Overview
An Out-of-bounds Write vulnerability in the InsydeH2O IHISI software used by HP PC BIOS can allow a local attacker with OS kernel privileges to perform arbitrary physical memory writes, including writes to System Management RAM (SMRAM). Because the vulnerable code executes in System Management Mode (SMM), successful exploitation can allow an attacker to modify SMM-protected memory and potentially achieve arbitrary code execution in SMM. Depending on the platform's memory and firmware configuration, the arbitrary physical memory write primitive may also have implications for UEFI firmware update or flash-related operations.
Description
HP PC BIOS is firmware that initializes and manages hardware components during the boot process and provides runtime services to the operating system and provide the ability to update firmware. The affected system uses InsydeH2O Kernel version 5.5 or earlier. The BIOS includes custom HP SMM handlers that execute in System Management Mode (SMM), a highly privileged CPU execution mode that is isolated from the operating system.
CVE-2026-12855: An Out-of-bounds Write vulnerability in the H19WMIHandlerSmm module (GUID f1946499-571b-44c3-9b9c-cc55210b0c02) allows a local attacker with OS kernel privileges to read or write arbitrary physical memory, including SMRAM, through a Software SMI handler.
An attacker with kernel-level privileges can trigger the vulnerable SMM handler by issuing a Software System Management Interrupt (SMI) through I/O port 0xB2 and supplying specially crafted CPU register values. The vulnerable handler does not adequately validate the supplied parameters before performing memory operations, allowing the attacker to influence the physical address and data involved in the operation.
Because the affected handler executes in SMM, the resulting arbitrary physical memory write can target memory regions that are normally inaccessible to software executing outside SMM, including SMRAM. Modifying SMM code or data may allow an attacker to alter subsequent SMM execution and potentially achieve arbitrary code execution in SMM. The ability to affect firmware or ROM contents is platform-dependent and is not assumed as a direct consequence of this vulnerability.
Impact
An attacker with privileged OS kernel access (ring 0) can exploit the vulnerability by raising Software SMI interrupts through I/O port 0xB2 with crafted CPU register values. Modifying SMM code or data may allow an attacker to alter subsequent SMM execution and potentially achieve arbitrary code execution and persistence via modifying SMRAM.
Solution
Users should check HP's security bulletins to determine whether their system is affected. Insyde advisory is available at https://www.insyde.com/security-pledge/sa-2026009/
Acknowledgements
Thank you to Zhenyu Liu for reporting these vulnerabilities. This document was written by Vijay Sarvepalli.
Other Information
| CVE IDs: | CVE-2026-12855 |
| API URL: | VINCE JSON | CSAF |
| Date Public: | 2026-10-01 |
| Date First Published: | 2026-10-01 |
| Date Last Updated: | 2026-10-01 14:33 UTC |
| Document Revision: | 1 |