CriticalCVE-2020-3161

    Cisco IP Phones Web Server Remote Code Execution and Denial of Service Vulnerability (CVE-2020-3161)

    Published In CISA KEV since Sources: CVE.org

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    What is affected, what to do

    A vulnerability in the web server for Cisco IP Phones could allow an unauthenticated, remote attacker to execute code with root privileges or cause a reload of an affected IP phone, resulting in a denial of service (DoS) condition. The vulnerability is due to a lack of proper input validation of HTTP requests.

    The vulnerability is being actively exploited; treat it as a priority. The available sources name no fixed version. Check the vendor guidance in the references below. Immediate actions, mitigation and detection rules are in the runbook below.

    Affected
    • Cisco IP Phone 8865 Firmware 9.3(4)SR3 eq 10.3\(1\)es14..10.3\(1\)es14
    • Cisco IP Phone 8865 Firmware 9.3(4)SR3 eq 11.0\(1\)..11.0\(1\)
    • Cisco IP Phone 8865 Firmware 9.3(4)SR3 eq 11.0\(5\)sr1..11.0\(5\)sr1
    • Cisco IP Phone 8851 Firmware 11.0(4)SR2 eq 10.3\(1\)es14..10.3\(1\)es14
    • Cisco IP Phone 8851 Firmware 11.0(4)SR2 eq 11.0\(1\)..11.0\(1\)
    • Cisco IP Phone 8851 Firmware 11.0(4)SR2 eq 11.0\(5\)sr1..11.0\(5\)sr1
    • Cisco IP Phone 7841 Firmware 10.3(1)SR4B eq 11.0\(1\)..11.0\(1\)
    • Cisco IP Phone 7821 Firmware 10.3(1)SR4B eq 11.0\(1\)..11.0\(1\)
    • Cisco IP Phone 8811 Firmware 10.3(1)SR4B eq 10.3\(1\)es14..10.3\(1\)es14
    • Cisco IP Phone 8811 Firmware 10.3(1)SR4B eq 11.0\(1\)..11.0\(1\)
    • Cisco IP Phone 8811 Firmware 10.3(1)SR4B eq 11.0\(5\)sr1..11.0\(5\)sr1
    • Cisco IP Phone 8861 Firmware 11.0(4)SR2 eq 10.3\(1\)es14..10.3\(1\)es14
    • Cisco IP Phone 8861 Firmware 11.0(4)SR2 eq 11.0\(1\)..11.0\(1\)
    • Cisco IP Phone 8861 Firmware 11.0(4)SR2 eq 11.0\(5\)sr1..11.0\(5\)sr1
    • Cisco IP Phone 8845 Firmware 11.0(4)SR2 eq 10.3\(1\)es14..10.3\(1\)es14
    • Cisco IP Phone 8845 Firmware 11.0(4)SR2 eq 11.0\(1\)..11.0\(1\)
    • Cisco IP Phone 8845 Firmware 11.0(4)SR2 eq 11.0\(5\)sr1..11.0\(5\)sr1
    • Cisco IP Phone 7861 Firmware 9.3(4)SR3 eq 11.0\(1\)..11.0\(1\)
    • Cisco IP Phone 8841 Firmware 9.3(4)SR3 eq 10.3\(1\)es14..10.3\(1\)es14
    • Cisco IP Phone 8841 Firmware 9.3(4)SR3 eq 11.0\(1\)..11.0\(1\)
    • Cisco IP Phone 8841 Firmware 9.3(4)SR3 eq 11.0\(5\)sr1..11.0\(5\)sr1
    • Cisco IP Phone 7811 Firmware 11.0(4)SR2 eq 11.0\(1\)..11.0\(1\)
    • Cisco IP Phone 8821 Firmware 11.0(4)sr3 eq 10.3\(1\)es14..10.3\(1\)es14
    • Cisco IP Phone 8821 Firmware 11.0(4)sr3 eq 11.0\(1\)..11.0\(1\)
    • Cisco IP Phone 8821 Firmware 11.0(4)sr3 eq 11.0\(5\)sr1..11.0\(5\)sr1
    • Cisco IP Phone 8821-EX Firmware 11.0(4)sr3 eq 10.3\(1\)es14..10.3\(1\)es14
    • Cisco IP Phone 8821-EX Firmware 11.0(4)sr3 eq 11.0\(1\)..11.0\(1\)
    • Cisco IP Phone 8821-EX Firmware 11.0(4)sr3 eq 11.0\(5\)sr1..11.0\(5\)sr1
    • Cisco 8831 Firmware eq 10.3\(1\)es14..10.3\(1\)es14
    • Cisco 8831 Firmware eq 11.0\(1\)..11.0\(1\)
    • Cisco 8831 Firmware eq 11.0\(5\)sr1..11.0\(5\)sr1

    Not sure whether and how you are affected? Ask a question about this vulnerability

    Metrics

    9.8

    Priority

    XIEM Effective CVSS: highest available CVSS rating from NVD and vendor data.

