CVE-2026-16671mcu-tools mcuboot out-of-bounds write
CVE-2026-16671 is a out-of-bounds write in mcu-tools mcuboot, rated low (2.9 on CVSS 3.1).
Why this priority: Low severity, with no exploitation signal.
Published · Updated · NVD status: Received
What it is
Code writes past the end of a buffer, corrupting memory in ways an attacker can often turn into code execution.
How it is reached
An attacker can exploit it by someone who can already run code or sign in on the machine, without an account, and nobody has to click anything. Success also depends on conditions the attacker does not fully control.
What an attacker gains
The direct impact is limited: partial access to data or a partial outage. In practice, this kind of flaw lets an attacker crash the process, read memory, or in the worst case run code.
Is it being exploited?
There is no public report of exploitation yet.
What to do
- Upgrade mcu-tools mcuboot to 2.5.0 or later.
- Limit who can run code or sign in on the hosts that run it.
Description from NVD
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.
What this means for your architecture.
A CVE names a product. Whether it matters depends on where that product sits in your systems: what reaches it, which trust boundary it guards and what it can reach once compromised. Veriom maps your components, their relationships and your trust boundaries from your code, cloud and delivery evidence, so you can see where a component like this sits. The radar itself does not look at your systems.
Components it touches
- Native code: operating systems, browsers and firmware. The flaw sits wherever native code reads untrusted data.
- Network appliances and media or file parsers. The flaw sits wherever native code reads untrusted data.
Trust boundaries in play
- Host boundary. The attacker needs a foothold on the machine first; the flaw turns a small foothold into a bigger one.
- Wherever native code reads untrusted data. An attacker who crosses it can crash the process, read memory, or in the worst case run code.
Questions to answer about your own systems
- Where does mcu-tools mcuboot run in your systems, and at which version?
- Which of those hosts also run code or users you do not fully trust?
- What can it reach once compromised: which data stores, secrets and services?
Sources
- CVE records and CVSS scores from the NVD CVE API. This product uses the NVD API but is not endorsed or certified by the NVD.
- Exploitation status from CISA’s Known Exploited Vulnerabilities catalog.
- Exploit likelihood from FIRST’s Exploit Prediction Scoring System (EPSS), refreshed daily.
- Explanations are written by Veriom from fixed templates over each record’s data, not by a person reviewing each CVE. Descriptions are quoted from the records as published.