Cisco IOS and IOS XE SNMP Subsystem Remote Code Execution and DoS Vulnerability (CVE-2017-6736)
Executive & SecOps Summary
SECOPS ASSESSMENTAll Cisco devices running Cisco IOS and IOS XE Software with SNMP enabled and affected MIBs/OIDs not explicitly excluded
Authenticated remote attackers can execute arbitrary code with full system privileges or trigger a device reload, leading to complete infrastructure compromise or denial of service
Apply fixed software versions via Cisco IOS Software Checker, or implement workarounds including SNMP community ACLs, MIB view restrictions, or disabling SNMP
Affected Products & Version Scope
Remediation & Live Workaround Commands
PATCH & WORKAROUND1. Official Patch & Version Upgrade Commands
Administrators should verify the current software version and use the Cisco IOS Software Checker to identify and upgrade to a non-vulnerable release.
# Verify current IOS version and boot image
show version
show flash:
# Example: Download secure IOS image via TFTP and update boot path
copy tftp: flash:
# Address or name of remote host? 192.168.1.100
# Source filename? c3900e-universalk9-mz.SPA.157-3.M6.bin
# Destination filename [c3900e-universalk9-mz.SPA.157-3.M6.bin]?
configure terminal
no boot system
boot system flash c3900e-universalk9-mz.SPA.157-3.M6.bin
end
write memory
reload2. Temporary Workaround & Hardening Configuration
If patching is not immediately feasible, disable SNMP entirely or restrict SNMP access to trusted management hosts using Access Control Lists (ACLs).
configure terminal
# 1. Disable SNMP globally if not required (Most effective workaround)
no snmp-server
# 2. Define a strict ACL for SNMP access
ip access-list standard SNMP_ALLOW_ACL
permit 10.10.100.50
permit 10.10.100.51
deny any log
exit
# 3. Apply the ACL to the SNMP community string
snmp-server community SECURE_RO_STRING RO SNMP_ALLOW_ACL
end
write memory3. Log Audit & IoC Verification Commands
Inspect the device configuration and system logs to identify unauthorized SNMP configurations or unexpected system reloads.
# Check active SNMP communities and configurations
show snmp community
# Search system logs for SNMP anomalies or unexpected reloads
show logging | include SNMP|SYS-5-RELOAD|ALIGN-3-SPURIOUSStop Exposing VPN Ports to the Internet — Switch to Zero-Trust Access
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Technical Root Cause & Exploitation Mechanics
1. Vulnerability Overview & Root Cause
This vulnerability is a buffer overflow condition within the Simple Network Management Protocol (SNMP) subsystem of Cisco IOS and IOS XE Software. The root cause lies in the improper boundary validation of input data within crafted SNMP packets processed by the subsystem. The vulnerability affects all versions of SNMP (v1, v2c, and v3).
2. Exploitation Mechanics & Attack Scenario
An attacker can exploit this vulnerability by sending crafted SNMP packets via IPv4 or IPv6 directly to the target system. The exploitation requirements vary by SNMP version:
- SNMP v1 and v2c: The attacker must possess the valid read-only community string of the target system.
- SNMP v3: The attacker must have valid user credentials configured on the target system.
Upon receiving the malicious SNMP request, the parser fails to validate the length of the input, triggering a buffer overflow. This allows the attacker to overwrite critical memory structures, hijack the execution flow to run arbitrary code, or crash the device (causing a reload).
3. Infrastructure & Business Impact
With a CVSS score of 8.8 (High) and its inclusion in the CISA KEV (Known Exploited Vulnerabilities) catalog, this vulnerability poses a severe threat to enterprise network backbones. Successful exploitation allows full administrative control over core routing and switching infrastructure, enabling traffic interception, unauthorized network pivoting, and persistent backdoor installation. It can also be used to launch devastating Denial of Service (DoS) attacks by repeatedly reloading critical network nodes.
Official Vendor Advisories & References
The Simple Network Management Protocol (SNMP) subsystem of Cisco IOS and IOS XE Software contains multiple vulnerabilities that could allow an authenticated, remote attacker to remotely execute code on an affected system or cause an affected system to reload. An attacker could exploit these vulnerabilities by sending a crafted SNMP packet to an affected system via IPv4 or IPv6. Only traffic directed to an affected system can be used to exploit these vulnerabilities. The vulnerabilities are due to a buffer overflow condition in the SNMP subsystem of the affected software. The vulnerabilities affect all versions of SNMP - Versions 1, 2c, and 3. To exploit these vulnerabilities via SNMP Version 2c or earlier, the attacker must know the SNMP read-only community string for the affected system. To exploit these vulnerabilities via SNMP Version 3, the attacker must have user credentials for the affected system. A successful exploit could allow the attacker to execute arbitrary code and obtain full control of the affected system or cause the affected system to reload. Customers are advised to apply the workaround as contained in the Workarounds section below. Fixed software information is available via the Cisco IOS Software Checker. All devices that have enabled SNMP and have not explicitly excluded the affected MIBs or OIDs should be considered vulnerable. There are workarounds that address these vulnerabilities.
Frequently Asked SecOps Questions (CVE-2017-6736)
FAQQ1. How critical is CVE-2017-6736 and is it actively exploited in the wild?
CVE-2017-6736 is rated CVSS 8.8 (HIGH) with a 30-day EPSS exploitation probability of 70.3%. It is listed in the CISA Known Exploited Vulnerabilities (KEV) catalog, requiring immediate emergency remediation.
Q2. Which vendors and products are affected by CVE-2017-6736?
Primary impacted vendors include cisco across ios, ios xe. Scope summary: All versions of Cisco IOS and IOS XE Software where SNMP is enabled and affected MIBs/OIDs are not explicitly restricted
Q3. What temporary mitigations or workarounds exist for CVE-2017-6736 before patching?
[Mitigation] Apply fixed software versions via Cisco IOS Software Checker, or implement workarounds including SNMP community ACLs, MIB view restrictions, or disabling SNMP Follow the CLI remediation commands and interactive SecOps checklist on this page to restrict exposure prior to your maintenance window.
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