CVE-2026-20185 Overview
CVE-2026-20185 is a denial of service vulnerability in the Simple Network Management Protocol (SNMP) subsystem of Cisco 350 Series Managed Switches (SG350) and Cisco 350X Series Stackable Managed Switches (SG350X) firmware. An authenticated, remote attacker can send a crafted SNMP request that triggers improper error handling when the device parses response data. Successful exploitation forces the affected switch to reload unexpectedly, producing a denial of service condition. The vulnerability affects SNMP versions 1, 2c, and 3, and is tracked under [CWE-122] (Heap-based Buffer Overflow).
Critical Impact
A single malformed SNMP request from an authenticated user can reload the switch, disrupting all network traffic traversing the device until it returns to service.
Affected Products
- Cisco 350 Series Managed Switches (SG350) firmware
- Cisco 350X Series Stackable Managed Switches (SG350X) firmware
- Devices configured with SNMPv1, SNMPv2c, or SNMPv3
Discovery Timeline
- 2026-05-06 - CVE-2026-20185 published to NVD
- 2026-05-06 - Last updated in NVD database
Technical Details for CVE-2026-20185
Vulnerability Analysis
The flaw resides in the SNMP subsystem of SG350 and SG350X firmware. When the device processes a specific SNMP request, the response data parsing routine fails to handle error conditions correctly. This improper error handling corresponds to a heap-based memory safety issue ([CWE-122]) and causes the SNMP process to terminate abnormally. Because the SNMP subsystem is integral to switch management, the failure cascades into a full device reload. All switching and routing functions stop until the device finishes rebooting, which interrupts every connected workload.
Root Cause
The root cause is improper error handling during the parsing of response data for a specific SNMP request. The parser does not validate or contain malformed input safely, leading to memory corruption on the heap. The condition is reachable across SNMPv1, SNMPv2c, and SNMPv3, indicating the defect lies in shared response-handling logic rather than a version-specific code path.
Attack Vector
The attacker must be authenticated to the SNMP service. For SNMPv1 or SNMPv2c, exploitation requires knowledge of a valid read-only or read-write community string. For SNMPv3, the attacker must possess valid SNMP user credentials. Once authenticated, the attacker sends the specific SNMP request over the network to trigger the parsing fault. No user interaction on the target is required, and the impact crosses a security boundary, affecting traffic and services beyond the SNMP subsystem itself.
No public proof-of-concept code or in-the-wild exploitation has been reported. Refer to the Cisco Security Advisory for vendor-confirmed technical details.
Detection Methods for CVE-2026-20185
Indicators of Compromise
- Unexpected reloads of SG350 or SG350X switches with no scheduled maintenance window
- Crash logs or show reload history entries referencing the SNMP process
- SNMP authentication events from unusual source IPs immediately preceding a device reboot
- Gaps in SNMP polling data correlated with device unavailability
Detection Strategies
- Correlate device uptime resets with preceding SNMP traffic from non-management subnets
- Alert on SNMP requests from sources outside the approved network management station (NMS) allowlist
- Monitor syslog for SNMP engine errors, agent restarts, or unexpected %SYS-5-RELOAD messages on affected models
Monitoring Recommendations
- Forward switch syslog and SNMP trap data to a centralized SIEM for cross-device correlation
- Track baseline SNMP query volumes and flag anomalous bursts targeting SG350/SG350X devices
- Audit SNMP community string and SNMPv3 user usage to identify credential reuse or compromise
How to Mitigate CVE-2026-20185
Immediate Actions Required
- Apply the fixed firmware identified in the Cisco Security Advisory to all SG350 and SG350X devices
- Restrict SNMP access using infrastructure access control lists (iACLs) so only authorized NMS hosts can reach UDP/161
- Rotate SNMP community strings and SNMPv3 credentials, especially on devices with shared or legacy credentials
- Disable SNMP entirely on switches that do not require it for monitoring
Patch Information
Cisco has published fixed software releases for the SG350 and SG350X product lines. Administrators should consult the Cisco Security Advisory for version-specific upgrade guidance and to confirm the target release for each affected platform.
Workarounds
- Apply SNMP views that limit accessible Object Identifiers (OIDs) to only those required by the NMS
- Enforce SNMPv3 with authPriv to require authentication and encryption, eliminating plaintext community access
- Place affected switches on a dedicated, segmented management VLAN with strict ingress filtering
- Use snmp-server community ACL bindings to restrict source addresses permitted to query the agent
# Configuration example: restrict SNMP to a single NMS host
ip access-list extended SNMP-NMS-ONLY
permit udp host 10.10.10.5 any eq 161
deny udp any any eq 161 log
permit ip any any
!
snmp-server community <REDACTED-RO> RO SNMP-NMS-ONLY
snmp-server group ADMINS v3 priv
snmp-server user nmsuser ADMINS v3 auth sha <auth-pass> priv aes 128 <priv-pass>
!
line vty 0 15
access-class SNMP-NMS-ONLY in
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
