CVE-2025-20312: Cisco IOS XE SNMP DoS Vulnerability

CVE-2025-20312 Overview

CVE-2025-20312 is a denial of service (DoS) vulnerability in the Simple Network Management Protocol (SNMP) subsystem of Cisco IOS XE Software. An authenticated remote attacker can exploit improper error handling when the device parses a specific SNMP request. A successful exploit forces the affected device to reload unexpectedly, interrupting network operations. The flaw is tracked under [CWE-835] (Loop with Unreachable Exit Condition) and affects SNMP versions 1, 2c, and 3. Cisco published the advisory on September 24, 2025.

Critical Impact

An authenticated attacker with a valid SNMP community string or SNMPv3 credentials can trigger an unexpected device reload, causing network outages on Cisco IOS XE infrastructure.

Affected Products

• Cisco IOS XE Software with SNMP enabled
• Devices configured with SNMPv1 or SNMPv2c community strings (read-only or read-write)
• Devices configured with SNMPv3 user accounts

Discovery Timeline

• 2025-09-24 - CVE-2025-20312 published to NVD
• 2026-04-15 - Last updated in NVD database

Technical Details for CVE-2025-20312

Vulnerability Analysis

The vulnerability resides in the SNMP subsystem of Cisco IOS XE Software. When the SNMP engine receives a specifically crafted request, the parsing logic fails to handle an error condition correctly. This triggers a fault that causes the device to reload, dropping all traffic and active sessions during the recovery window.

The issue is classified as [CWE-835], indicating that the affected code path enters a loop or processing state without a reachable exit condition under attacker-controlled input. The condition propagates through all SNMP protocol versions supported on the platform, expanding the exposure surface across legacy and modern deployments.

Exploitation requires valid authentication. For SNMPv1 and SNMPv2c, an attacker must possess a community string with either read-only or read-write access. For SNMPv3, valid user credentials are required. The complexity is low and no user interaction is needed, but the authentication requirement constrains opportunistic attacks to environments where credentials have leaked or weak community strings remain in use.

Root Cause

Improper error handling during SNMP request parsing allows specific malformed input to drive the SNMP process into an unrecoverable state. The device cannot gracefully recover and reloads to restore operation.

Attack Vector

The attacker sends a crafted SNMP request over the network to UDP port 161 on the affected device. Authentication is performed using a community string (SNMPv1/v2c) or user credentials (SNMPv3). On successful parsing through the vulnerable code path, the device reloads. No code execution or data exfiltration occurs, but availability is lost for the duration of the reload cycle.

The vulnerability mechanism is described in the Cisco Security Advisory. No public proof-of-concept exploit is available at this time.

Detection Methods for CVE-2025-20312

Indicators of Compromise

• Unexpected device reloads correlated with inbound SNMP traffic from non-management sources
• Crash files or core dumps in flash referencing the SNMP process after a reboot event
• SNMP requests originating from unfamiliar source addresses targeting UDP port 161

Detection Strategies

• Monitor show logging and show version output for unexpected reload reasons, particularly those referencing the SNMP subsystem
• Correlate SNMP traffic flows with device uptime metrics to identify reload events triggered shortly after specific requests
• Audit SNMP community strings and SNMPv3 user accounts for unauthorized or unused entries that could serve as attack credentials

Monitoring Recommendations

• Enable syslog forwarding from network devices to a centralized collector to capture reload events in real time
• Apply NetFlow or IPFIX analysis to baseline normal SNMP polling patterns and flag deviations
• Alert on repeated SNMP requests from non-NMS sources or on SNMP authentication failures preceding reload events

How to Mitigate CVE-2025-20312

Immediate Actions Required

• Apply the fixed Cisco IOS XE Software release referenced in the Cisco Security Advisory
• Rotate all SNMP community strings and SNMPv3 credentials, especially on devices exposed beyond dedicated management networks
• Restrict SNMP access to trusted management hosts using access control lists (ACLs) and SNMP views

Patch Information

Cisco has released fixed software versions addressing CVE-2025-20312. Refer to the Cisco Security Advisory for the specific fixed releases applicable to each IOS XE train and platform. Upgrade affected devices during a planned maintenance window to avoid additional service disruption.

Workarounds

• Limit SNMP access using ACLs that permit only authorized network management stations to reach UDP port 161 on the device
• Disable SNMP entirely on devices that do not require it with no snmp-server until patched
• Use SNMPv3 with strong authentication and privacy settings and remove legacy SNMPv1 and SNMPv2c community strings where possible

# Example ACL restricting SNMP to a dedicated management subnet
ip access-list standard SNMP-ACL
 permit 10.10.0.0 0.0.0.255
 deny   any log
!
snmp-server community <STRONG_STRING> RO SNMP-ACL
! Remove unused community strings and legacy versions
no snmp-server community public
no snmp-server community private