Cisco Adaptive Security Appliance (ASA) SNMP Buffer Overflow Vulnerability (CVE-2016-6366)
Published In CISA KEV since Sources: CVE.org
What is affected, what to do
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.
The vulnerability is being actively exploited; treat it as a priority. Update Cisco Adaptive Security Appliance (ASA) Software to version 9.0.4.40, Cisco Adaptive Security Appliance (ASA) Software to version 9.1.7\(9\), Cisco Adaptive Security Appliance (ASA) Software to version 9.2.4\(14\). Immediate actions, mitigation and detection rules are in the runbook below.
- Affected
- Cisco Adaptive Security Appliance (ASA) Software
lt 7.2.1..9.0.4.40 - Cisco Adaptive Security Appliance (ASA) Software
lt 9.1.1..9.1.7\(9\) - Cisco Adaptive Security Appliance (ASA) Software
lt 9.2.0..9.2.4\(14\) - Cisco Adaptive Security Appliance (ASA) Software
lt 9.3.0..9.3.3\(10\) - Cisco Adaptive Security Appliance (ASA) Software
lt 9.4.0.115..9.4.3\(8\) - Cisco Adaptive Security Appliance (ASA) Software
lte 9.5.0..9.5\(3\) - Cisco Adaptive Security Appliance (ASA) Software
lt 9.6.0..9.6.1\(11\) - Cisco ASA 1000V Cloud Firewall Software
eq 8.7.1..8.7.1 - Cisco ASA 1000V Cloud Firewall Software
eq 8.7.1.1..8.7.1.1
- Cisco Adaptive Security Appliance (ASA) Software
- Fixed in
- Cisco Adaptive Security Appliance (ASA) Software
9.0.4.40 - Cisco Adaptive Security Appliance (ASA) Software
9.1.7\(9\) - Cisco Adaptive Security Appliance (ASA) Software
9.2.4\(14\) - Cisco Adaptive Security Appliance (ASA) Software
9.3.3\(10\) - Cisco Adaptive Security Appliance (ASA) Software
9.4.3\(8\) - Cisco Adaptive Security Appliance (ASA) Software
9.6.1\(11\)
- Cisco Adaptive Security Appliance (ASA) Software
Not sure whether and how you are affected? Ask a question about this vulnerability
Metrics
8.8
Priority
XIEM Effective CVSS: highest available CVSS rating from NVD and vendor data.
Source: NVD, CVSS v3
8.8
CVSS v3
Severity High on the scale from 0 to 10.
Source: CVE.org
87.6 %
EPSS score
High — model estimates ≥ 50% chance of real-world exploitation within 30 days.
Source: FIRST EPSS, percentile 99.76
Show all metrics
- Severity
- Critical
Priority (0–325)
The priority score is a consolidated metric that condenses several individual signals into a single, action-guiding ranking: technical severity (CVSS), exploitation likelihood (EPSS), known-exploited status (KEV) and the context of the affected asset. The aim is to answer the only truly important question in vulnerability management: what first?
Read in glossary- 223.62 Methodology
Exploit maturity
An exploit is code or a technique that specifically uses a concrete vulnerability to force unintended behaviour — such as executing code or escalating privileges. It is the tool that turns a theoretical flaw into a practical attack.
Read in glossary- actively exploited
XIEM Effective CVSS
XIEM® is the tiered managed-security operating model from NEOSEC and a registered trademark of NEOSEC GmbH. It extends classic SIEM — the central collection, normalisation and correlation of security-relevant events — with endpoint forensics, network visibility and deception technology, and consolidates them into three building tiers: Sentry, Orchestrate and Command.
Read in glossary- 8.8 Methodology
CVSS v3
CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions.
Read in glossary- 8.8
EPSS score
The EPSS score is the absolute model output for an individual vulnerability: the estimated probability of real-world exploitation within the next 30 days, expressed as a value between 0 and 1 (i.e. 0 % to 100 %). A score of 0.42 means „the model estimates a 42 % chance of real-world exploitation within 30 days“.
Read in glossary- 87.6 %
- High — model estimates ≥ 50% chance of real-world exploitation within 30 days.
EPSS percentile
The EPSS percentile is the relative position of a vulnerability within all EPSS-scored CVEs, expressed as a value between 0 and 1. A percentile of 0.90 means „this CVE has a higher EPSS score than 90 % of all other scored CVEs“. Unlike the EPSS score, it is not a probability — but a positioning within a ranking.
Read in glossary- 99.76
- Critical — this CVE ranks in the top fifth of all CVEs scored today (rank ≥ 80%).
