CriticalCVE-2026-7273

    Zyxel GS1900 Series Switches Stack-Based Buffer Overflow Vulnerability (CVE-2026-7273)

    Published Last modified In CISA KEV since Sources: CVE.org

    What is affected, what to do

    A stack-based buffer overflow vulnerability in the CGI program of Zyxel GS1900-48HPv2 firmware versions through 2.90(ABTQ.1)C0 could allow a LAN-based, unauthenticated attacker to exploit the flaw and potentially execute OS commands via a crafted HTTP request.

    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 GS1900-48HPv2 firmware lte *..2.90(ABTQ.1)C0
    • Zyxel GS1900-8 firmware lte *..2.90(AAHH.1)C0
    • Zyxel GS1900-8HP firmware lte *..2.90(AAHI.1)C0
    • Zyxel GS1900-10HP firmware lte *..2.90(AAZI.1)C0
    • Zyxel GS1900-16 firmware lte *..2.90(AAHJ.1)C0
    • Zyxel GS1900-24 firmware lte *..2.90(AAHL.1)C0
    • Zyxel GS1900-24E firmware lte *..2.90(AAHK.1)C0
    • Zyxel GS1900-24EP firmware lte *..2.90(ABTO.1)C0
    • Zyxel GS1900-24HPv2 firmware lte *..2.90(ABTP.1)C0
    • Zyxel GS1900-48 firmware lte *..2.90(AAHN.1)C0

    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

    2.8 %

    EPSS score

    Moderate — model estimates 1-10% exploitation likelihood.

    Source: FIRST EPSS, percentile 86.3

    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
    222.60 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 exploitedSource: VulnCheck KEV (opens in a new tab) How we classify
    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
    2.8 %
    Moderate — model estimates 1-10% 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
    86.3
    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 17 June 2026
    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-121

    Response and mitigation

    Why act now?

    Prioritisation rationale

    This flaw gives an unauthenticated attacker on an adjacent network OS command execution on the switch, with full loss of confidentiality, integrity and availability. In practice, attackers chain bugs like this after an initial foothold on a compromised endpoint. They use them to persist on poorly monitored network gear, sniff traffic via port mirroring or bridge VLAN boundaries. Exploitation likelihood is elevated, and CISA does not flag known ransomware use. The GS1900 line is common in SMBs, branch offices and edge/OT-adjacent sites. If the management interface is reachable from user or guest VLANs, treat this as urgent and patch within days as part of NIS2 risk management. If management is strictly isolated, the fix can go into the next regular maintenance window.

    1. Runbook step 1

      Immediate response (0-24 h)

      • Inventory: Identify every Zyxel GS1900 switch (affected per context include GS1900-10HP, -16, -24, -24E, -24EP, -24HPv2, -48, -48HPv2). Collect firmware levels via the web GUI, SNMP (sysDescr) or your CMDB. The NVD description says "through 2.90(ABTQ.1)C0", while the CPE list shows 2.90(xxxx.2)C0 builds. Treat any build without a confirmed fix as vulnerable until the Zyxel advisory clarifies.
      • Apply the firmware fix: A patch is available from the vendor — look up the fixed version for CVE-2026-7273 in the Zyxel security advisory and map it per model variant (ABTQ, AAHL, etc.). Prioritise switches in user, guest, wireless and OT-adjacent segments.
      • Lock down the management interface: Until patched, permit web management (HTTP/TCP 80, HTTPS/TCP 443) only from a dedicated management VLAN or admin jump hosts. If that cannot be done quickly, disable HTTP management and fall back to console or out-of-band access.
      • Compromise check: Diff the running config against the last known-good backup. Look for new local accounts, changed VLAN or port-mirroring settings, new syslog/SNMP targets and unexplained reboots.
      • Rotate secrets: After patching, rotate admin passwords, SNMP communities and any RADIUS/TACACS shared secrets on affected devices. If compromise is suspected, factory-reset, reflash the firmware and restore configuration from a verified backup.

