Description
When MCUboot is built with the TinyCrypt crypto backend (MCUBOOT_USE_TINYCRYPT), bootutil_aes_ctr_drop() in boot/bootutil/include/bootutil/crypto/aes_ctr_tinycrypt.h was an empty stub. The AES-CTR context for this backend is the fully expanded AES-128 key schedule (struct tc_aes_key_sched_struct). Generic code relies on this hook to release and clear key state, so with TinyCrypt nothing was cleared. The mbedTLS and PSA backends do free or destroy their key state here.
As a result, boot_state_clear(), which boot_go() calls before handing off to the application, left the image's AES content-encryption key schedule in the static boot_data state. This is the case after an encrypted upgrade in single-image builds, where boot_enc_zeroize() is not called. boot_decrypt_key() likewise left the ECIES-derived key-encryption key schedule on the stack. The affected build uses MCUBOOT_ENC_IMAGES with TinyCrypt, which is the default ECDSA and Ed25519 backend in the Zephyr port.
The first AES-128 round key equals the cipher key, so anyone who can read the leftover bootloader RAM after boot can recover the image content key and decrypt that encrypted image. That includes a physical attacker with debug or RAM access, or code later running on the device. This weakens image-encryption key confidentiality (and so the confidentiality of the encrypted image's plaintext). The device's long-term private decryption key is not exposed, and image authenticity is not affected. The fix wipes the context with bootutil_wipe_memory().
As a result, boot_state_clear(), which boot_go() calls before handing off to the application, left the image's AES content-encryption key schedule in the static boot_data state. This is the case after an encrypted upgrade in single-image builds, where boot_enc_zeroize() is not called. boot_decrypt_key() likewise left the ECIES-derived key-encryption key schedule on the stack. The affected build uses MCUBOOT_ENC_IMAGES with TinyCrypt, which is the default ECDSA and Ed25519 backend in the Zephyr port.
The first AES-128 round key equals the cipher key, so anyone who can read the leftover bootloader RAM after boot can recover the image content key and decrypt that encrypted image. That includes a physical attacker with debug or RAM access, or code later running on the device. This weakens image-encryption key confidentiality (and so the confidentiality of the encrypted image's plaintext). The device's long-term private decryption key is not exposed, and image authenticity is not affected. The fix wipes the context with bootutil_wipe_memory().
Analysis
No analysis available yet.
Default status is the baseline for the product, each version can override it (e.g. patched versions marked unaffected).
| Vendor | Product | Default status | Versions | ||||||
|---|---|---|---|---|---|---|---|---|---|
| mcu-tools | mcuboot | unaffected |
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Remediation
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Advisories
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References
History
Fri, 13 Nov 2026 00:45:00 +0000
| Type | Values Removed | Values Added |
|---|---|---|
| Description | When MCUboot is built with the TinyCrypt crypto backend (MCUBOOT_USE_TINYCRYPT), bootutil_aes_ctr_drop() in boot/bootutil/include/bootutil/crypto/aes_ctr_tinycrypt.h was an empty stub. The AES-CTR context for this backend is the fully expanded AES-128 key schedule (struct tc_aes_key_sched_struct). Generic code relies on this hook to release and clear key state, so with TinyCrypt nothing was cleared. The mbedTLS and PSA backends do free or destroy their key state here. As a result, boot_state_clear(), which boot_go() calls before handing off to the application, left the image's AES content-encryption key schedule in the static boot_data state. This is the case after an encrypted upgrade in single-image builds, where boot_enc_zeroize() is not called. boot_decrypt_key() likewise left the ECIES-derived key-encryption key schedule on the stack. The affected build uses MCUBOOT_ENC_IMAGES with TinyCrypt, which is the default ECDSA and Ed25519 backend in the Zephyr port. The first AES-128 round key equals the cipher key, so anyone who can read the leftover bootloader RAM after boot can recover the image content key and decrypt that encrypted image. That includes a physical attacker with debug or RAM access, or code later running on the device. This weakens image-encryption key confidentiality (and so the confidentiality of the encrypted image's plaintext). The device's long-term private decryption key is not exposed, and image authenticity is not affected. The fix wipes the context with bootutil_wipe_memory(). | |
| Title | AES key schedule not cleared by the TinyCrypt AES-CTR backend in MCUboot bootutil | |
| Weaknesses | CWE-226 | |
| References |
| |
| Metrics |
cvssV3_1
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Subscriptions
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MITRE
Status: PUBLISHED
Assigner: zephyr
Published:
Updated: 2026-11-13T00:22:58.572Z
Reserved: 2026-11-12T21:38:51.005Z
Link: CVE-2026-128091
Vulnrichment
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NVD
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Redhat
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OpenCVE Enrichment
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Weaknesses
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CWE-226
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