Cisco IOS and IOS-XE SNMP Buffer Overflow Remote Code Execution Vulnerability (CVE-2017-6739)
Executive & SecOps Summary
SECOPS ASSESSMENTCisco IOS and IOS-XE devices with active SNMP services and exposed community strings or user credentials.
Authenticated remote attackers can trigger a buffer overflow via crafted SNMP packets, leading to system reload (DoS) or full remote code execution (RCE) with administrative privileges.
Apply official Cisco software updates, restrict SNMP access using strict Access Control Lists (ACLs), or disable SNMP services if not required.
Affected Products & Version Scope
Remediation & Live Workaround Commands
PATCH & WORKAROUND1. Official Patch & Version Upgrade Commands
Download and upgrade to the latest suggested Cisco IOS/IOS-XE release from the Cisco Software Center.
# Download the secure IOS image from TFTP server and set boot path
copy tftp: flash:
# Address or name of remote host? 192.168.1.100
# Source filename? c3900e-universalk9-mz.SPA.157-3.M9.bin
# Destination filename [c3900e-universalk9-mz.SPA.157-3.M9.bin]?
configure terminal
no boot system
boot system flash c3900e-universalk9-mz.SPA.157-3.M9.bin
end
write memory
reload2. Temporary Workaround & Hardening Configuration
If patching cannot be performed immediately, disable SNMP or restrict access to authorized Network Management Systems (NMS) only.
configure terminal
# Option A: Completely disable SNMP (Recommended)
no snmp-server
# Option B: Restrict SNMP access to a trusted IP (e.g., 10.10.10.50) and change community string
ip access-list standard SECURE_SNMP_ACL
permit 10.10.10.50
deny any log
exit
snmp-server community HardenedReadString RO SECURE_SNMP_ACL
end
write memory3. Log Audit & IoC Verification Commands
Verify if SNMP is enabled and check for unexpected system crashes or reloads.
# Check SNMP configuration and active community strings
show running-config | include snmp-server
# Check for system crash history and crashinfo files
show stacks
show flash: | include crashinfoStop Exposing VPN Ports to the Internet — Switch to Zero-Trust Access
Eliminate public-facing VPN concentrators and firewall management ports with identity-first Zero-Trust Network Access.
Technical Root Cause & Exploitation Mechanics
1. Vulnerability Overview & Root Cause
This vulnerability exists in the SNMP packet processing engine of Cisco IOS and IOS-XE software. It is caused by a buffer overflow condition (CWE-119) when parsing specifically crafted SNMP packets. The root cause is insufficient input validation of packet bounds, allowing an attacker to overwrite adjacent memory structures (stack/heap) by sending a payload that exceeds the allocated buffer size.
2. Exploitation Mechanics & Attack Scenario
To exploit this vulnerability, an attacker must possess valid credentials for the target device's SNMP service. This requires knowing the SNMP read-only community string for SNMP v1/v2c, or valid user credentials for SNMPv3. Once authenticated, the attacker transmits a crafted SNMP GET or SET request packet to UDP port 161 of the target device. When the SNMP daemon processes this malformed packet, the buffer overflow is triggered. This allows the attacker to hijack the execution flow, execute arbitrary shellcode in the context of the operating system, or crash the device.
3. Infrastructure & Business Impact
As a vulnerability listed in the CISA Known Exploited Vulnerabilities (KEV) catalog, this flaw has been actively exploited in the wild. Successful exploitation allows complete compromise of the network infrastructure. Attackers can intercept network traffic, manipulate routing tables, or pivot deeper into the internal network. Additionally, causing a device reload results in immediate network downtime, severely impacting business continuity.
Official Vendor Advisories & References
A vulnerability in the SNMP implementation of could allow an authenticated, remote attacker to cause a reload of the affected system or to remotely execute code. An attacker could exploit this vulnerability by sending a crafted SNMP packet to the affected device. The vulnerability is due to a buffer overflow in the affected code area. The vulnerability affects all versions of SNMP (versions 1, 2c, and 3). The attacker must know the SNMP read only community string (SNMP version 2c or earlier) or the user credentials (SNMPv3). An exploit could allow the attacker to execute arbitrary code and obtain full control of the system or to cause a reload of the affected system. Only traffic directed to the affected system can be used to exploit this vulnerability.
Frequently Asked SecOps Questions (CVE-2017-6739)
FAQQ1. How critical is CVE-2017-6739 and is it actively exploited in the wild?
CVE-2017-6739 is rated CVSS 8.8 (HIGH) with a 30-day EPSS exploitation probability of 10.8%. It is listed in the CISA Known Exploited Vulnerabilities (KEV) catalog, requiring immediate emergency remediation.
Q2. Which vendors and products are affected by CVE-2017-6739?
Primary impacted vendors include cisco across ios, ios-xe. Scope summary: All versions of Cisco IOS and IOS-XE software with SNMP (v1, v2c, and v3) enabled.
Q3. What temporary mitigations or workarounds exist for CVE-2017-6739 before patching?
[Mitigation] Apply official Cisco software updates, restrict SNMP access using strict Access Control Lists (ACLs), or disable SNMP services if not required. Follow the CLI remediation commands and interactive SecOps checklist on this page to restrict exposure prior to your maintenance window.
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