    Source: NVD, CVSS v3

    9.8

    CVSS v3

    Severity Critical on the scale from 0 to 10.

    Source: CVE.org

    83.9 %

    EPSS score

    High — model estimates ≥ 50% chance of real-world exploitation within 30 days.

    Source: FIRST EPSS, percentile 99.69

    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
    228.50 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
    9.8 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
    9.8
    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
    83.9 %
    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
    99.69
    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 · 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-20

    Response and mitigation

    Why act now?

    Prioritisation rationale

    The flaw is reachable over the network without authentication or user interaction and yields root-level code execution on the handset. Its exploitation likelihood is very high, placing it among the CVEs most likely to be attacked. Compromised IP phones give attackers a poorly monitored foothold inside the internal network: they can be used for call eavesdropping, as a pivot towards CUCM, or for persistence. The same bug also makes disrupting voice services trivial, which matters especially for KRITIS operators running emergency or control-room telephony. CISA does not list known ransomware use. Organisations whose voice VLAN is reachable from user networks, or that expose phones externally, should still treat this as high priority. Well-segmented environments with phone web access disabled can schedule the upgrade in a prompt but regular patch window.

    1. Runbook step 1

      Immediate response (0-24 h)

      • Inventory: Export all Cisco IP Phone 7811, 7821, 7841, 7861, 8811 and 8831 devices from CUCM (Device > Phone) or your asset inventory and check their firmware loads. Releases explicitly listed as affected are 11.0(1), 11.0(5)SR1 and 10.3(1)ES14 (8831).
      • Upgrade firmware: A patch is available from Cisco. Look up the fixed release in the Cisco Security Advisory for CVE-2020-3161 and roll it out via CUCM Device Defaults or the per-phone Load Name. Prioritise phones in lobbies, meeting rooms and branch sites.
      • Restrict HTTP reachability now: At the Layer-3 boundary (ACL or firewall), block TCP/80 and TCP/443 towards phone subnets or the voice VLAN from every source except dedicated admin hosts.
      • Disable phone web access: In CUCM (Product Specific Configuration, "Web Access"), set the option to "Disabled" wherever it is not operationally required, then push it with Apply Config/Reset. Confirm in the vendor advisory whether this fully neutralises the flaw.
      • Check internet exposure: Scan your external attack surface (Shodan/Censys or your own scanner) for reachable phone web servers, for example remote-worker phones that bypass VPN Phone or sit behind misconfigured NAT/port forwards. Shut down any hits immediately.
      • Triage suspected compromise: Isolate phones showing unexplained reboots or odd traffic. Factory-reset them, re-provision them with clean firmware, and rotate any credentials stored in the phone configuration (SSH, admin, 802.1X).

    2. Runbook step 2

      Mitigation layers

      • Network segmentation: Run a dedicated voice VLAN that only permits SIP/SCCP, RTP, TFTP/HTTP config to CUCM, DHCP and DNS. Apply default-deny to inbound HTTP/HTTPS from user, guest and Wi-Fi segments.
      • Egress filtering: Allow phones to reach only CUCM, TFTP, NTP and media gateways. Block outbound traffic to the internet and to workstation segments, which defeats reverse shells after a successful RCE.
      • IPS/NGFW: Enable IPS inspection on HTTP flows to phone IPs and check your Cisco Firepower/Snort and Suricata ET rulesets for CVE-2020-3161 signatures. Also drop abnormally long URIs or parameters and non-browser user agents aimed at the voice segment.
      • Network access control: Enforce 802.1X/MAB through Cisco ISE or an equivalent, so only known phone MACs or certificates land in the voice VLAN. This stops an attacker from simply plugging into a wall jack and hopping into the segment.
      • Configuration hardening: Disable unneeded features in CUCM (Web Access, PC Port, Span to PC Port, SSH). Change default admin passwords and audit the hardening parameters regularly via BAT export.

