CVE-2026-20124: Cisco IOS XE SNMP DoS Vulnerability

CVE-2026-20124 Overview

CVE-2026-20124 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 send a malformed SNMP request to trigger an unexpected device reload. The flaw affects SNMP versions 1, 2c, and 3 due to improper error handling when parsing SNMP requests. The vulnerability maps to [CWE-772] (Missing Release of Resource after Effective Lifetime).

Critical Impact
A single malformed SNMP request from an authenticated user can force an affected Cisco IOS XE device to reload, disrupting network availability across the routed environment.

Affected Products

Cisco IOS XE Software (all versions supporting SNMPv1, v2c, and v3)
Devices with SNMP read-only or read-write community strings configured
Devices configured with SNMPv3 user credentials

Discovery Timeline

2026-08-05 - CVE-2026-20124 published to NVD
2026-08-05 - Last updated in NVD database

Technical Details for CVE-2026-20124

Vulnerability Analysis

The vulnerability resides in the SNMP request parsing logic of Cisco IOS XE Software. When the SNMP subsystem receives a malformed SNMP protocol data unit (PDU), the error-handling path fails to properly release allocated resources or recover cleanly. This condition causes the affected device to reload, interrupting all traffic and services running on the platform.

The flaw affects all three SNMP versions supported by IOS XE. SNMPv1 and SNMPv2c rely on community strings for access control, while SNMPv3 uses user-based authentication. In each case, an attacker must possess valid credentials — either a community string or SNMPv3 user credentials — to reach the vulnerable code path. The scope change reflected in the CVSS vector indicates that exploitation impacts availability beyond the SNMP process itself, resulting in a full device restart.

Root Cause

The root cause is improper error handling during SNMP request parsing, aligned with [CWE-772]. When malformed input triggers an error condition, the affected code does not release internal resources or return gracefully. Instead, the fault propagates and causes the device to reload as a protective measure.

Attack Vector

Exploitation requires network access to an SNMP-enabled interface on the target device and valid SNMP credentials. An attacker sends a crafted, malformed SNMP request over UDP to the SNMP listener. Because the attacker only needs read-only community string privileges or basic SNMPv3 user credentials, low-privileged accounts with SNMP polling access are sufficient to cause outage. The attack does not require user interaction on the device.

No public proof-of-concept code has been released. Refer to the Cisco Security Advisory for authoritative technical details.

Detection Methods for CVE-2026-20124

Indicators of Compromise

Unexpected device reloads on Cisco IOS XE platforms with SNMP enabled, accompanied by crashinfo or traceback files referencing the SNMP process.
SNMP traffic from unexpected source IPs immediately preceding a reload event, particularly requests with unusual PDU structure or oversized varbind lists.
Repeated SNMP authentication events from a single source followed by device unavailability.

Detection Strategies

Correlate device syslog reload messages (%SYS-5-RELOAD, %SYS-2-CRASHED) with prior SNMP request logs from the same time window.
Inspect SNMP polling traffic on UDP port 161 for malformed PDUs using packet capture or network detection tooling.
Alert on SNMP authentication activity originating from hosts outside the approved network management subnet.

Monitoring Recommendations

Forward IOS XE syslog and SNMP trap data to a centralized log platform and build detections for reload events adjacent to SNMP traffic.
Monitor device availability metrics and NMS polling gaps to identify DoS attempts against multiple devices in sequence.
Track the source addresses of all SNMP GET, GETNEXT, GETBULK, and SET operations to establish a baseline of authorized pollers.

How to Mitigate CVE-2026-20124

Immediate Actions Required

Apply the fixed Cisco IOS XE Software release identified in the Cisco Security Advisory.
Rotate SNMPv1 and SNMPv2c community strings and reset SNMPv3 user credentials, especially where credentials may have been shared or exposed.
Restrict SNMP access to trusted management hosts using access control lists on the SNMP configuration and infrastructure ACLs at the network edge.

Patch Information

Cisco has published security advisory cisco-sa-iosxe-snmp-dos-ZAqNm4MD. Consult the advisory for the list of fixed software versions and upgrade paths applicable to each affected platform. Administrators should validate the fixed release against their current IOS XE version before scheduling maintenance.

Workarounds

Apply an SNMP access list restricting polling to specific NMS IP addresses using the snmp-server community and snmp-server group access-list options.
Disable SNMP entirely on devices that do not require it by removing all snmp-server configuration statements.
Where feasible, migrate to SNMPv3 with strong authentication and privacy, and remove SNMPv1 and v2c community strings to reduce the exposed credential surface.

bash

# Configuration example - restrict SNMP access to authorized NMS hosts
ip access-list standard SNMP-NMS-ONLY
 permit host 10.10.10.10
 permit host 10.10.10.11
 deny   any log
!
snmp-server community <REDACTED-STRING> RO SNMP-NMS-ONLY
snmp-server group SECURE-GRP v3 priv access SNMP-NMS-ONLY
!
! Verify configuration
show snmp
show access-lists SNMP-NMS-ONLY