which could negatively impact user traffic and management access. When the attack stops, the device will recover without manual intervention.

Critical Impact
Unauthenticated remote attackers can cause denial of service conditions affecting call processing and management access across critical enterprise communications infrastructure.

Affected Products

Cisco Emergency Responder 14su3
Cisco Prime Collaboration Deployment 14su3
Cisco Unified Communications Manager 12.5(1)su7 and 14su3
Cisco Unified Communications Manager IM & Presence Service 12.5(1)su7 and 14su3
Cisco Unity Connection 14su3

Discovery Timeline

October 4, 2023 - CVE-2023-20259 published to NVD
November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2023-20259

Vulnerability Analysis

CVE-2023-20259 is classified as a Resource Exhaustion vulnerability (CWE-400) affecting the API endpoint of multiple Cisco Unified Communications products. The vulnerability allows unauthenticated remote attackers to trigger high CPU utilization by sending specially crafted HTTP requests to a vulnerable API endpoint.

The attack is network-based and requires no privileges or user interaction, making it particularly dangerous in enterprise environments where Cisco Unified Communications products are often internet-facing or accessible from broad network segments. While the vulnerability does not compromise confidentiality or integrity, the availability impact is significant—affecting both user traffic (call processing) and administrative access to the web-based management interface.

Notably, the affected API is not used for device management and is unlikely to be utilized during normal device operations, which may reduce the likelihood of detection through routine monitoring. The device will recover automatically once the attack ceases, indicating that the DoS condition is transient and does not cause permanent damage to the system.

Root Cause

The root cause of this vulnerability stems from two related issues: improper API authentication and incomplete validation of API requests. The affected API endpoint fails to properly authenticate incoming requests, allowing unauthenticated users to interact with the API. Additionally, the API does not adequately validate request parameters, enabling malformed or specially crafted requests to consume excessive CPU resources.

This combination of authentication bypass and input validation flaws creates conditions where an attacker can repeatedly submit resource-intensive requests without authorization, leading to CPU exhaustion and subsequent denial of service.

Attack Vector

The attack vector for CVE-2023-20259 is network-based, requiring the attacker to have network access to the vulnerable Cisco Unified Communications product. The exploitation process involves:

Reconnaissance: The attacker identifies a target running a vulnerable version of Cisco Unified Communications Manager, Emergency Responder, Prime Collaboration Deployment, IM & Presence Service, or Unity Connection.
Crafted Request Submission: The attacker sends specially crafted HTTP requests to the specific vulnerable API endpoint. These requests are designed to trigger intensive processing operations on the server.
Resource Exhaustion: The device processes these malformed requests, consuming excessive CPU resources. As CPU utilization increases, legitimate users experience degraded service quality.
Service Impact: The denial of service manifests as delays in call processing and reduced accessibility to the web-based management interface, impacting both end-users and administrators.

The attack does not require authentication, making it accessible to any attacker with network connectivity to the target system. For detailed technical information about the specific API endpoint and request format, refer to the Cisco Security Advisory.

Detection Methods for CVE-2023-20259

Indicators of Compromise

Unusual spikes in CPU utilization on Cisco Unified Communications products without corresponding legitimate traffic increases
Multiple HTTP requests to uncommonly used API endpoints from single or distributed source IP addresses
User reports of call processing delays or dropped connections
Administrators experiencing slow or unresponsive web-based management interfaces
Abnormal HTTP request patterns in access logs targeting specific API endpoints

Detection Strategies

Implement network-based intrusion detection rules to identify high-volume HTTP requests targeting Cisco UC API endpoints
Configure CPU utilization thresholds with alerting on Cisco Unified Communications appliances to detect resource exhaustion attacks
Monitor web server access logs for anomalous request patterns, particularly to non-standard or administrative API endpoints
Deploy behavioral analysis tools to baseline normal API usage and alert on deviations

Monitoring Recommendations

Enable SNMP monitoring for CPU and memory utilization metrics on all affected Cisco UC products
Implement centralized logging with real-time correlation to identify attack patterns across multiple systems
Configure alerting for web-based management interface availability degradation
Review access logs periodically for unauthorized API access attempts from unexpected source addresses

How to Mitigate CVE-2023-20259

Immediate Actions Required

Review the Cisco Security Advisory for specific fixed software versions and upgrade guidance
Identify all instances of affected Cisco Unified Communications products in your environment and determine their current software versions
Prioritize patching for internet-facing or externally accessible Cisco UC deployments
Implement network access controls to restrict API endpoint access to authorized management networks only

Patch Information

Cisco has released software updates that address this vulnerability. Organizations should consult the Cisco Security Advisory (cisco-sa-cucm-apidos-PGsDcdNF) for detailed information on fixed software releases for each affected product. The advisory provides specific version information for:

Cisco Emergency Responder
Cisco Prime Collaboration Deployment
Cisco Unified Communications Manager
Cisco Unified Communications Manager IM & Presence Service
Cisco Unity Connection

Workarounds

Implement firewall rules or access control lists to restrict access to the vulnerable API endpoint from untrusted networks
Deploy a web application firewall (WAF) or reverse proxy to filter and rate-limit requests to Cisco UC API endpoints
Segment Cisco Unified Communications infrastructure on isolated network segments with strict ingress/egress controls
Monitor and implement IP-based blocking for source addresses exhibiting attack patterns

bash

# Example ACL configuration to restrict API access (adapt to your environment)
# Restrict HTTP/HTTPS access to management interfaces from trusted networks only
access-list CUCM_MGMT_ACL permit tcp 10.0.0.0 0.255.255.255 host 192.168.1.100 eq 443
access-list CUCM_MGMT_ACL deny tcp any host 192.168.1.100 eq 443
access-list CUCM_MGMT_ACL deny tcp any host 192.168.1.100 eq 80