    3. Runbook step 3

      Detection rules

      • Network telemetry (Zeek/Suricata): Alert on HTTP requests to voice VLAN hosts from anything other than admin networks. Zeek: http.log | where id.resp_h in voice_subnet and id.orig_h !in admin_hosts Treat very long URIs, unusual HTTP methods or binary content in parameters as high-confidence signals.
      • Unexplained reboots (DoS/exploit indicator): Correlate RTMT DeviceUnregistered/DeviceRegistered alerts and CUCM syslog for phones that cycle registration repeatedly without a change ticket. SPL: index=cucm "DeviceUnregistered" | stats count by DeviceName | where count>3
      • Outbound traffic from phones: Hunt firewall and NetFlow logs for phone IPs connecting to anything other than CUCM, TFTP or NTP, especially outbound TCP to high ports or internet destinations. KQL: CommonSecurityLog | where SourceIP in (voice_ips) and DestinationIP !in (cucm_ips) and DeviceAction == "allow"
      • Scanning: Flag internal hosts that sweep TCP/80 or TCP/443 across many voice IPs, using a Suricata threshold or a Zeek conn.log aggregation on source IP with a high distinct id.resp_h count in the voice range.
      • Integrity drift: Compare the Active Load ID that CUCM reports against your baseline. Unknown or unexpected loads indicate tampering.

    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

    A vulnerability in the web server for Cisco IP Phones could allow an unauthenticated, remote attacker to execute code with root privileges or cause a reload of an affected IP phone, resulting in a denial of service (DoS) condition. The vulnerability is due to a lack of proper input validation of HTTP requests. An attacker could exploit this vulnerability by sending a crafted HTTP request to the web server of a targeted device. A successful exploit could allow the attacker to remotely execute code with root privileges or cause a reload of an affected IP phone, resulting in a DoS condition.

    Source: NVD (NIST), cvelistv5, CISA KEV Catalog

    Vendors and products

    Affected operating systems

    Affected operating systems
    Othercisco8831_firmware10.3\(1\)es14
    Othercisco8831_firmware11.0\(1\)
    Othercisco8831_firmware11.0\(5\)sr1
    Otherciscoip_phone_7811_firmware11.0\(1\)
    Otherciscoip_phone_7821_firmware11.0\(1\)
    Otherciscoip_phone_7841_firmware11.0\(1\)
    Otherciscoip_phone_7861_firmware11.0\(1\)
    Otherciscoip_phone_8811_firmware10.3\(1\)es14
    Otherciscoip_phone_8811_firmware11.0\(1\)
    Otherciscoip_phone_8811_firmware11.0\(5\)sr1
    Otherciscoip_phone_8821-ex_firmware10.3\(1\)es14
    Otherciscoip_phone_8821-ex_firmware11.0\(1\)
    Otherciscoip_phone_8821-ex_firmware11.0\(5\)sr1
    Otherciscoip_phone_8821_firmware10.3\(1\)es14
    Otherciscoip_phone_8821_firmware11.0\(1\)
    Otherciscoip_phone_8821_firmware11.0\(5\)sr1
    Otherciscoip_phone_8841_firmware10.3\(1\)es14
    Otherciscoip_phone_8841_firmware11.0\(1\)
    Otherciscoip_phone_8841_firmware11.0\(5\)sr1
    Otherciscoip_phone_8845_firmware10.3\(1\)es14
    Otherciscoip_phone_8845_firmware11.0\(1\)
    Otherciscoip_phone_8845_firmware11.0\(5\)sr1
    Otherciscoip_phone_8851_firmware10.3\(1\)es14
    Otherciscoip_phone_8851_firmware11.0\(1\)

    Weakness classes (CWE)

    • CWE-20Improper Input Validation

      Class

      The product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly.

      Entry at MITRE CWE-20

    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.

    Public exploit references
    CISA KEV CatalogCISA Known Exploited Vulnerability (opens in a new tab)cisa.govactively exploited
    Packet StormCisco IP Phone 11.7 Denial Of Service (opens in a new tab)packetstormsecurity.comPoC (publicly reported)reported, not verified—

    References & sources

    • tools.cisco.com20200415 Cisco IP Phones Web Server Remote Code Execution and Denial of Service VulnerabilityVendor advisory, x_refsource_CISCO, Vendor advisory
    • packetstormsecurity.comfiles/157265/Cisco-IP-Phone-11.7-Denial-Of-Service.htmlx_refsource_MISC, Exploit, Third-party advisory, Vulnerability database
    • cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2020-3161government-resource, US government resource
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    <a href="https://intel.neosec.eu/en/cve/CVE-2020-3161"><img src="https://intel.neosec.eu/badge/cve/CVE-2020-3161.svg?lang=en" alt="CVE-2020-3161 – NEOSEC Intel" height="20"></a>