KEV status
The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.
Read in glossary- Actively exploited
- Published
In CISA KEV since
The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.
Read in glossary- CISA assessment (SSVC)
- Exploitation: active · not automatable · Technical impact: total
- Source: CISA Vulnrichment, as of 8 November 2024
CWE
CWE is a catalogue of typical weakness types in software and hardware — such as flawed input validation, unsafe memory handling or poor authentication. Where a CVE names a concrete vulnerability in a concrete product, a CWE describes the underlying class of defect.
Read in glossary- CWE-120
Response and mitigation
Why act now?
Prioritisation rationale
EXTRABACON has been a publicly available, weaponised exploit since 2016 and carries a very high exploitation likelihood. The "authenticated" precondition only means knowing an SNMP community string, which in many networks is weak, default or sniffable in cleartext. Typical targets are perimeter firewalls at public bodies, utilities and mid-market firms running long-unpatched ASA releases or EoL hardware. Success yields code execution or passwordless admin access on the core security control, and with it full control over VPN and segmentation. Treat it as critical if SNMP is reachable from untrusted networks or the device runs an unpatched or EoL release. If SNMP is strictly confined to an OOB management network using SNMPv3, a scheduled upgrade in the next maintenance window is sufficient.
Runbook step 1
Immediate response (0-24 h)
- Inventory and version check: Enumerate every ASA 5500/5500-X, ASA Services Module, ASAv, Firepower 9300 ASA Security Module and ASA 1000V, plus legacy PIX and FWSM units. Record the running release with
show version. - Patch (Bug ID CSCva92151): Per the context, upgrade to at least 9.0.4.40, 9.1.7(9), 9.2.4(14), 9.3.3(10), 9.4.3(8) or 9.6.1(11). No fixed release is listed for the 9.5 train, so check the vendor advisory for the correct version or move to a fixed train. ASA 1000V 8.7.1.1, PIX and FWSM are end-of-life. Retire them, or disable SNMP outright (
no snmp-server enable). - Rotate SNMP community strings now: Exploitation requires a valid SNMPv1/v2c community (or SNMPv3 credentials). Replace defaults and weak values such as "public", "private" or hostname-derived strings. Update your NMS accordingly.
- Lock SNMP to management hosts: Permit only named stations via
snmp-server host <interface> <NMS-IP> community <string>. Remove SNMP from all internet- or DMZ-facing interfaces. - Integrity check where compromise is suspected: Treat a unit as potentially compromised if you see unexplained reloads or admin logins you cannot attribute. Verify the image hash with
verify /md5 disk0:<image>against Cisco's published value and preserveshow crashinfo. Then reload from a verified image and rotate all local admin, enable and VPN credentials.
- Inventory and version check: Enumerate every ASA 5500/5500-X, ASA Services Module, ASAv, Firepower 9300 ASA Security Module and ASA 1000V, plus legacy PIX and FWSM units. Record the running release with
Runbook step 2
Mitigation layers
- Network segmentation: Allow UDP 161 to ASA interfaces only from a dedicated management VLAN or out-of-band network. Upstream ACLs on routers and core switches should drop SNMP to the firewall from every other segment. Example pattern:
deny udp any host <ASA-mgmt-IP> eq 161, with explicit permits for NMS IPs only. - Protocol hardening: Migrate to SNMPv3 authPriv (SHA/AES) and disable v1/v2c. This removes the easy path of guessed or sniffed community strings.
- IPS/IDS: Enable the Cisco Talos signatures for CVE-2016-6366/EXTRABACON in Snort, Suricata or Firepower. Add a custom rule that alerts on IPv4 SNMP to ASA addresses carrying abnormally long OIDs or oversized varbinds.
- Management-plane hardening: Restrict SSH with
ssh <IP> <mask> <if>to the admin subnet, disable Telnet, and enforce TACACS+/RADIUS AAA. The known payload bypasses local password checks, so tight management reachability limits what an attacker can do next. - Lifecycle: Log EoL platforms (PIX, FWSM, ASA 1000V) in the risk register with a dated replacement plan. No vendor fix will ever exist for them.
