CriticalCVE-2024-40891

    Zyxel DSL CPE OS Command Injection Vulnerability (CVE-2024-40891)

    Published In CISA KEV since Sources: pwn.watch, CVE.org

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    What is affected, what to do

    An authenticated attacker can exploit a post-authentication command injection vulnerability in the management commands of Zyxel DSL CPE devices to execute OS commands via Telnet. The vulnerability affects specific firmware versions, including VMG4325-B10A Firmware 1.00(AAFR.4)C0_20170615. It is actively exploited in attack campaigns. Affected products include several Zyxel models such as VMG1312-B10A/B/B10E, VMG3312-B10A, VMG3313-B10A, VMG3926-B10B, VMG4325-B10A, and VMG4380-B10A. Immediate patching is urgently required to secure the devices.

    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
    • Zyxel VMG4325-B10A firmware lte *..1.00(AAFR.4)C0_20170615

    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: pwn.watch, CVE.org

    22.9 %

    EPSS score

    Elevated — model estimates 10-50% exploitation likelihood.

    Source: FIRST EPSS, percentile 97.8

    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
    228.08 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“.

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    22.9 %
    Elevated — model estimates 10-50% exploitation likelihood.
    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
    97.8
    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 11 February 2025
    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-78

    Response and mitigation

    Why act now?

    Prioritisation rationale

    The flaw is network-reachable with low attack complexity and needs only a low-privilege account. On devices with default credentials and Telnet exposed, that amounts to unauthenticated remote takeover with full loss of confidentiality, integrity and availability. Exploitation likelihood is among the highest of all scored CVEs: attackers mass-scan for exposed Telnet to recruit botnet nodes, build proxy infrastructure, or gain a foothold into the network behind the router. CISA does not flag known ransomware use. Because no fix will ever ship, treat every internet-facing unit as top priority and replace rather than update it. Internal-only, Telnet-disabled units in bridge mode can follow, but should still be replaced within the current quarter. For NIS2 entities, running end-of-life perimeter hardware is also a risk that must be documented under risk-management obligations.

    1. Runbook step 1

      Immediate response (0-24 h)

      • No patch available: The affected devices (VMG4325-B10A firmware 1.00(AAFR.4)C0_20170615, VMG1312-B10A/B10B/B10E, SBG3300-N000/NB00, SBG3500-N000/NB00) were already end-of-life when the CVE was assigned. Do not wait for a firmware fix; raise an urgent change to replace them with supported hardware.
      • Find every unit: Locate all affected Zyxel DSL CPE, including branch offices, home-office links and OT remote-maintenance lines. Use the CMDB and ISP contracts, and run an external scan of your own address space for TCP/23 and Zyxel HTTP banners.
      • Disable Telnet (TCP/23) now on both WAN and LAN interfaces. If the device UI won't allow it, block TCP/23 to the device on the upstream firewall or ACL. Also turn off WAN-side remote management (HTTP/HTTPS/SSH).
      • Rotate all local credentials: Set unique, long passwords on every local account, explicitly including service, supervisor and user accounts still on vendor defaults. The flaw requires authentication, but default credentials make that requirement meaningless.
      • Assume compromise where Telnet was exposed: Back up the running config, then check for rogue accounts, altered DNS servers and unexpected port forwards. Factory-reset the device and either rebuild it with a hardened config or swap it out immediately.

    2. Runbook step 2

      Mitigation layers

      • Segmentation: Run the CPE in pure bridge/modem mode and move routing, NAT and filtering to a supported firewall behind it. Expose the management interface only to a dedicated admin VLAN.
      • Perimeter ACL: Drop inbound TCP/23, TCP/2323 and management ports (80/443/8080) to the CPE WAN address. If the ISP manages the device, ask them to filter upstream.
      • Egress control: Restrict traffic sourced from the CPE's own management IP (not client traffic) to approved DNS/NTP servers only. This breaks the usual post-injection payload fetch via wget/curl/tftp.
      • IPS signature: Use a Suricata/Snort rule on Telnet sessions to CPE addresses that matches shell metacharacters plus download commands, e.g. content:"wget"; content:"/tmp/"; or pcre:"/[;|]\s*(wget|curl|tftp|busybox|sh)\b/". Add a threshold` rule for Telnet login brute-force bursts.
      • Access control: Permit local administration only from a jump-host segment, and never reuse admin passwords across sites.

