CVE-2025-20176: Cisco IOS SNMP DoS Vulnerability

CVE-2025-20176 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 is due to improper error handling when parsing SNMP requests. An attacker could exploit this vulnerability by sending a crafted SNMP request to an affected device. A successful exploit could allow the attacker to cause the device to reload unexpectedly, resulting in a DoS condition.

This 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 remote attackers can cause network device reloads, resulting in service disruption and potential network outages affecting critical infrastructure.

Affected Products

• Cisco IOS Software (multiple versions from 15.0 through 15.9)
• Cisco IOS XE Software (versions 3.3.x through 17.16.x)
• Network devices running vulnerable SNMP configurations

Discovery Timeline

• February 5, 2025 - CVE-2025-20176 published to NVD
• July 3, 2025 - Last updated in NVD database

Technical Details for CVE-2025-20176

Vulnerability Analysis

This vulnerability (CWE-248: Uncaught Exception) exists in the SNMP subsystem's request parsing logic within Cisco IOS and IOS XE Software. The flaw occurs when the SNMP subsystem fails to properly handle certain error conditions during the parsing of incoming SNMP requests. When a malformed or specially crafted SNMP request is received, the improper error handling causes an uncaught exception that triggers an unexpected device reload.

The attack requires authentication—either knowledge of a valid SNMP community string (for SNMPv1 or v2c) or valid SNMP user credentials (for SNMPv3). This authentication requirement limits the attack surface but does not eliminate the risk, as SNMP credentials are often shared across network management systems and may be obtained through reconnaissance, configuration file exposure, or credential theft.

The network-based attack vector with low complexity means that any attacker with valid SNMP credentials and network access to the management interface can trigger the vulnerability remotely. The impact is purely to availability—there is no confidentiality or integrity impact—but the ability to cause unplanned device reloads can severely disrupt network operations.

Root Cause

The root cause is improper error handling (CWE-248: Uncaught Exception) in the SNMP request parsing code. When the parser encounters unexpected or malformed data within an SNMP request, the software fails to catch and properly handle the resulting exception. Instead of gracefully rejecting the malformed request and logging an error, the uncaught exception propagates to a level that causes the device to reload. This represents a failure to implement defensive programming practices that would ensure all error conditions are properly handled without impacting system stability.

Attack Vector

The attack is network-based and can be executed remotely by any authenticated attacker with access to the SNMP service on the target device. The attack flow involves:

1. The attacker identifies a Cisco device running vulnerable IOS or IOS XE software with SNMP enabled
2. The attacker obtains valid SNMP credentials (community string for v1/v2c or user credentials for v3)
3. The attacker crafts a malicious SNMP request designed to trigger the parsing error
4. Upon receiving the crafted request, the vulnerable SNMP subsystem fails to properly handle the parsing error
5. The uncaught exception causes the device to reload, resulting in denial of service

The vulnerability can be repeatedly exploited to cause sustained service disruption, as each malicious request will trigger a reload cycle.

Detection Methods for CVE-2025-20176

Indicators of Compromise

• Unexpected device reloads or crashinfo files indicating SNMP-related crashes
• Multiple SNMP-triggered reload events in device logs in a short time period
• Abnormal SNMP traffic patterns from unexpected source addresses
• Device crash logs referencing SNMP subsystem errors or uncaught exceptions

Detection Strategies

• Monitor syslog messages for device reload events and correlate with SNMP traffic
• Implement network-based SNMP traffic analysis to detect anomalous request patterns
• Configure SNMP trap receivers to alert on device reload events
• Review crashinfo files for SNMP-related stack traces indicating exploitation attempts

Monitoring Recommendations

• Enable comprehensive SNMP logging on all Cisco IOS and IOS XE devices
• Deploy network monitoring to track SNMP request volumes and source addresses
• Establish baseline device stability metrics to quickly identify reload anomalies
• Configure alerting for repeated device restarts within short timeframes

How to Mitigate CVE-2025-20176

Immediate Actions Required

• Review the Cisco Security Advisory for fixed software versions
• Restrict SNMP access to trusted management stations using access control lists (ACLs)
• Implement SNMPv3 with strong authentication and encryption where possible
• Audit and rotate SNMP credentials to limit exposure if credentials have been compromised

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 deployment. The advisory provides detailed guidance on determining whether devices are vulnerable and identifying the appropriate upgrade path.

Workarounds

• Implement access control lists (ACLs) to restrict SNMP access to authorized management stations only
• Disable SNMP if not required for network management operations
• Use infrastructure ACLs (iACLs) to limit SNMP access at network boundaries
• Consider network segmentation to isolate management traffic from untrusted networks

# Example ACL to restrict SNMP access (apply to appropriate interface)
access-list 10 permit 192.168.1.0 0.0.0.255
snmp-server community YOUR_COMMUNITY ro 10
snmp-server community YOUR_COMMUNITY rw 10