    2. Runbook step 2

      Mitigation layers

      • Network segmentation: Move switch management IPs into a dedicated, non-routed management VLAN. Add an ACL on the L3 gateway — permit tcp <admin-net> -> <switch-mgmt-net> eq 80,443, deny everything else. The attack vector is adjacent-network, so this directly shrinks the reachable population.
      • On-device access restriction: Where the firmware supports it, restrict management access to admin source networks and disable unused services (HTTP, Telnet, unneeded SNMP).
      • LAN access control (802.1X/NAC): Enforce port-based authentication so unmanaged, guest and unknown devices cannot reach the management segment at layer 2. Administratively shut down unused ports.
      • East-west IPS rule: On internal Suricata/Snort sensors, flag HTTP requests to switch management IPs that hit CGI paths (typically /cgi-bin/ on GS1900) with abnormally long parameters or bodies, e.g. alert http any any -> $SWITCH_MGMT any (http.uri; content:"/cgi-bin/"; urilen:>512; ...). Tune the threshold against legitimate GUI traffic.
      • Egress control: Block outbound connections from switch management IPs to the internet and client subnets. Allow only NTP, syslog, RADIUS and firmware downloads to defined destinations. This kills reverse shells and second-stage downloads after successful command execution.
      • Lifecycle: Replace devices with no available fix or out of support, and record the residual risk and deadline in your ISMS risk register (ISO 27001 A.8.8).

    3. Runbook step 3

      Detection rules

      • Network telemetry (Zeek http.log): Look for HTTP to switch management IPs from outside admin subnets. Example SPL: index=zeek sourcetype=zeek_http id.resp_h IN ($switch_mgmt$) NOT id.orig_h IN ($admin_nets$) | stats count by id.orig_h, uri. Also flag outliers in request_body_len or URI length to /cgi-bin/.
      • Switch-originated connections (NetFlow/Zeek conn.log): Any session sourced from a switch management IP to a non-approved destination, especially external IPs or unusual ports. This is a strong indicator of command execution or a reverse shell.
      • Device syslog/SNMP traps: Unexpected reboots (SNMP coldStart/warmStart, boot messages) may indicate failed overflow attempts. Alert on config changes, new users and logins outside maintenance windows with a Sigma rule (logsource network/syslog, filtered on switch hostnames plus config/user-change keywords).
      • Scanning patterns (Suricata/Zeek): A single client probing many switch management IPs on TCP 80/443 suggests internal recon or lateral movement. Pivot to EDR on the source host and check process ancestry (curl, python, script interpreters on non-admin endpoints).
      • Config integrity: Run a daily hash comparison of exported running configs (e.g. Oxidized/RANCID). Any drift without a change ticket is an incident.

    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 stack-based buffer overflow vulnerability in the CGI program of Zyxel GS1900-48HPv2 firmware versions through 2.90(ABTQ.1)C0 could allow a LAN-based, unauthenticated attacker to exploit the flaw and potentially execute OS commands via a crafted HTTP request.

    Source: cvelistv5, NVD (NIST), 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
    ZyxelGS1900-10HP firmware
    • ≤ 2.90(AAZI.1)C0
    • —
    ZyxelGS1900-16 firmware
    • ≤ 2.90(AAHJ.1)C0
    • —
    ZyxelGS1900-24 firmware
    • ≤ 2.90(AAHL.1)C0
    • —
    ZyxelGS1900-24E firmware
    • ≤ 2.90(AAHK.1)C0
    • —
    ZyxelGS1900-24EP firmware
    • ≤ 2.90(ABTO.1)C0
    • —
    ZyxelGS1900-24HPv2 firmware
    • ≤ 2.90(ABTP.1)C0
    • —
    ZyxelGS1900-48 firmware
    • ≤ 2.90(AAHN.1)C0
    • —
    ZyxelGS1900-48HPv2 firmware
    • ≤ 2.90(ABTQ.1)C0
    • —
    ZyxelGS1900-8 firmware
    • ≤ 2.90(AAHH.1)C0
    • —
    ZyxelGS1900-8HP firmware
    • ≤ 2.90(AAHI.1)C0
    • —

    Vendors and products

    Affected operating systems

    Affected operating systems
    Otherzyxelgs1900-10hp_firmware—
    Otherzyxelgs1900-16_firmware—
    Otherzyxelgs1900-24_firmware—
    Otherzyxelgs1900-24e_firmware—
    Otherzyxelgs1900-24ep_firmware—
    Otherzyxelgs1900-24hpv2_firmware—
    Otherzyxelgs1900-48_firmware—
    Otherzyxelgs1900-48hpv2_firmware—
    Otherzyxelgs1900-8_firmware—
    Otherzyxelgs1900-8hp_firmware—

    Weakness classes (CWE)

    • CWE-121Stack-based Buffer Overflow

      Variant

      A stack-based buffer overflow condition is a condition where the buffer being overwritten is allocated on the stack (i.e., is a local variable or, rarely, a parameter to a function).

