SB2026021079 - Multiple vulnerabilities in Microsoft Windows Hyper-V

Description

This security bulletin contains information about 4 vulnerabilities.

1) Improper access control (CVE-ID: CVE-2026-21255)

CWE-ID: CWE-284 - Improper Access Control

CVSSv4: CVSS:4.0/AV:L/AC:L/AT:N/PR:L/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N/E:U/U:Clear

The vulnerability allows a local user to gain unauthorized access to otherwise restricted functionality.

The vulnerability exists due to improper access restrictions in Windows Hyper-V. A local user can bypass implemented security restrictions and gain unauthorized access to the application.

2) Input validation error (CVE-ID: CVE-2026-21247)

CWE-ID: CWE-20 - Improper input validation

CVSSv4: CVSS:4.0/AV:L/AC:L/AT:N/PR:L/UI:A/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N/E:U/U:Green

The vulnerability allows a remote attacker to execute arbitrary code on the target system.

The vulnerability exists due to insufficient validation of user-supplied input in Windows Hyper-V. A remote user can trick a victim to load a specially crafted file and execute arbitrary code on the system.

3) Heap-based buffer overflow (CVE-ID: CVE-2026-21248)

CWE-ID: CWE-122 - Heap-based Buffer Overflow

CVSSv4: CVSS:4.0/AV:L/AC:L/AT:N/PR:L/UI:A/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N/E:U/U:Green

The vulnerability allows a remote attacker to execute arbitrary code on the target system.

The vulnerability exists due to a boundary error in Windows Hyper-V. A remote user can trick a victim to load a specially crafted file, trigger a heap-based buffer overflow and execute arbitrary code on the target system.

Successful exploitation of this vulnerability may result in complete compromise of vulnerable system.

4) Heap-based buffer overflow (CVE-ID: CVE-2026-21244)

CWE-ID: CWE-122 - Heap-based Buffer Overflow

CVSSv4: CVSS:4.0/AV:L/AC:L/AT:N/PR:L/UI:A/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N/E:U/U:Green

The vulnerability allows a remote attacker to execute arbitrary code on the target system.

The vulnerability exists due to a boundary error in Windows Hyper-V. A remote user can trick a victim to load a specially crafted file, trigger a heap-based buffer overflow and execute arbitrary code on the target system.

Successful exploitation of this vulnerability may result in complete compromise of vulnerable system.

Remediation

Install update from vendor's website.

References

• https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-21255
• https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-21247
• https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-21248
• https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-21244