CVE-2025-20173: Cisco IOS SNMP DoS Vulnerability

CVE-2025-20173 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

Successful exploitation causes network devices to reload unexpectedly, disrupting network connectivity and potentially affecting critical infrastructure services dependent on Cisco routing and switching equipment.

Affected Products

• Cisco IOS Software (multiple versions from 12.2 through 15.9)
• Cisco IOS XE Software (versions 3.2.x through 17.15.x)
• Network devices running vulnerable SNMP configurations

Discovery Timeline

• February 5, 2025 - CVE-2025-20173 published to NVD
• July 3, 2025 - Last updated in NVD database

Technical Details for CVE-2025-20173

Vulnerability Analysis

This vulnerability resides in the SNMP subsystem's request parsing functionality within Cisco IOS and IOS XE software. The flaw stems from improper error handling mechanisms that fail to properly validate and process malformed SNMP requests. When the SNMP subsystem encounters a specially crafted request, the inadequate error handling causes the device to enter an unstable state, ultimately triggering an unexpected device reload.

The vulnerability affects all three major versions of the SNMP protocol (v1, v2c, and v3), though exploitation requires different prerequisites depending on the protocol version. For SNMPv1 and SNMPv2c, an attacker needs knowledge of a valid community string (either read-only or read-write). For SNMPv3, valid user credentials are required. This authentication requirement provides some mitigation, as it requires the attacker to have some level of legitimate or previously compromised access to the SNMP service.

The impact extends across the changed scope boundary, meaning successful exploitation affects resources beyond the vulnerable component's security scope. While no confidentiality or integrity impact exists, the availability impact is significant as it results in complete service disruption until the device completes its reload cycle.

Root Cause

The root cause of this vulnerability is classified as CWE-248: Uncaught Exception. The SNMP parsing code fails to properly catch and handle exceptions that occur when processing malformed or unexpected request structures. Instead of gracefully handling the error and rejecting the malformed request, the unhandled exception propagates through the system, causing a critical failure that triggers the device's watchdog mechanism to initiate a reload.

Attack Vector

The attack vector is network-based, requiring the attacker to have network connectivity to the target device's SNMP service (typically UDP port 161). The attack requires low privilege (valid SNMP credentials or community string) and no user interaction. The attack complexity is low once the prerequisite credentials are obtained.

An attacker would craft a specially formatted SNMP request designed to trigger the parsing error. This malformed request, when processed by the vulnerable SNMP subsystem, causes the exception handling failure that leads to device reload. The attack can be repeated to maintain a persistent denial of service condition.

The vulnerability mechanism can be described as follows: when the SNMP daemon receives a request, it passes the data to parsing routines that interpret the SNMP protocol data units (PDUs). A crafted PDU with unexpected values or structures causes the parser to encounter an error condition that is not properly handled, resulting in an uncaught exception that destabilizes the device. For detailed technical information, refer to the Cisco Security Advisory.

Detection Methods for CVE-2025-20173

Indicators of Compromise

• Unexpected device reloads with crash logs showing SNMP-related exceptions or memory access violations
• System logs containing SNMP subsystem errors immediately preceding device restarts
• Unusual SNMP traffic patterns or malformed SNMP packets in network captures targeting UDP port 161
• Multiple consecutive device reloads within short time periods indicating potential attack persistence

Detection Strategies

• Monitor syslog messages for SNMP-related errors and exception conditions on Cisco IOS/IOS XE devices
• Configure SNMP traps to alert on device reload events and correlate with SNMP access logs
• Implement network intrusion detection rules to identify malformed SNMP packets targeting affected devices
• Review crash dump files and traceback information for patterns consistent with SNMP parsing failures

Monitoring Recommendations

• Enable enhanced logging for SNMP operations to capture request details that precede any failures
• Deploy network monitoring to track SNMP traffic volume and source addresses accessing network infrastructure
• Establish baseline device uptime metrics and alert on deviations that may indicate exploitation attempts
• Configure automated alerting for device reload events, particularly those occurring outside maintenance windows

How to Mitigate CVE-2025-20173

Immediate Actions Required

• Review and verify SNMP configurations on all Cisco IOS and IOS XE devices to identify exposure
• Restrict SNMP access to trusted management networks using access control lists (ACLs)
• Rotate SNMP community strings and SNMPv3 credentials as a precautionary measure if compromise is suspected
• Prioritize patching for internet-facing or critical infrastructure devices

Patch Information

Cisco has released software updates that address this vulnerability. Customers are advised to consult the Cisco Security Advisory for specific fixed software versions applicable to their deployment. The advisory contains detailed information about fixed releases for both Cisco IOS and IOS XE software trains.

Workarounds

• Disable SNMP if it is not required for network management operations
• Implement strict ACLs to limit SNMP access to only authorized management stations
• Use SNMPv3 with strong authentication and encryption to reduce the attack surface

# Example: Restrict SNMP access to trusted management subnet
access-list 10 permit 10.0.0.0 0.0.0.255
snmp-server community your-community-string RO 10
snmp-server host 10.0.0.100 traps your-community-string

# Example: Disable SNMP if not required
no snmp-server