CVE-2017-6736

What Is Cisco IOS SNMP?

Simple Network Management Protocol (SNMP) is the standard protocol for network device monitoring and management, implemented in all Cisco IOS and IOS XE devices. SNMP uses community strings (version 1/2c) or user credentials (version 3) for authentication. In June 2017, Cisco disclosed a batch of buffer overflow vulnerabilities in the IOS SNMP subsystem — each affecting a different SNMP Object Identifier (OID) handler — collectively addressed by advisory cisco-sa-20170629-snmp. CVE-2017-6736 is one of multiple distinct CVEs in this advisory, each representing a separate OID-specific buffer overflow that can allow code execution or device reload.

Overview

CVE-2017-6736 is one of multiple buffer overflow vulnerabilities in the SNMP subsystem of Cisco IOS and IOS XE disclosed in June 2017 advisory cisco-sa-20170629-snmp. The vulnerability affects a specific SNMP OID handler and can allow an attacker with a valid SNMP community string (v1/v2c) or user credentials (v3) to execute arbitrary code or cause a device reload. Related CVEs in the same advisory include CVE-2017-6737, 6738, 6739, 6740, 6742, 6743, and 6744, each affecting different OID handlers. CISA added CVE-2017-6736 to the KEV catalog in March 2022 reflecting nation-state exploitation of Cisco infrastructure vulnerabilities.

Affected Versions

Cisco IOS and IOS XE with SNMP enabled and using vulnerable OID handlers. Use the Cisco IOS Software Checker and advisory cisco-sa-20170629-snmp for specific version identification. All SNMP-enabled IOS versions before the patched releases in the advisory are potentially affected.

Technical Details

Root Cause: Buffer Overflow in SNMP OID Handler

CVE-2017-6736 is a buffer overflow vulnerability (CWE-119) in the Cisco IOS SNMP subsystem. The SNMP implementation processes incoming SNMP GET, SET, and other requests that reference specific OIDs. The handler for the OID associated with CVE-2017-6736 fails to validate the size of incoming SNMP data before copying it into a fixed-size buffer, enabling overflow of the buffer and potential control of instruction execution.

Privilege requirement (PR:L): Exploitation requires a valid SNMP community string (in SNMPv1/v2c) or SNMP v3 user credentials. SNMP community strings are often set to default values ("public", "private") or are weak and easily guessed — lowering the effective barrier for exploitation in practice.

The full SNMP advisory (cisco-sa-20170629-snmp): Cisco disclosed a total of 8+ distinct SNMP buffer overflow CVEs (6736-6744) in the same June 2017 advisory, each affecting a different OID. Organizations must apply the advisory patch to address all CVEs simultaneously — partial patching of individual OIDs is not possible.

Exploitation Context

• Nation-state network infrastructure targeting: The Cisco IOS SNMP vulnerabilities represent a class of attack specifically interesting to intelligence agencies — SNMP is a management protocol that gives insight into device state, routing tables, and configuration; buffer overflows in SNMP handlers provide persistent access to network infrastructure
• Default SNMP community strings: Many enterprise networks still use default SNMP community strings ("public", "private"); combined with SNMP buffer overflows, default credentials eliminate the authentication barrier
• CIA and NSA tooling: Intelligence agency toolkits have historically incorporated SNMP-based exploitation of network devices; the CISA KEV addition reflects government awareness of continued nation-state use of these vectors
• CISA KEV (2022): Added March 3, 2022 alongside the other Cisco IOS SNMP CVEs from the same advisory

Remediation

1. Apply Cisco IOS security update for cisco-sa-20170629-snmp — this single advisory patch addresses all SNMP CVEs (6736-6744); apply the appropriate IOS/IOS XE version per the advisory.

2. Disable SNMP if not required — if SNMP monitoring is not in active use, disable the SNMP service (no snmp-server) to eliminate the entire attack surface.

3. Use SNMPv3 with authentication and encryption — replace SNMPv1/v2c community strings with SNMPv3 users using authpriv security level; SNMPv3 with unique, complex credentials significantly raises the exploitation bar.

4. Apply SNMP ACLs — restrict SNMP access to specific trusted management hosts:

   snmp-server community <string> RO <acl>
   

5. Change all default SNMP community strings — replace "public" and "private" community strings immediately; default strings make PR:L effectively equivalent to PR:N.