Cisco ASA SNMP Buffer Overflow Remote Code Execution Vulnerability (CVE-2016-6366, EXTRABACON)
Executive & SecOps Summary
SECOPS ASSESSMENTCisco Adaptive Security Appliance (ASA) Software through 9.4.2.3 on ASA 5500, 5500-X, ASAv, Firepower 9300, PIX, and FWSM devices
Remote authenticated attackers can exploit this vulnerability to execute arbitrary code (RCE) with elevated privileges or cause a denial of service (DoS) via crafted IPv4 SNMP packets
Apply official Cisco software updates immediately, restrict SNMP access using strict Access Control Lists (ACLs), and rotate SNMP community strings
Affected Products & Version Scope
Remediation & Live Workaround Commands
PATCH & WORKAROUND1. Official Patch & Version Upgrade Commands
Administrators should upgrade to the latest patched Cisco ASA software release. Below is an example of upgrading the ASA image via TFTP.
# Enter privileged mode and copy the patched image
enable
copy tftp://192.168.1.100/asa943-k8.bin flash:/asa943-k8.bin
# Update boot configuration and reload
configure terminal
no boot system disk0:/asa942-k8.bin
boot system disk0:/asa943-k8.bin
write memory
reload2. Temporary Workaround & Hardening Configuration
If patching cannot be applied immediately, restrict SNMP access to trusted management hosts only or disable SNMP entirely.
configure terminal
# Remove default or weak SNMP community strings
no snmp-server community public
no snmp-server community private
# Restrict SNMP access to a specific trusted host (e.g., 10.1.1.50)
access-list SNMP_ALLOW standard permit host 10.1.1.50
snmp-server community Strong_Community_String_Here
snmp-server host inside 10.1.1.50 community Strong_Community_String_Here
write memory3. Log Audit & IoC Verification Commands
Verify the current running version and inspect the system logs for unexpected reboots or crash dumps that may indicate exploitation attempts.
# Check the current ASA software version
show version | include Software Version
# Inspect crash logs and memory status
show crashinfo
show memory detailStop 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 (CVE-2016-6366, widely known as EXTRABACON) is a critical buffer overflow (CWE-120) in the Simple Network Management Protocol (SNMP) code of Cisco Adaptive Security Appliance (ASA) Software. The root cause lies in the improper validation of input data length during the parsing of SNMP packets, allowing an attacker to overflow the system memory buffer.
2. Exploitation Mechanics & Attack Scenario
To exploit this vulnerability, an attacker must send a specially crafted IPv4 SNMP packet containing a valid SNMP community string (either read-only or read-write) to the affected ASA device. The malformed packet triggers a buffer overflow during processing, allowing the attacker to overwrite memory structures, hijack the execution flow to run arbitrary shellcode, or crash the system.
3. Infrastructure & Business Impact
As this vulnerability was publicly disclosed as part of the Shadow Brokers leak and is actively tracked in the CISA KEV catalog, it poses an extreme risk. Successful exploitation grants full administrative control over the perimeter firewall, enabling attackers to intercept network traffic, bypass security controls, and pivot into the internal corporate network.
Official Vendor Advisories & References
Buffer overflow in Cisco Adaptive Security Appliance (ASA) Software through 9.4.2.3 on ASA 5500, ASA 5500-X, ASA Services Module, ASA 1000V, ASAv, Firepower 9300 ASA Security Module, PIX, and FWSM devices allows remote authenticated users to execute arbitrary code via crafted IPv4 SNMP packets, aka Bug ID CSCva92151 or EXTRABACON.
Frequently Asked SecOps Questions (CVE-2016-6366)
FAQQ1. How critical is CVE-2016-6366 and is it actively exploited in the wild?
CVE-2016-6366 is rated CVSS 8.8 (HIGH) with a 30-day EPSS exploitation probability of 87.6%. It is listed in the CISA Known Exploited Vulnerabilities (KEV) catalog, requiring immediate emergency remediation.
Q2. Which vendors and products are affected by CVE-2016-6366?
Primary impacted vendors include cisco across adaptive_security_appliance_software, asa_5500, asa_5500-x, asa_services_module, asa_1000v, asav, firepower_9300_asa_security_module, pix, fwsm. Scope summary: Cisco ASA Software versions 9.4.2.3 and prior running on ASA 5500, 5500-X, ASA Services Module, ASA 1000V, ASAv, Firepower 9300, PIX, and FWSM platforms
Q3. What temporary mitigations or workarounds exist for CVE-2016-6366 before patching?
[Mitigation] Apply official Cisco software updates immediately, restrict SNMP access using strict Access Control Lists (ACLs), and rotate SNMP community strings Follow the CLI remediation commands and interactive SecOps checklist on this page to restrict exposure prior to your maintenance window.
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