CVE-2017-6739: Cisco IOS and IOS XE Software SNMP Remote Code Execution Vulnerability

CVE-2017-6739 is a remote code execution vulnerability in the SNMP subsystem of Cisco IOS and IOS XE Software. An authenticated remote attacker who can reach SNMP on an affected device may execute code or force a reload, which can disrupt network operations or give an attacker a foothold on core infrastructure.

Because routers and switches running IOS/IOS XE often sit at the center of enterprise and service-provider networks, successful abuse can affect availability and open a path to further compromise. Confirm exact impact and fixed releases against the vendor advisory.

How it works

The weakness is classified as CWE-119 (improper restriction of operations within the bounds of a memory buffer). In broad terms for this class, flawed handling of SNMP-related input can corrupt memory in the SNMP process on the device.

According to the CISA summary, an authenticated, remote attacker who can interact with the SNMP subsystem may trigger the flaw to execute code on the affected system or cause it to reload. Public detail in the provided record does not describe packet formats, OID specifics, or exploit mechanics; treat any such claims as unconfirmed unless they appear in the vendor advisory. Authentication is required, so the attacker needs valid SNMP credentials (or equivalent access) and network reachability to the SNMP service.

Am I affected? How to find it in your systems

Cisco IOS and IOS XE commonly run on enterprise and service-provider routers, switches, and related network platforms. Inventory every device that might still run these operating systems, including branch, campus, data-center, and management-plane gear.

• Pull software version and feature set from each device (for example via show version or your configuration-management / NAC inventory) and compare against the fixed releases listed in the Cisco advisory for CVE-2017-6739.
• Identify whether SNMP is enabled, which versions (v1/v2c/v3) are in use, and which communities or users are configured; note any exposure of UDP/161 (and related SNMP ports) from untrusted networks.
• Search change-management and CMDB records for “IOS,” “IOS XE,” and SNMP configuration so nothing is missed in labs or out-of-band management networks.
• For exploitation signs, review device logs and SIEM data for unexpected SNMP authentication activity, SNMP-related process crashes or traceback messages, unexplained reloads, or configuration changes following SNMP access. Correlating sudden device reloads with SNMP traffic from unusual sources is a practical starting point; exact log signatures should be confirmed with Cisco guidance.

How to remediate

Patch first. Apply the updates Cisco designates for CVE-2017-6739, following the vendor’s installation and reload procedures for IOS and IOS XE. CISA’s required action is to apply updates per vendor instructions.

• Schedule maintenance windows appropriate to the device role; verify the image hash and post-upgrade SNMP and routing health.
• After patching, re-validate SNMP configuration: prefer SNMPv3 with strong authentication and encryption, remove unused communities, and restrict SNMP to management stations only.
• Harden generally for this class: keep management-plane ACLs tight, disable SNMP where it is not required, and ensure devices are not reachable for management from the internet or untrusted segments.

If you can't patch immediately

Reduce exposure until you can install the vendor fix.

• Segment and filter: restrict SNMP (typically UDP/161) with infrastructure ACLs or firewalls so only authorized management hosts can reach it; block it from user and guest VLANs and from the internet.
• Disable or limit the feature: turn off SNMP if operationally feasible, or disable unnecessary SNMP views/communities and move to SNMPv3 only.
• Virtual patching / monitoring: if you operate a network IDS/IPS or management gateway that can filter SNMP, apply vendor or community rules that address this CVE only after validating them; increase alerting on SNMP brute-force, anomalous OID access, and device reload events.
• Credential hygiene: rotate SNMP community strings and SNMPv3 credentials, and treat any shared read-write communities as high risk.
• Compensating detection: forward device syslogs and SNMP-server logs to a central SIEM and alert on crashes, unexpected reloads, and SNMP access from new sources.

If your data may have been exposed

Actively exploited remote-code-execution flaws on network devices can lead to broader intrusion and data exposure even when ransomware use is not documented for this CVE. If you suspect compromise, isolate affected devices, preserve logs and memory/crashinfo where possible, rotate credentials and keys that traversed the device, and follow your incident-response process. You can also run a free exposure scan of your email addresses against known breach data to see whether associated accounts appear in prior breaches while you complete containment and recovery.