      Entry at MITRE CWE-121

    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

    References & sources

    • zyxel.comglobal/en/support/security-advisories/zyxel-security-advisory-f…Vendor advisory, Vendor advisory
    • greynoise.ioblog/open-season-on-kapibala-attacker-steals-government-records…Third-party advisory, Exploit, Third-party advisory
    • cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2026-7273government-resource, Third-party advisory, US government resource

    Changes to this CVE

    Who changed this CVE and when: NVD (analysis, affected products, CVSS), the CVE publisher (CNA) and additional providers such as CISA (ADP).

    1. Initial analysis by NVDNVD

      Newly listed as affected:
      Show 13 original changes
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Product configurationadded
      —
      AND OR *cpe:2.3:o:zyxel:gs1900-8_firmware:*:*:*:*:*:*:*:* versions up to (excluding) 2.90(aahh.2)c0 OR cpe:2.3:h:zyxel:gs1900-8:-:*:*:*:*:*:*:*
      Product configurationadded
      —
      AND OR *cpe:2.3:o:zyxel:gs1900-8hp_firmware:*:*:*:*:*:*:*:* versions up to (excluding) 2.90(aahi.2)c0 OR cpe:2.3:h:zyxel:gs1900-8hp:-:*:*:*:*:*:*:*
      Product configurationadded
      —
      AND OR *cpe:2.3:o:zyxel:gs1900-10hp_firmware:*:*:*:*:*:*:*:* versions up to (excluding) 2.90(aazi.2)c0 OR cpe:2.3:h:zyxel:gs1900-10hp:-:*:*:*:*:*:*:*
      Product configurationadded
      —
      AND OR *cpe:2.3:o:zyxel:gs1900-16_firmware:*:*:*:*:*:*:*:* versions up to (excluding) 2.90(aahj.2)c0 OR cpe:2.3:h:zyxel:gs1900-16:-:*:*:*:*:*:*:*
      Product configurationadded
      —
      AND OR *cpe:2.3:o:zyxel:gs1900-24_firmware:*:*:*:*:*:*:*:* versions up to (excluding) 2.90(aahl.2)c0 OR cpe:2.3:h:zyxel:gs1900-24:-:*:*:*:*:*:*:*
      Product configurationadded
      —
      AND OR *cpe:2.3:o:zyxel:gs1900-24e_firmware:*:*:*:*:*:*:*:* versions up to (excluding) 2.90(aahk.2)c0 OR cpe:2.3:h:zyxel:gs1900-24e:-:*:*:*:*:*:*:*
      Product configurationadded
      —
      AND OR *cpe:2.3:o:zyxel:gs1900-24ep_firmware:*:*:*:*:*:*:*:* versions up to (excluding) 2.90(abto.2)c0 OR cpe:2.3:h:zyxel:gs1900-24ep:-:*:*:*:*:*:*:*
      Product configurationadded
      —
      AND OR *cpe:2.3:o:zyxel:gs1900-24hpv2_firmware:*:*:*:*:*:*:*:* versions up to (excluding) 2.90(abtp.2)c0 OR cpe:2.3:h:zyxel:gs1900-24hpv2:-:*:*:*:*:*:*:*
      Product configurationadded
      —
      AND OR *cpe:2.3:o:zyxel:gs1900-48_firmware:*:*:*:*:*:*:*:* versions up to (excluding) 2.90(aahn.2)c0 OR cpe:2.3:h:zyxel:gs1900-48:-:*:*:*:*:*:*:*
      Product configurationadded
      —
      AND OR *cpe:2.3:o:zyxel:gs1900-48hpv2_firmware:*:*:*:*:*:*:*:* versions up to (excluding) 2.90(abtq.2)c0 OR cpe:2.3:h:zyxel:gs1900-48hpv2:-:*:*:*:*:*:*:*
      Reference typeadded
      —
      Zyxel Corporation: https://www.zyxel.com/global/en/support/security-advisories/zyxel-security-advisory-for-stack-based-buffer-overflow-vulnerability-in-gs1900-series-switches-06-16-2026 Types: Vendor Advisory
      Reference typeadded
      —
      CISA-ADP: https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2026-7273 Types: Third Party Advisory, US Government Resource
      Reference typeadded
      —
      CISA-ADP: https://www.greynoise.io/blog/open-season-on-kapibala-attacker-steals-government-records-wordpress-exploitation Types: Exploit, Third Party Advisory

