CVE-2026-127916mcu-tools mcuboot out-of-bounds read
CVE-2026-127916 is a out-of-bounds read 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 reads past the end of a buffer, leaking memory contents or crashing.
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
bootutil_img_hash() computes the image digest over ih_hdr_size + ih_img_size + ih_protect_tlv_size bytes, all three taken from the candidate image's header, and reads that span out of the slot without checking that it fits in the flash area. The three sizes are also summed without an overflow check. Nothing earlier on the validation path bounds the whole span either: bootutil_img_validate() compares the image size against bootutil_max_image_size() only after the hash has been computed.
The callers bound part of the span, or none of it:
On the swap and overwrite upgrade path, boot_validate_slot() first runs the image header check (boot_is_header_valid(), later boot_check_header_valid()). Before MCUboot 2.2.0 that check covered only ih_hdr_size + ih_img_size, so for every image the protected-TLV area (ih_protect_tlv_size, a uint16_t, up to 65535 bytes) was hashed without being bounded. From 2.2.0 the check includes ih_protect_tlv_size, except for a compressed image in the secondary slot when MCUBOOT_DECOMPRESS_IMAGES (CONFIG_BOOT_DECOMPRESSION) is enabled, which leaves the same gap of up to 64 KB. The serial recovery image-list and image-state handlers (bs_list() and bs_set() in boot/boot_serial/src/boot_serial.c) and Zephyr's single-slot and firmware loaders (boot/zephyr/single_loader.c, boot/zephyr/firmware_loader.c) call bootutil_img_validate() with no header check at all, so on those paths ih_img_size is unbounded as well.
The image is hashed before its signature is checked, so anyone who can place an image in a slot (an OTA or SMP update delivered through the application, or serial recovery) reaches this read before authentication.
Whether a malformed header produces an actual read beyond the area depends on the flash backend:
Backends whose flash_area_read() bounds-checks the request (Zephyr, Espressif, NuttX, Mbed) return an error once the chunked read crosses the end of the area. Validation fails and the image is rejected, with no over-read. With MCUBOOT_HASH_STORAGE_DIRECTLY (Zephyr CONFIG_BOOT_IMG_HASH_DIRECTLY_ON_STORAGE, available from 2.2.0, not enabled by default) the span is hashed through a single direct pointer into memory-mapped flash, with no bounds check. The Cypress PSoC6 port's flash_area_read() (boot/cypress/cy_flash_pal/cy_flash_map.c, from 1.6.0) does not bound the read: its assert()s compare against the area's absolute offset rather than its size, and it copies with memcpy().
The bytes read past the area feed only the digest, which is compared internally and never returned, so nothing is disclosed. The realistic effect is a read fault during image validation: up to 64 KB past the slot on the upgrade path, and potentially much further on the paths that skip the header check. Where validation is repeated at every boot (a pending upgrade, or the single-slot loader checking its image), the fault can recur on each reset and keep the device from booting.
The fix makes bootutil_img_hash() add the three sizes with boot_u32_safe_add() and reject an image whose span, including the sector offset used in swap-using-offset mode, does not fit in the flash area, before any of it is hashed. Because the bound is applied where the bytes are read, it covers every caller.
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.