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

Should you act?

Exploited: patch now if you run it

CISA has confirmed exploitation in the wild. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

• Yes, if you run Zyxel GS1900 Series Switches. CISA added this CVE to its Known Exploited Vulnerabilities catalog on 2026-09-21, which means there is reliable evidence of exploitation in the wild. US federal agencies must remediate by 2026-09-24; treat that as the outer limit, not the target.
• EPSS 2.5%, percentile 85: FIRST’s model estimates a 2.5% chance of exploitation activity in the next 30 days, higher than 85% of all scored CVEs (elevated). EPSS is a probability, not a verdict.
• Editorial verdict: Check exposure. Zyxel GS1900 Switches: Exploited CGI Overflow (KEV) — Patch every GS1900 to the June firmware, but the urgency depends on reach: if the switch's web interface is exposed to the internet, a guest network or a flat LAN, treat it as compromised until checked; if only a locked-down admin VLAN can reach it, patch on a normal cycle this month.

What it is

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.

CISA’s summary

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.

Weakness type: CWE-121.

Product: Zyxel GS1900 Series Switches · Assigned by: Zyxel · Published: 2026-06-16

Affected and fixed versions

Severity scores

• Attack vector: adjacent network only
• Attack complexity: low
• Privileges required: none (no login)
• User interaction: none
• Scope: unchanged

CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.

Exploitation and urgency

• CISA KEV: added 2026-09-21; federal due date 2026-09-24. Known ransomware use: Unknown.
• CISA 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 SSVC assessment: exploitation active, automatable no, technical impact total.
• FIRST EPSS (model of 2026-10-21): 2.50% probability of exploitation activity in the next 30 days; percentile 85.

Timeline

• 2026-06-16 — CVE record published by Zyxel
• 2026-06-16 — erroneo.us brief: Zyxel GS1900 Switches: Exploited CGI Overflow (KEV)
• 2026-09-21 — Added to CISA’s Known Exploited Vulnerabilities catalog
• 2026-09-22 — CVE record last updated
• 2026-09-24 — CISA remediation deadline for US federal agencies

Sources and references

Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.

Vendor and project advisories, patches

1. zyxel.com zyxel.com

Vulnerability records

1. CVE.org record CVE-2026-7273 cve.org
2. NVD entry CVE-2026-7273 nvd.nist.gov

Government

1. CISA Known Exploited Vulnerabilities catalog entry cisa.gov
2. Listed in CISA KEV notes (cisa.gov) cisa.gov
3. Listed in CISA KEV notes (cisa.gov) cisa.gov

Other references from the CVE record and advisory

1. greynoise.io greynoise.io

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