Mcu-tools Mcuboot: Use of Uninitialized Variable (CVE-2026-16662)
Published Last modified Sources: EUVD, CVE.org
What is affected, what to do
boot_save_boot_status() in boot/bootutil/src/boot_record.c reads the image's EXPECTED_HASH_TLV into a stack buffer image_hash[IMAGE_HASH_SIZE] that is never zero-initialised. The TLV handler validates only the upper bound (len > sizeof(image_hash)) and then reads exactly len bytes, so a TLV shorter than IMAGE_HASH_SIZE leaves the tail of image_hash holding stale stack content.
Update Mcu-tools Mcuboot to version 2.5.0.
- Affected
- Mcu-tools Mcuboot
lt 1.6.0..2.5.0
- Mcu-tools Mcuboot
- Fixed in
- Mcu-tools Mcuboot
2.5.0
- Mcu-tools Mcuboot
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Metrics
2.4
XIEM Effective CVSS
Highest available CVSS rating from NVD and vendor data.
Source: Vendor (CNA), CVSS v3
2.4
CVSS v3
Severity Low on the scale from 0 to 10.
Source: EUVD, CVE.org
—
EPSS score
No value is available for this vulnerability.
Show all metrics
- Severity
- Low
Priority (0–325)
The priority score is a consolidated metric that condenses several individual signals into a single, action-guiding ranking: technical severity (CVSS), exploitation likelihood (EPSS), known-exploited status (KEV) and the context of the affected asset. The aim is to answer the only truly important question in vulnerability management: what first?
Read in glossary- 42.00 Methodology
Exploit maturity
An exploit is code or a technique that specifically uses a concrete vulnerability to force unintended behaviour — such as executing code or escalating privileges. It is the tool that turns a theoretical flaw into a practical attack.
Read in glossary- no public PoC known
XIEM Effective CVSS
XIEM® is the tiered managed-security operating model from NEOSEC and a registered trademark of NEOSEC GmbH. It extends classic SIEM — the central collection, normalisation and correlation of security-relevant events — with endpoint forensics, network visibility and deception technology, and consolidates them into three building tiers: Sentry, Orchestrate and Command.
Read in glossary- 2.4 Methodology
CVSS v3
CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions.
Read in glossary- 2.4
KEV status
The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.
Read in glossary- —
- Published
CWE
CWE is a catalogue of typical weakness types in software and hardware — such as flawed input validation, unsafe memory handling or poor authentication. Where a CVE names a concrete vulnerability in a concrete product, a CWE describes the underlying class of defect.
Read in glossary- CWE-457
Detection rules
No NEOSEC detection rule is available for this vulnerability yet.
Classification under NIS2 and CRA
Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.
NIS2
NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management.
Read in glossary- Handle the vulnerability in regular vulnerability handling under Art. 21(2)(e) NIS2.
CRA
The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates.
Read in glossary- From 11.12.2027, Annex I Part II CRA requires manufacturers of affected products with digital elements to handle vulnerabilities without delay and provide security updates.
ISO/IEC 27001
ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security.
Read in glossary- Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.
Description
boot_save_boot_status() in boot/bootutil/src/boot_record.c reads the image's EXPECTED_HASH_TLV into a stack buffer image_hash[IMAGE_HASH_SIZE] that is never zero-initialised. The TLV handler validates only the upper bound (len > sizeof(image_hash)) and then reads exactly len bytes, so a TLV shorter than IMAGE_HASH_SIZE leaves the tail of image_hash holding stale stack content. The subsequent copy into the boot record, memcpy(buf + offset, image_hash, sizeof(image_hash)), uses the fixed buffer size rather than len, so up to IMAGE_HASH_SIZE - 1 bytes of uninitialised bootloader stack are written into the measured-boot record. The defect is reachable only under MCUBOOT_MEASURED_BOOT, and only when the image reaching boot_save_boot_status() was not hash-validated on that boot. bootutil_img_validate() rejects a hash TLV whose length is not exactly sizeof(hash) (boot/bootutil/src/image_validate.c:345), and primary-slot validation is on by default (Zephyr's CONFIG_BOOT_VALIDATE_SLOT0 defaults to y), so in a default build the malformed image never reaches this code. The direct-XIP and RAM-load strategies in boot/bootutil/src/loader.c validate the selected slot unconditionally and are not affected. The short TLV survives only where the integrator has turned primary-slot validation off: the swap and overwrite loader built without MCUBOOT_VALIDATE_PRIMARY_SLOT, which checks only the image magic before calling boot_add_shared_data(), and the Zephyr single-slot loader (boot/zephyr/single_loader.c) built without it, including the MCUBOOT_VALIDATE_PRIMARY_SLOT_ONCE variant, which skips validation once the trailer records an earlier successful one. It is also reachable through a time-of-check/time-of-use change to tamperable external flash between validation and the boot-record read. Placing the crafted image requires physical or flash-write access to the device; in a single-slot build with serial recovery enabled, the serial-recovery upload writes the primary slot directly and can place it as well. In these configurations the boot record's measurement is already not trustworthy: with validation off, the hash copied into the record is the image's own unverified EXPECTED_HASH_TLV, which the attacker chooses. The defect therefore does not weaken attestation integrity any further. What it adds is a path for bootloader memory to leave the device: up to 63 bytes (IMAGE_HASH_SIZE - 1) of uninitialised bootloader stack are embedded in the measurement field that a downstream attestation service sends to a remote verifier. In the configurations analysed the residue is bootloader internal state (TLV offsets and lengths, flash addresses, loop state), not key material, but what occupies that stack region depends on configuration and on which code ran before boot_save_boot_status(). It is reported as a low-severity, defense-in-depth disclosure with no integrity or availability impact. This is distinct from the already-fixed record_len integer underflow in the same routine (GHSA-8hrv-4cp5-4rg3). The fix rejects any EXPECTED_HASH_TLV whose length is not exactly IMAGE_HASH_SIZE.