    3. Runbook step 3

      Detection rules

      • Zeek/NetFlow – inbound Telnet: Watch for external sources hitting TCP/23 on CPE addresses. KQL: ZeekConn | where id_resp_p == 23 and not(ipv4_is_private(id_orig_h)) and id_resp_h in (cpe_list).
      • Device-originated outbound traffic: The CPE management IP connecting to unknown hosts over HTTP, TFTP (UDP/69) or high ports, or itself scanning for Telnet, strongly suggests a bot implant. SPL: index=netflow src_ip IN (cpe_ips) NOT dest_ip IN (dns_ntp_allowlist) | stats dc(dest_ip) by src_ip.
      • Suricata Telnet payload alerts: Flag session content containing ;, |, $( or backticks together with wget, curl, chmod +x, /tmp/ or busybox. Sigma shape: logsource: product=suricata; detection: dest_port=23 AND payload|contains|all: ['/tmp/','chmod'].
      • Device syslog (if forwarded to the SIEM): Look for bursts of failed Telnet logins followed by a success, logins with service/supervisor accounts, and unexpected changes to DNS, port forwarding or remote-management settings.
      • DNS telemetry: Clients behind the CPE suddenly resolving through unfamiliar resolvers, or returning anomalous answers, may indicate DNS hijacking after takeover.

    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

    **UNSUPPORTED WHEN ASSIGNED** A post-authentication command injection vulnerability in the management commands of the legacy DSL CPE Zyxel VMG4325-B10A firmware version 1.00(AAFR.4)C0_20170615 could allow an authenticated attacker to execute operating system (OS) commands on an affected device via Telnet.

    Source: pwn.watch, NVD (NIST), cvelistv5, CISA KEV Catalog, CVE.org (MITRE)

    Affected products

    The following versions are affected. Older releases of the same product line are also vulnerable unless stated otherwise.

    Affected products
    Affected versionsFixed in
    ZyxelVMG4325-B10A firmware
    • ≤ 1.00(AAFR.4)C0_20170615
    • —

    Vendors and products

    Affected operating systems

    Affected operating systems
    Otherzyxelsbg3300-n000_firmware—
    Otherzyxelsbg3300-nb00_firmware—
    Otherzyxelsbg3500-n000_firmware—
    Otherzyxelsbg3500-nb00_firmware—
    Otherzyxelvmg1312-b10a_firmware—
    Otherzyxelvmg1312-b10b_firmware—
    Otherzyxelvmg1312-b10e_firmware—
    Otherzyxelvmg3312-b10a_firmware—
    Otherzyxelvmg3313-b10a_firmware—
    Otherzyxelvmg3926-b10b_firmware—
    Otherzyxelvmg4325-b10a_firmware—
    Otherzyxelvmg4380-b10a_firmware—
    Otherzyxelvmg8324-b10a_firmware—
    Otherzyxelvmg8924-b10a_firmware—

    Weakness classes (CWE)

    • CWE-78Improper Neutralization of Special Elements used in an OS Command ('OS Command Injection')

      Base

      The product constructs all or part of an OS command using externally-influenced input from an upstream component, but it does not neutralize or incorrectly neutralizes special elements that could modify the intended OS command when it is sent to a downstream component.

      Entry at MITRE CWE-78

    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
    PoC-in-GitHub (nomi-sec)zyxel-p870hn-hardware-hacking (opens in a new tab)github.comPoC (publicly reported)reported, not verified

    References & sources

    • zyxel.comZyxel advisory: Zyxel DSL CPE DevicesVendor advisory, Vendor advisory, pwn-watch
    • cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2024-40891government-resource, US government resource
    • pwn.watchpwn.watch: Zyxel DSL CPE Devices: 2 exploited vulnerabilities (CVE-2024-40890, CVE-2024-40891)pwn-watch
    • zyxel.comZyxel advisory: Zyxel DSL CPE Devicespwn-watch
    • NVD (NIST)NVD: CVE-2024-40890pwn-watch
    • NVD (NIST)NVD: CVE-2024-40891pwn-watch
    • cisa.govCISA Known Exploited Vulnerabilities Catalogpwn-watch
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