Jailbreaking a Car's Nav System to Fix the Clock

When Changing the Clock Requires a Jailbreak

In a saga that will resonate with anyone who has wrestled with proprietary automotive software, a developer known as nns_ee has documented a painstaking reverse-engineering effort to change the timezone on a 2014 Renault Latitude. The car's TomTom Carminat LIVE head unit, codenamed 'beaumont', was designed to hide its clock settings behind a DRM-locked navigation SD card—a card the previous owner had thrown away. Without it, the 'Set time' option simply didn't exist in any menu, and the unit showed a persistent 'No maps found!' error.

The developer's journey, detailed in a blog post, began with disbelief and escalated into a full-blown hacking project. After hours of searching menus and online forums, he concluded that the previous owner was right: the menu definition pointing to the settings applet was stored on the SD card itself. The system also enforced DRM checks via the card's CID, making it impossible to clone or spoof. Facing a €60 shipping cost for a replacement card, he decided to root the device instead—a decision that would consume weeks of effort.

The Hunt for an Entry Point

The unit runs a Linux 2.6.28.10 kernel on a Samsung S5P6440 SoC, with a custom TomTom OS. The developer started by extracting the firmware from official update files, which revealed a ttsystem ext3 filesystem. His initial hope was to modify the update image and flash it via SD, but a signature verification layer (likely DSA with SHA-1) rejected any alterations. To test attacks, he built a QEMU emulator with a patched kernel and a CAN emulator—a step that proved invaluable for understanding the system's behavior.

In the emulator, he discovered that most partitions were mounted read-only or noexec, a surprisingly robust security posture for 2011. The SD card itself was mounted noexec, preventing direct execution of custom scripts. He explored master.script, a custom scripting language that automates UI actions, but found no command injection. A SHEX command could execute arbitrary binaries, but only with no arguments, limiting its usefulness. A LOG command, however, allowed writing ASCII to any path—including a path traversal vulnerability that could write outside the intended directory.

The Core_pattern Trick

The breakthrough came from borrowing a kernel exploit technique: overwriting /proc/sys/kernel/core_pattern to pipe a coredump to a shell script. The developer realized he could use the LOG command to write |/bin/sh /media/sdcard/payload.sh to that file. Then, by triggering a crash in /bin/dbus-monitor (which aborts if it can't connect to the system bus), the kernel would execute his script as root. The final master.script was elegantly simple:

#define LOGFILE ../../proc/sys/kernel/core_pattern
LOG |/bin/sh /media/sdcard/payload.sh
SHEX /bin/dbus-monitor

The script worked, causing the unit to reboot when he inserted the SD card and approved execution. With root access, he backed up the internal eMMC and located the timezone setting in /mnt/flash/settings.dat. After some reverse engineering, he identified the offset holding the timezone offset in minutes and changed it from +180 (UTC+3) to +120 (UTC+2), finally fixing the clock.

Why This Matters

This story is more than a quirky hack; it highlights a growing frustration with automotive software locks. Automakers and suppliers like TomTom often restrict even basic functions behind proprietary hardware and DRM, forcing owners to pay for unnecessary accessories or live with broken features. The developer's approach—treating the car's infotainment system as a research target—demonstrates both the resilience of hobbyist reverse engineering and the absurdity of the initial lockout. As cars become more software-defined, such restrictions will likely become more common, and the community's response will be critical in pushing for more open systems.

Community Reaction and Future Work

The write-up gained traction on Reddit's r/ReverseEngineering and r/netsec, with users praising the non-AI-generated, detailed approach. One user, who had previously attempted to root the same unit and gave up, appreciated the work and shared his own GitHub research. The developer is now exploring the system's signedloop kernel module, which hooks mount operations to enforce noexec on unsigned images, and is investigating whether he can insmod a module to bypass these hooks. He also plans to potentially remove the 'No maps found!' screen entirely, a follow-up that many owners would welcome.

For now, his victory is a testament to the lengths one must go to for a simple convenience—and a cautionary tale for automakers who underestimate the determination of their users.