Source: ENISA EUVD, NVD (NIST), cvelistv5
Affected products
The following versions are affected. Older releases of the same product line are also vulnerable unless stated otherwise.
| Affected versions | Fixed in | ||
|---|---|---|---|
| mcu-tools | mcuboot |
|
|
Vendors and products
Weakness classes (CWE)
CWE-457Use of Uninitialized Variable
Variant
The code uses a variable that has not been initialized, leading to unpredictable or unintended results.
Entry at MITRE CWE-457
References & sources
- github.comFix commiteuvd, Patch
- github.comGHSA-779w-4x76-5chfeuvd
- NVD (NIST)vuln/detail/CVE-2026-16662euvd
Changes to this CVE
Who changed this CVE and when: NVD (analysis, affected products, CVSS), the CVE publisher (CNA) and additional providers such as CISA (ADP).
Receivedzephyrproject.org
CVSS V3.1: AV:P/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N
Weakness type: CWE-457
Description added or changed
Show 6 original changes
Individual changes of this event as published by NVD Type Action Before After Description added —boot_save_boot_status() in boot/bootutil/src/boot_record.c reads the image's EXPECTED_HASH_TLV into a stack buffer image_hash[IMAGE_HASH_SIZE] that is never zero-initialised. The TLV handler validates only the upper bound (len > sizeof(image_hash)) and then reads exactly len bytes, so a TLV shorter than IMAGE_HASH_SIZE leaves the tail of image_hash holding stale stack content. The subsequent copy into the boot record, memcpy(buf + offset, image_hash, sizeof(image_hash)), uses the fixed buffer size rather than len, so up to IMAGE_HASH_SIZE - 1 bytes of uninitialised bootloader stack are written into the measured-boot record. The defect is reachable only under MCUBOOT_MEASURED_BOOT, and only when the image reaching boot_save_boot_status() was not hash-validated on that boot. bootutil_img_validate() rejects a hash TLV whose length is not exactly sizeof(hash) (boot/bootutil/src/image_validate.c:345), and primary-slot validation is on by default (Zephyr's CONFIG_BOOT_VALIDATE_SLOT0 defaults to y), so in a default build the malformed image never reaches this code. The direct-XIP and RAM-load strategies in boot/bootutil/src/loader.c validate the selected slot unconditionally and are not affected. The short TLV survives only where the integrator has turned primary-slot validation off: the swap and overwrite loader built without MCUBOOT_VALIDATE_PRIMARY_SLOT, which checks only the image magic before calling boot_add_shared_data(), and the Zephyr single-slot loader (boot/zephyr/single_loader.c) built without it, including the MCUBOOT_VALIDATE_PRIMARY_SLOT_ONCE variant, which skips validation once the trailer records an earlier successful one. It is also reachable through a time-of-check/time-of-use change to tamperable external flash between validation and the boot-record read. Placing the crafted image requires physical or flash-write access to the device; in a single-slot build with serial recovery enabled, the serial-recovery upload writes the primary slot directly and can place it as well. In these configurations the boot record's measurement is already not trustworthy: with validation off, the hash copied into the record is the image's own unverified EXPECTED_HASH_TLV, which the attacker chooses. The defect therefore does not weaken attestation integrity any further. What it adds is a path for bootloader memory to leave the device: up to 63 bytes (IMAGE_HASH_SIZE - 1) of uninitialised bootloader stack are embedded in the measurement field that a downstream attestation service sends to a remote verifier. In the configurations analysed the residue is bootloader internal state (TLV offsets and lengths, flash addresses, loop state), not key material, but what occupies that stack region depends on configuration and on which code ran before boot_save_boot_status(). It is reported as a low-severity, defense-in-depth disclosure with no integrity or availability impact. This is distinct from the already-fixed record_len integer underflow in the same routine (GHSA-8hrv-4cp5-4rg3). The fix rejects any EXPECTED_HASH_TLV whose length is not exactly IMAGE_HASH_SIZE.CVSS added —AV:P/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:NWeakness type (CWE) added —CWE-457Affected versions (publisher) added —New affected value received. <a href="https://github.com/CVEProject/cvelistV5/blob/main/cves/2026/16xxx/CVE-2026-16662.json">CVE-2026-16662</a>Reference added —https://github.com/mcu-tools/mcuboot/commit/5098d8eef1139abd3d821d8138a6321d656dcd3eReference added —https://github.com/mcu-tools/mcuboot/security/advisories/GHSA-779w-4x76-5chfSource identifier: vulnerabilities@zephyrproject.org
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