CVE-2025-20171: Cisco IOS SNMP DoS Vulnerability

CVE-2025-20171 Overview

A vulnerability in the SNMP subsystem of Cisco IOS Software and Cisco IOS XE Software could allow an authenticated, remote attacker to cause a Denial of Service (DoS) condition on an affected device. This vulnerability stems from improper error handling when parsing SNMP requests. An attacker could exploit this vulnerability by sending a crafted SNMP request to an affected device, potentially causing the device to reload unexpectedly and resulting in a DoS condition.

The vulnerability affects SNMP versions 1, 2c, and 3. To exploit this vulnerability through SNMP v2c or earlier, the attacker must know a valid read-write or read-only SNMP community string for the affected system. To exploit this vulnerability through SNMP v3, the attacker must have valid SNMP user credentials for the affected system.

Critical Impact

Authenticated attackers can crash Cisco network devices by sending malformed SNMP requests, causing unexpected reloads and network disruption.

Affected Products

• Cisco IOS (multiple versions from 12.2 through 15.9)
• Cisco IOS XE (versions 3.2.x through 17.16.x)

Discovery Timeline

• 2025-02-05 - CVE-2025-20171 published to NVD
• 2025-07-03 - Last updated in NVD database

Technical Details for CVE-2025-20171

Vulnerability Analysis

This vulnerability exists within the Simple Network Management Protocol (SNMP) subsystem of Cisco IOS and IOS XE operating systems. When the affected devices process specially crafted SNMP requests, improper error handling in the parsing logic can lead to an unhandled exception condition (CWE-248). This causes the device to reload unexpectedly, disrupting network operations.

The attack requires network access to the SNMP service on the target device and valid authentication credentials. For SNMP v1 and v2c, this means knowing a valid community string (either read-only or read-write). For SNMP v3, the attacker needs valid user credentials. While this authentication requirement limits the attack surface, SNMP community strings are often weakly configured or use default values, potentially lowering the barrier to exploitation.

The network-accessible nature of SNMP services, combined with the potential for cross-scope impact (affecting services beyond the vulnerable component), makes this vulnerability particularly concerning for enterprise network infrastructure.

Root Cause

The root cause is improper error handling (CWE-248: Uncaught Exception) within the SNMP request parsing functionality. When the SNMP subsystem encounters malformed or specially crafted request data, it fails to properly handle the resulting exception condition, leading to an uncontrolled device reload rather than graceful error recovery.

Attack Vector

The attack is conducted remotely over the network by sending crafted SNMP requests to an affected Cisco device. The attacker must have:

1. Network connectivity to the device's SNMP service (typically UDP port 161)
2. Valid SNMP credentials (community string for v1/v2c or user credentials for v3)
3. A specially crafted SNMP request designed to trigger the parsing error

Once the malformed request is received and processed by the SNMP subsystem, the improper exception handling causes the device to reload, resulting in service disruption for all network traffic passing through or managed by that device.

Detection Methods for CVE-2025-20171

Indicators of Compromise

• Unexpected device reloads with crash dumps referencing SNMP processing functions
• Syslog entries indicating SNMP-related errors immediately before device crashes
• Abnormal SNMP traffic patterns from unexpected source IP addresses
• Multiple rapid device reload events in a short time period

Detection Strategies

• Monitor for unexpected device reloads and review crash information for SNMP-related components
• Implement SNMP access control lists (ACLs) and log all SNMP access attempts
• Configure SNMP trap destinations to alert on device reload events
• Use network intrusion detection systems (IDS) to identify malformed SNMP packets

Monitoring Recommendations

• Enable syslog logging for all SNMP events and forward to a central SIEM platform
• Set up automated alerting for device reload events across network infrastructure
• Monitor SNMP traffic volume and source addresses for anomalies
• Regularly review SNMP access logs for unauthorized access attempts

How to Mitigate CVE-2025-20171

Immediate Actions Required

• Apply Cisco security patches as soon as they become available for affected IOS/IOS XE versions
• Restrict SNMP access to trusted management networks using access control lists (ACLs)
• Change default SNMP community strings to strong, unique values
• Consider migrating from SNMP v1/v2c to SNMP v3 with authentication and encryption

Patch Information

Cisco has released security updates to address this vulnerability. Administrators should consult the Cisco Security Advisory for specific fixed software versions applicable to their deployed hardware and current IOS/IOS XE version. Use Cisco's Software Checker tool to identify the appropriate fixed release.

Workarounds

• Implement infrastructure ACLs to restrict SNMP access to authorized management stations only
• If SNMP is not required, disable the SNMP service entirely on affected devices
• Use Control Plane Policing (CoPP) to rate-limit SNMP traffic to the device
• Segment network management traffic onto a dedicated management VLAN or out-of-band network

# Example Cisco IOS ACL to restrict SNMP access
access-list 99 permit 10.0.0.0 0.0.0.255
access-list 99 deny any log

snmp-server community YOUR_SECURE_STRING RO 99
snmp-server community YOUR_SECURE_RW_STRING RW 99