---
id: CVE-2026-16659
title: >-
  Pre-authentication NULL pointer dereference in MCUboot serial-recovery image
  upload (encrypted-image builds)
summary: >-
  MCUboot's serial-recovery image-upload handler bs_upload() in
  boot/boot_serial/src/boot_serial.c declares const struct flash_area *fap =
  NULL and assigns it only via flash_area_open() part-way through the function.
  Two early validation f…
severity: medium
cvss: 4
cvssVector: 'CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L'
cvssSource: cna
cwe:
  - CWE-476
vendor: mcu-tools
product: mcuboot
affected:
  - mcuboot >= 2.0.0 < 2.5.0
published: '2026-11-13'
updated: '2026-11-13'
sourceUpdated: '2026-11-13T00:23:03.881Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2026-16659'
references:
  - url: >-
      https://github.com/mcu-tools/mcuboot/commit/400f9a7242641655948b08acac19932f56d9881f
    label: Fix commit
  - url: >-
      https://github.com/mcu-tools/mcuboot/security/advisories/GHSA-777r-3chm-p78c
    label: GHSA-777r-3chm-p78c
tags:
  - cve.org
ingestedAt: '2026-11-13T01:19:02.074Z'
---

## Overview

MCUboot's serial-recovery image-upload handler bs_upload() in boot/boot_serial/src/boot_serial.c declares const struct flash_area *fap = NULL and assigns it only via flash_area_open() part-way through the function. Two early validation failures (goto out_invalid_data) — a CBOR map that fails to decode, or an upload packet missing the off/data members — branch past the open while fap is still NULL, then fall into the shared epilogue. In MCUBOOT_ENC_IMAGES builds that epilogue unconditionally calls boot_handle_enc_fw(fap), which reaches boot_image_load_header() and flash_area_read() and dereferences the NULL flash_area.

The defect is reachable pre-authentication: the trigger is the very first serial-recovery upload command carrying a malformed payload. The guarding conditions are default-true on a fresh recovery session — img_num, curr_off and img_size are zero-initialized statics, so curr_off == img_size and the primary-slot test both hold. It requires a non-default build: encrypted images (MCUBOOT_ENC_IMAGES / Zephyr CONFIG_BOOT_ENCRYPT_IMAGE) together with an active serial-recovery transport, on the Zephyr or Mynewt port (the two that build both boot/boot_serial/src/boot_serial.c and boot_serial_encryption.c with MCUBOOT_ENC_IMAGES).

The impact is a transient denial of service of the bootloader during recovery. fap is a hard NULL rather than an attacker-controlled value, so there is no memory-corruption or information-leak primitive and image authentication is unaffected. Whether the dereference faults is platform-dependent — on targets where address 0x0 is mapped readable flash the read merely returns an error and execution continues, while targets that trap NULL (MPU / null-pointer-exception detection) take a hard fault. Because serial recovery is unauthenticated by design and already permits the console attacker to write the secondary slot, the added capability is only a low-value transient crash.

The related unconditional flash_area_close(fap) call in the same epilogue is a safe no-op on the ports that reach it, so it does not contribute to the dereference.

The affected ports are the Zephyr port — boot/zephyr/CMakeLists.txt compiles boot_serial.c under CONFIG_MCUBOOT_SERIAL and boot_serial_encryption.c under CONFIG_BOOT_ENCRYPT_IMAGE — and the Mynewt port, whose mcuboot_config.h defines MCUBOOT_SERIAL unconditionally and MCUBOOT_ENC_IMAGES under any BOOTUTIL_ENCRYPT_ syscfg, with boot/boot_serial/pkg.yml declaring no explicit source list so the whole directory is built. The Espressif port is not affected**: it compiles boot_serial.c but not boot_serial_encryption.c, and defines no MCUBOOT_ENC_IMAGES, so the #ifdef MCUBOOT_ENC_IMAGES block containing the call is preprocessed away. The NuttX, Cypress and Mbed ports do not build boot_serial.c at all.

## Affected

- `mcuboot >= 2.0.0 < 2.5.0`

## Remediation

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