A detailed technical examination conducted recently has revealed a number of serious security vulnerabilities in the well-known Thermomix TM5 kitchen equipment, which raises questions regarding the integrity and safety of its firmware and related ecosystem.
The vulnerabilities, which impact the TM5’s main board and its connected modules, potentially allow attackers to bypass firmware protections, downgrade software versions, and even achieve persistent code execution on the device.
Weaknesses in Cryptography
The Thermomix TM5 relies on a sophisticated architecture featuring an i.MX28 system-on-chip and multiple secure storage and peripheral interfaces.
Core to its operation are the main board, which controls the user interface and system logic, and a power board handling physical operations.
Analysis began with a teardown of the main board NAND flash, revealing the use of custom drivers and hardware-level integrity checks.
However, investigators found that the NAND flash could be dumped and parsed using open-source tools, exposing sensitive cryptographic keys and configuration files.
Particularly troubling is the discovery of weak cryptographic binding in the device’s firmware update process.
The firmware images for the TM5 are organized by independently encrypted and signed sections, with each section’s authentication data (RSA signature, AES-EAX nonce, and tag) stored in the header.
The vulnerability arises from the fact that the nonce is not included in the RSA signature, allowing attackers to manipulate it for controlled decryption.
With the AES key extractable from the device’s binaries, malicious actors can use nonce tampering to control the plaintext output of the decrypted firmware version block, allowing forced downgrades and bypassing intended anti-rollback protections.
Exploitation Pathways
Security researchers demonstrated that, by using dumped cryptographic material and reverse engineering the proprietary kernel drivers (which leverage a dedicated cryptographic coprocessor), it is possible to decrypt and modify the file systems, including those used by the TM5’s “cook sticks” recipe chips.
These chips and the Cook Key Wi-Fi module, originally designed to securely add content or updates, are found to store sensitive data in an encrypted but ultimately compromiseable manner.
Attackers, once in possession of the required keys which were recoverable from NAND dumps and kernel images can mount, modify, or even emulate these devices to inject arbitrary content.
The firmware update process, a critical component of the TM5’s security posture, is particularly susceptible to manipulation.
Prior to firmware version 2.14, the update package structure allowed for section swapping and crafted updates as long as basic version string criteria were met a limitation easily overcome with brute-force approaches due to weak signature enforcement.
More significantly, the TM5 lacks comprehensive secure boot validation: the root filesystem, the heart of the device’s operating system, is neither signed nor thoroughly verified during the boot process.
This opens the door to persistent rootkit-style attacks, where a compromised root filesystem can maintain unauthorized system access even after reboots or further updates.
While Vorwerk, the manufacturer, has addressed some of these vulnerabilities in firmware version 2.14 by strengthening the cryptographic linkage of update sections and improving anti-downgrade mechanisms, devices on older versions remain exposed.
Importantly, the vulnerabilities cannot be exploited remotely without significant reversing and physical access to the device, somewhat mitigating the overall risk to end-users.
However, they do highlight substantial weaknesses in the device’s cryptographic and firmware design raising questions about best practices in embedded systems security.
Experts recommend that vendors implement holistic secure boot processes, robust digital signatures on all system-critical files, and binding all cryptographic parameters (including nonces and tags) to their signatures.
The Thermomix case serves as a cautionary tale for smart appliance manufacturers, emphasizing the need for baked-in security in both hardware and software from the earliest design stages.
With the proliferation of Internet-connected devices in everyday life, such weaknesses could have broader repercussions if left unaddressed across the industry.
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