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Used in attacks· fix by 2026-09-248.8High

Zyxel GS1900 Series Switches

CVE-2026-7273

Zyxel GS1900 series switches contain a stack-based buffer overflow vulnerability in the CGI program which could allow a LAN-based, unauthenticated attacker to exploit the flaw and potentially execute OS commands via a crafted HTTP request.

What this means for your business

  • It affects Zyxel GS1900 Series Switches. It matters if your company, or a supplier that handles your data, runs it.
  • Attackers are already using it. CISA added it to its list of flaws used in real attacks on 21 September 2026.
  • An attacker can use it from inside the same network, without a login, and without anyone at your company clicking anything.
  • FIRST's prediction model gives it a 3% chance of attack attempts being seen in the next 30 days, ranking above 86% of all known flaws.

What to do

  1. 1Check whether your company or your suppliers run Zyxel GS1900 Series Switches, and which version. The affected versions are listed further down this page.
  2. 2If you do, apply the vendor's fix. A patch or vendor advisory has been published.
  3. 3Treat it as urgent. US federal agencies must fix it by 24 September 2026. That deadline does not apply to private companies, but it shows how seriously CISA takes it.
  4. 4Because attackers have used it, check the affected systems for signs of a break-in, not only for the update.

Fastnexa security experts

Attackers are already using this flaw. Is your Zyxel GS1900 Series Switches exposed?

Tell us where you run Zyxel GS1900 Series Switches and a Fastnexa penetration tester will check whether this flaw, or others like it, can be used against your websites, apps and network.

The full test is free for our first 10 founding clients until 31 December 2026. See the offer

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Required action

Apply mitigations in accordance with vendor instructions, ensuring compliance with CISA’s BOD 26-04 Prioritizing Security Updates Based on Risk (see URL in Notes) guidance and CISA’s “Forensics Triage Requirements” (see URL in Notes). Follow applicable BOD 26-04 guidance for cloud services or discontinue use of the product if mitigations are unavailable. Stakeholders are responsible for evaluating each asset's internet exposure and ensuring adherence to BOD 26-04 patching guidelines.

CISA’s wording, unedited. The notes it mentions are in CISA’s catalogue entry.

Scoring

CVSS
8.8 (v3.1)
Vector
CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
Weakness
CWE-121
Assigned by
security@zyxel.com.tw

Dates

Published
2026-09-21
Last modified
2026-09-22
Added to KEV
2026-09-21
Remediation due
2026-09-24
Sources
CISA KEV, NVD

Affected products

  • Zyxel Gs1900-8 Firmware- 2.90\(aahh.2\)c0
  • Zyxel Gs1900-8all versions
  • Zyxel Gs1900-8hp Firmware- 2.90\(aahi.2\)c0
  • Zyxel Gs1900-8hpall versions
  • Zyxel Gs1900-10hp Firmware- 2.90\(aazi.2\)c0
  • Zyxel Gs1900-10hpall versions
  • Zyxel Gs1900-16 Firmware- 2.90\(aahj.2\)c0
  • Zyxel Gs1900-16all versions
  • Zyxel Gs1900-24 Firmware- 2.90\(aahl.2\)c0
  • Zyxel Gs1900-24all versions
  • Zyxel Gs1900-24e Firmware- 2.90\(aahk.2\)c0
  • Zyxel Gs1900-24eall versions

As listed in the NVD configuration data. Not a statement about your estate.

Coverage

References