Zyxel GS1900 Series Switches Stack-Based Buffer Overflow Vulnerability

[ Why it matters ]Priority signals

• Exploited in the wild: on CISA's Known Exploited Vulnerabilities list since 2026-09-21

[ Description ]Official 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.

CISA: 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.

[ Fixes ]Fixes and patches

CISA required action (US federal deadline 24 Sep 2026): 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.

No fix or interim patch is linked from our sources yet. Check the vendor references below and limit exposure of affected systems until a fix is available.

[ References ]References

• www.zyxel.com/global/en/support/security-advisories/zyxel-security-advisory-for-stack-b…Vendor Advisory
• www.cisa.gov/known-exploited-vulnerabilities-catalogThird Party AdvisoryUS Government Resource
• www.greynoise.io/blog/open-season-on-kapibala-attacker-steals-government-records-wordpr…ExploitThird Party Advisory