CVE-2024-20451: Cisco SPA 301 Firmware DoS Vulnerability

CVE-2024-20451 Overview

CVE-2024-20451 affects the web-based management interface of Cisco Small Business SPA300 Series and SPA500 Series IP Phones. An unauthenticated, remote attacker can send a crafted HTTP packet to the device and force an unexpected reload. The flaw exists because HTTP packets are not properly checked for errors, resulting in a denial-of-service (DoS) condition on the affected hardware. The weakness is classified under [CWE-120] (Buffer Copy without Checking Size of Input).

Critical Impact
A single crafted HTTP request from a network-adjacent or remote attacker can reload an affected IP phone, disrupting voice communications across SPA300 and SPA500 deployments.

Affected Products

Cisco Small Business SPA300 Series IP Phones (SPA301, SPA303)
Cisco Small Business SPA500 Series IP Phones (SPA501G, SPA502G, SPA504G, SPA508G, SPA509G)
Cisco Small Business SPA500 Series IP Phones (SPA512G, SPA514G, SPA525G, SPA525G2)

Discovery Timeline

2024-08-07 - CVE-2024-20451 published to NVD
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2024-20451

Vulnerability Analysis

The vulnerability resides in the HTTP processing logic of the web-based management interface exposed by SPA300 and SPA500 Series IP Phones. The interface does not perform sufficient error checking on incoming HTTP packets. When the device receives a malformed or otherwise unexpected HTTP request, the parser fails in a way that triggers an unexpected reload of the phone.

Because the affected devices are voice endpoints, any reload interrupts active calls and disrupts call registration with the SIP infrastructure. Repeated exploitation can keep a target phone offline for as long as the attacker can deliver crafted packets to its management interface.

Cisco has confirmed the issue in the security advisory and indicated that the SPA300 and SPA500 Series have reached end-of-life status, so a software fix is not planned. Operators must rely on network controls and device replacement to mitigate the risk.

Root Cause

The root cause is improper validation of HTTP input within the management interface, aligned with [CWE-120] buffer-copy weaknesses. The HTTP request handler trusts attacker-controlled fields without enforcing length or format constraints, causing a fault condition that the device handles by restarting.

Attack Vector

Exploitation requires only network reachability to the phone's management interface. No authentication and no user interaction are needed. An attacker sends a crafted HTTP packet to the device, triggering the reload. The vulnerability impacts availability only; confidentiality and integrity are not affected. Refer to the Cisco Security Advisory for the official technical description.

Detection Methods for CVE-2024-20451

Indicators of Compromise

Unexpected reboots or registration drops on SPA300 or SPA500 Series IP phones with no corresponding administrative action.
Inbound HTTP or HTTPS traffic to phone management interfaces from unexpected sources, including hosts outside the voice VLAN.
Spikes in SIP re-registration events from phone subnets that correlate in time with HTTP requests to the device management port.

Detection Strategies

Capture and inspect HTTP traffic destined for IP phone management interfaces, looking for malformed requests, unusual headers, or oversized fields.
Correlate phone uptime and reboot logs with network flow data to detect patterns where reloads follow inbound HTTP packets.
Alert when any host outside the management network initiates HTTP sessions to phone IP addresses.

Monitoring Recommendations

Forward syslog and SNMP trap data from voice infrastructure to a centralized log platform for trend analysis on phone reboot counts.
Monitor call manager registration churn per device and threshold-alert on phones that repeatedly drop and re-register.
Track HTTP request volumes to phone management ports and baseline normal administrative activity to surface anomalies.

How to Mitigate CVE-2024-20451

Immediate Actions Required

Restrict access to the web-based management interface of SPA300 and SPA500 phones to a dedicated management VLAN with strict access control lists.
Block inbound HTTP and HTTPS traffic to phone IP addresses from user, guest, and untrusted network segments.
Inventory all SPA300 and SPA500 Series devices and plan replacement, since these models are end-of-life and will not receive a fix.

Patch Information

Cisco has not released and does not plan to release software updates that address this vulnerability for the affected SPA300 and SPA500 Series IP Phones, as these products have entered the end-of-life process. Customers are advised to migrate to supported Cisco IP phone platforms. Consult the Cisco Security Advisory cisco-sa-spa-http-vulns-RJZmX2Xz for migration guidance.

Workarounds

Disable the web-based management interface on affected phones where the deployment workflow allows configuration to be managed via provisioning files only.
Enforce firewall rules at the access layer that permit management traffic to phones only from authorized administrator workstations.
Place IP phones on an isolated voice VLAN that prevents lateral HTTP traffic from endpoints sharing the data network.

bash

# Example access control list restricting HTTP/HTTPS to phone management interface
# Replace 10.10.20.0/24 with your phone subnet and 10.10.99.10 with your admin host
access-list 150 permit tcp host 10.10.99.10 10.10.20.0 0.0.0.255 eq 80
access-list 150 permit tcp host 10.10.99.10 10.10.20.0 0.0.0.255 eq 443
access-list 150 deny   tcp any 10.10.20.0 0.0.0.255 eq 80
access-list 150 deny   tcp any 10.10.20.0 0.0.0.255 eq 443
access-list 150 permit ip any any