- Network segmentation: Allow UDP 161 to ASA interfaces only from a dedicated management VLAN or out-of-band network. Upstream ACLs on routers and core switches should drop SNMP to the firewall from every other segment. Example pattern:
Runbook step 3
Detection rules
- Network telemetry (Zeek/Suricata): In
snmp.log, flag SNMP to ASA IPs from sources outside the NMS allowlist. Example logic:snmp.log | where id.resp_h in ASA_IPS and id.orig_h !in NMS_IPS. Also alert on unusually large SNMP packets (roughly > 500 bytes) aimed at firewall management IPs. - ASA syslog, unexpected reloads: Correlate boot or crash messages and uptime resets outside change windows. Failed exploit attempts typically crash the device. Example SPL:
index=network sourcetype=cisco:asa ("reload" OR "crashinfo" OR "Booting") | stats count by host. - Admin logins without AAA: Match ASA
%ASA-6-605005(login permitted) against TACACS+/RADIUS logs. A successful SSH or Telnet session with no corresponding AAA authentication strongly suggests an active payload. - Config drift: Use NCM, RANCID or Oxidized to alert on changes to
snmp-server,ssh,aaaandusernamelines, and on any deviation ofshow running-configfrom the baseline. - SNMP failures: Bursts of SNMP authentication failures, or requests from unauthorised hosts in ASA syslog, point to community-string guessing ahead of exploitation.
- Network telemetry (Zeek/Suricata): In
Detection rules
No NEOSEC detection rule is available for this vulnerability yet.
Classification under NIS2 and CRA
Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.
NIS2
NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management.
Read in glossary- Actively exploited: treat the vulnerability as a priority in vulnerability handling under Art. 21(2)(e) NIS2. If it is exploited in your environment, check the reporting obligation under Art. 23 (early warning within 24 hours).
CRA
The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates.
Read in glossary- Since 11.09.2026, manufacturers of affected products with digital elements must report actively exploited vulnerabilities under Art. 14 CRA, with an early warning within 24 hours via ENISA's single reporting platform.
ISO/IEC 27001
ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security.
Read in glossary- Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.
Description
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.
Source: NVD (NIST), cvelistv5, CISA KEV Catalog, CVE.org (MITRE)
Vendors and products
Affected operating systems
| Other | cisco | adaptive_security_appliance_software | — |
| Other | cisco | asa_1000v_cloud_firewall_software | 8.7.1 |
| Other | cisco | asa_1000v_cloud_firewall_software | 8.7.1.1 |
| Other | cisco | pix_firewall_software | — |
Weakness classes (CWE)
CWE-120Buffer Copy without Checking Size of Input ('Classic Buffer Overflow')
Base
The product copies an input buffer to an output buffer without verifying that the size of the input buffer is less than the size of the output buffer.
Entry at MITRE CWE-120
Public exploit references
Publicly reported proof-of-concepts, exploit modules and detection templates for this vulnerability. NEOSEC links to the original sources only and does not provide any exploit code.
| CISA KEV Catalog | CISA Known Exploited Vulnerability (opens in a new tab)cisa.gov | actively exploited | |
| Reference tagged as exploit | blogs.cisco.com (opens in a new tab) | PoC (publicly reported)reported, not verified | — |
| Exploit-DB | EDB-ID 40258 (opens in a new tab)exploit-db.com | PoC (publicly reported)reported, not verified | — |
References & sources
- securityfocus.com92521Vulnerability database, x_refsource_BID, Broken link, Not applicable, Third-party advisory, Vulnerability database
- github.comoffensive-security/exploit-database-bin-sploits/raw/master/splo…x_refsource_MISC, Broken link, Exploit
- blogs.cisco.comsecurity/shadow-brokersx_refsource_CONFIRM, Exploit, Press coverage, Vendor advisory
- tools.cisco.com20160817 Cisco Adaptive Security Appliance SNMP Remote Code Execution VulnerabilityVendor advisory, x_refsource_CISCO, Vendor advisory
- tools.cisco.comsecurity/center/viewErp.x?alertId=ERP-56516x_refsource_CONFIRM, Vendor advisory
- zerosum0x0.blogspot.com2016/09/reverse-engineering-cisco-asa-for.htmlx_refsource_MISC, Exploit, Technical description
- exploit-db.com40258Exploit, x_refsource_EXPLOIT-DB, Third-party advisory, Vulnerability database
- securitytracker.com1036637Vulnerability database, x_refsource_SECTRACK, Broken link, Third-party advisory, Vulnerability database
- cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2016-6366government-resource, US government resource
Embed badge
Show the severity and KEV status of this vulnerability on your site – with a link back here. The image is updated hourly.
<a href="https://intel.neosec.eu/en/cve/CVE-2016-6366"><img src="https://intel.neosec.eu/badge/cve/CVE-2016-6366.svg?lang=en" alt="CVE-2016-6366 – NEOSEC Intel" height="20"></a>