      Source identifier: nvd@nist.gov

    2. Record modifiedCISA ADP

      CISA assessment (SSVC): exploitation: active, automatable: no

      Show the original change
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      SSVC assessmentchanged
      {"id":"CVE-2026-7273","role":"CISA Coordinator","options":[{"exploitation":"active"},{"automatable":"no"},{"technicalImpact":"total"}],"version":"2.0.3","timestamp":"2026-09-21T19:40:05.862596Z"}
      {"id":"CVE-2026-7273","role":"CISA Coordinator","options":[{"exploitation":"active"},{"automatable":"no"},{"technicalImpact":"total"}],"version":"2.0.3","timestamp":"2026-06-17T00:00:00+00:00"}

      Source identifier: 134c704f-9b21-4f2e-91b3-4a467353bcc0

    3. CISA KEV entry updated9119a7d8…

      4 individual changes: Date Added, Due Date, Required Action, Vulnerability Name

      Show 4 original changes
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Date Addedadded
      —
      2026-09-21
      Due Dateadded
      —
      2026-09-21
      Required Actionadded
      —
      2026-09-21
      Vulnerability Nameadded
      —
      2026-09-21

      Source identifier: 9119a7d8-5eab-497f-8521-727c672e3725

    4. Record modifiedzyxel.com.tw

      Show 2 original changes
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Referenceadded
      —
      https://www.zyxel.com/global/en/support/security-advisories/zyxel-security-advisory-for-stack-based-buffer-overflow-vulnerability-in-gs1900-series-switches-06-16-2026
      Referenceremoved
      https://www.zyxel.com/global/en/support/security-advisories/zyxel-security-advisory-for-stack-based-buffer-overflow-vulnerability-in-gs1900-series-switches-06-16-2026
      —

      Source identifier: security@zyxel.com.tw

    5. Record modifiedCISA ADP

      CISA assessment (SSVC): exploitation: active, automatable: no

      Show 3 original changes
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Referenceadded
      —
      https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2026-7273
      Referenceadded
      —
      https://www.greynoise.io/blog/open-season-on-kapibala-attacker-steals-government-records-wordpress-exploitation
      SSVC assessmentchanged
      {"id":"CVE-2026-7273","role":"CISA Coordinator","options":[{"exploitation":"none"},{"automatable":"no"},{"technicalImpact":"total"}],"version":"2.0.3","timestamp":"2026-06-17T00:00:00+00:00"}
      {"id":"CVE-2026-7273","role":"CISA Coordinator","options":[{"exploitation":"active"},{"automatable":"no"},{"technicalImpact":"total"}],"version":"2.0.3","timestamp":"2026-09-21T19:40:05.862596Z"}

      Source identifier: 134c704f-9b21-4f2e-91b3-4a467353bcc0

    Show one older change

    1. Receivedzyxel.com.tw

      CVSS V3.1: AV:A/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

      Weakness type: CWE-121

      Description added or changed

      Show 4 original changes
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Descriptionadded
      —
      A stack-based buffer overflow vulnerability in the CGI program of Zyxel GS1900-48HPv2 firmware versions through 2.90(ABTQ.1)C0 could allow a LAN-based, unauthenticated attacker to exploit the flaw and potentially execute OS commands via a crafted HTTP request.
      CVSSadded
      —
      AV:A/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
      Weakness type (CWE)added
      —
      CWE-121
      Referenceadded
      —
      https://www.zyxel.com/global/en/support/security-advisories/zyxel-security-advisory-for-stack-based-buffer-overflow-vulnerability-in-gs1900-series-switches-06-16-2026

      Source identifier: security@zyxel.com.tw

    One NVD translation into another language is hidden.

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