# CVE-2026-16671: Out-of-bounds stack write in MCUboot split-image validation with SHA-384/SHA-512 builds

Status: published. Published: 2026-11-13. Updated: 2026-11-13.

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.

## Remediation

- fix: The reporting source identifies this reference as a patch. Check the advisory for affected versions and supported release branches. (source: CNA: zephyr) — https://github.com/mcu-tools/mcuboot/commit/9fb70f00173766d73cdadd82266bd61e4ddd0252

## Affected products and versions

- mcu-tools / mcuboot (mcuboot): ≥ 2.0.0; < 2.5.0 (affected, semver). Source: CNA: zephyr

## Severity and prioritization

- CVSS 3.1: 2.9 (low); CVSS:3.1/AV:L/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L; source: CNA: zephyr

- CISA KEV: No listing in collected sources

- EPSS: Not available

- SSVC: Not available

- Weaknesses: CWE-787

## Exploit evidence

No exploit evidence in collected sources. This is not proof of no exploitation.

## References

- https://github.com/mcu-tools/mcuboot/commit/9fb70f00173766d73cdadd82266bd61e4ddd0252 (CNA: zephyr; patch)

- https://github.com/mcu-tools/mcuboot/security/advisories/GHSA-vj6q-r752-fpqv (CNA: zephyr)

## Provenance

- CNA: zephyr: https://www.cve.org/CVERecord?id=CVE-2026-16671; retrieved 2026-11-13; source updated 2026-11-13

Revision: ce6fde791422524a5a79e4686f97268853d906c536345129b2f5fb53a4968ce4

Canonical: https://y2.dev/cve/CVE-2026-16671

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