CVE-2026-21239
Published: 10 February 2026
Summary
CVE-2026-21239 is a high-severity Heap-based Buffer Overflow (CWE-122) vulnerability in Microsoft Windows 10 21H2. Its CVSS base score is 7.8 (High).
Operationally, exploitation aligns with the MITRE ATT&CK technique Exploitation for Privilege Escalation (T1068); ranked at the 11.6th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.
The strongest mitigations our analysis identified are NIST 800-53 SI-16 (Memory Protection) and SI-2 (Flaw Remediation).
Threat & Defense at a Glance
Threat & Defense Details
Mitigating Controls (NIST 800-53 r5)AI
Timely remediation of flaws through patching directly addresses the heap-based buffer overflow vulnerability in the Windows Kernel as per Microsoft's update guidance.
Memory protection mechanisms such as ASLR, DEP, and heap isolation directly mitigate exploitation of heap-based buffer overflows in the kernel.
Enforcement of least privilege limits the scope and impact of local low-privilege attackers attempting kernel privilege escalation.
MITRE ATT&CK Enterprise TechniquesAI
Why these techniques?
Local kernel heap buffer overflow directly enables privilege escalation from low-privileged local context.
NVD Description
Heap-based buffer overflow in Windows Kernel allows an authorized attacker to elevate privileges locally.
Deeper analysisAI
CVE-2026-21239 is a heap-based buffer overflow vulnerability in the Windows Kernel, published on 2026-02-10T18:16:24.780. It carries a CVSS v3.1 base score of 7.8 (AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H) and is associated with CWE-122 (Heap-based Buffer Overflow) and CWE-787 (Out-of-bounds Write).
An authorized local attacker with low privileges (PR:L) can exploit this vulnerability due to its low attack complexity (AC:L) and lack of required user interaction (UI:N). Successful exploitation enables local privilege escalation, granting high impacts on confidentiality, integrity, and availability (C:H/I:H/A:H) without changing scope (S:U).
Microsoft provides mitigation guidance in their update guide at https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-21239.
Details
- CWE(s)