---
id: CVE-2026-16671
title: >-
  Out-of-bounds stack write in MCUboot split-image validation with
  SHA-384/SHA-512 builds
summary: >-
  split_image_check() in boot/bootutil/src/loader.c declares a fixed uint8_t
  loader_hash[32] buffer and passes it as the out_hash out-parameter to
  bootutil_img_validate(). That routine copies IMAGE_HASH_SIZE bytes into
  out_hash (memcpy(out…
severity: low
cvss: 2.9
cvssVector: 'CVSS:3.1/AV:L/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L'
cvssSource: cna
cwe:
  - CWE-787
vendor: mcu-tools
product: mcuboot
affected:
  - mcuboot >= 2.0.0 < 2.5.0
published: '2026-11-13'
updated: '2026-11-13'
sourceUpdated: '2026-11-13T00:23:02.120Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2026-16671'
references:
  - url: >-
      https://github.com/mcu-tools/mcuboot/commit/9fb70f00173766d73cdadd82266bd61e4ddd0252
    label: Fix commit
  - url: >-
      https://github.com/mcu-tools/mcuboot/security/advisories/GHSA-vj6q-r752-fpqv
    label: GHSA-vj6q-r752-fpqv
tags:
  - cve.org
ingestedAt: '2026-11-13T01:19:02.073Z'
---

## Overview

split_image_check() in boot/bootutil/src/loader.c declares a fixed uint8_t loader_hash[32] buffer and passes it as the out_hash out-parameter to bootutil_img_validate(). That routine copies IMAGE_HASH_SIZE bytes into out_hash (memcpy(out_hash, hash, IMAGE_HASH_SIZE) in boot/bootutil/src/image_validate.c), where IMAGE_HASH_SIZE is a compile-time constant of 64 for MCUBOOT_SHA512 or 48 for MCUBOOT_SIGN_EC384/SHA-384 (boot/bootutil/include/bootutil/crypto/sha.h). The buffer tracks SHA-256 (32 bytes) while the copy length tracks the configured algorithm, so the write overruns the buffer by 16 or 32 bytes.

The overflow is only present in a non-default configuration: the legacy Mynewt split-image feature must be compiled (split_go()/split_image_check(), i.e. neither MCUBOOT_DIRECT_XIP nor MCUBOOT_RAM_LOAD), the digest must be larger than 32 bytes (MCUBOOT_SHA512 or MCUBOOT_SIGN_EC384), and MCUBOOT_SIGN_PURE must be off. split_go() is a public API with no caller inside MCUboot; it is invoked from a Mynewt application. In the default SHA-256 build there is no overflow.

The over-written bytes are the one-way digest of the loader image and the overflow length is fixed at build time, so the contents are not meaningfully attacker-controlled and the size is not attacker-chosen. The realistic impact is bounded, uncontrolled corruption of the bootloader's stack on the split-boot path, manifesting as instability or failed boot in the affected configuration rather than reliable code execution. The fix is to size loader_hash to IMAGE_HASH_SIZE and pass IMAGE_HASH_SIZE as the matching seed length.

## Affected

- `mcuboot >= 2.0.0 < 2.5.0`

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.