CVE-2025-20350: Cisco Desk Phone 9871 DOS Vulnerability

CVE-2025-20350 Overview

A stack-based buffer overflow vulnerability exists in the web UI of Cisco Desk Phone 9800 Series, Cisco IP Phone 7800 and 8800 Series, and Cisco Video Phone 8875 running Cisco SIP Software. This vulnerability could allow an unauthenticated, remote attacker to cause a Denial of Service (DoS) condition on affected devices by sending specially crafted HTTP packets.

The vulnerability arises from improper handling of HTTP packets by the device's web interface. When the affected device processes maliciously crafted HTTP input, it triggers a buffer overflow that causes the device to reload, resulting in service disruption. This vulnerability affects enterprise VoIP infrastructure and could impact business communications if exploited.

Critical Impact

Unauthenticated remote attackers can force affected Cisco IP phones to reload, causing communication disruption across enterprise VoIP deployments.

Affected Products

• Cisco Desk Phone 9800 Series (9841, 9851, 9861, 9871)
• Cisco IP Phone 7800 Series (7811, 7821, 7841, 7861)
• Cisco IP Phone 8800 Series (8811, 8821, 8832, 8841, 8845, 8851, 8861, 8865)
• Cisco Video Phone 8875

Discovery Timeline

• October 15, 2025 - CVE-2025-20350 published to NVD
• December 4, 2025 - Last updated in NVD database

Technical Details for CVE-2025-20350

Vulnerability Analysis

This vulnerability is classified as CWE-121 (Stack-based Buffer Overflow), a memory corruption issue where data written to a buffer exceeds its allocated size on the stack. When the web UI component of affected Cisco phones processes HTTP packets, insufficient bounds checking allows an attacker to write beyond the allocated buffer space.

The exploitation requires that the phone be registered to Cisco Unified Communications Manager and have Web Access enabled. While Web Access is disabled by default, organizations that enable this feature for remote administration or monitoring purposes expose the vulnerability surface.

A successful exploit causes the affected device to reload, temporarily disrupting phone services. In enterprise environments with numerous affected devices, a coordinated attack could significantly impact business communications infrastructure.

Root Cause

The root cause of CVE-2025-20350 is inadequate input validation and boundary checking in the HTTP packet processing code within the Cisco SIP Software web UI. When processing incoming HTTP requests, the software fails to properly validate the size of input data before copying it into a fixed-size stack buffer. This allows an attacker to overflow the buffer by sending oversized or malformed HTTP data, corrupting adjacent stack memory and causing the device to crash.

Attack Vector

The attack vector for this vulnerability is network-based, requiring no authentication or user interaction. An attacker with network access to an affected device's web interface can exploit this vulnerability by:

1. Identifying Cisco IP phones with Web Access enabled on the network
2. Crafting malicious HTTP requests designed to trigger the buffer overflow
3. Sending the crafted packets to the target device's web UI
4. Causing the device to reload and temporarily lose service

The attack can be executed remotely from anywhere on the network that can reach the phone's web interface. Since no authentication is required, any network-adjacent attacker or an attacker who has compromised the internal network can exploit this vulnerability.

Detection Methods for CVE-2025-20350

Indicators of Compromise

• Unexpected phone reboots or service interruptions without administrative action
• Unusual HTTP traffic patterns targeting Cisco IP phone web interfaces
• Multiple connection attempts to phone web UI ports from unexpected sources
• Crash logs on affected devices indicating buffer overflow conditions

Detection Strategies

• Monitor network traffic for anomalous HTTP requests to Cisco IP phone web interfaces (typically port 80 or 443)
• Implement intrusion detection rules to identify oversized or malformed HTTP packets targeting phone endpoints
• Configure alerts for unexpected device reboots within Cisco Unified Communications Manager
• Deploy network segmentation monitoring to detect unauthorized access attempts to voice VLAN segments

Monitoring Recommendations

• Enable logging on Cisco Unified Communications Manager to track phone registration events and unexpected reloads
• Implement network flow analysis to baseline normal HTTP traffic to IP phones and alert on deviations
• Configure SNMP traps for phone reboot events across the enterprise
• Review access logs on network firewalls for connections to phone web interfaces from unauthorized sources

How to Mitigate CVE-2025-20350

Immediate Actions Required

• Disable Web Access on affected Cisco IP phones if not required for business operations
• Implement network access controls to restrict access to phone web interfaces to authorized management systems only
• Segment voice networks from general user networks to limit attack surface
• Apply firmware updates from Cisco as they become available

Patch Information

Cisco has released a security advisory addressing this vulnerability. Organizations should consult the Cisco Security Advisory for specific firmware versions that address CVE-2025-20350. Contact Cisco TAC or your Cisco support representative for guidance on obtaining and deploying the appropriate firmware updates for your specific phone models.

Workarounds

• Disable Web Access on all affected devices via Cisco Unified Communications Manager phone configuration
• Implement firewall rules blocking external access to IP phone web interfaces
• Deploy network access control lists (ACLs) limiting web UI access to trusted management subnets only
• Consider using out-of-band management networks for phone administration where possible

# Example: Verify Web Access status on Cisco IP Phone via CLI
# Access the phone's admin page and navigate to:
# Settings > Security Configuration > Web Access
# Set Web Access to "Disabled" if not required

# For Cisco Unified Communications Manager bulk configuration:
# Navigate to Device > Phone > Find and List Phones
# Select affected devices and modify Web Access setting to "Disabled"