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1Fix the driver behind crashes, sound loss and screen glitches2Repair Windows errors before they cause bigger problems3Scan for outdated or missing drivers - takes under a minuteYes, Doom really ran on a SumUp Solo payment terminal—but the novelty was only the final act. In a 2023 hardware-security project, Aaron Christophel opened an examined Solo, reached its embedded Linux system through exposed UART test points, and obtained an unauthenticated root shell. That privileged access made it possible to compile Doom and redirect its graphics to the terminal’s framebuffer.
The more important finding was not that a card reader could play a 1993 game. It was that a production payment device reportedly exposed a powerful software and debugging path locally, even though its anti-tamper design separately protected critical payment keys.
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The Solo is a small Linux computer disguised as a card terminal
The SumUp Solo is a compact touchscreen payment terminal for card and NFC transactions. It has a display, battery, payment hardware, wireless connectivity and the software needed to communicate with payment services. Its consumer-facing job is narrow, but internally it is a networked embedded computer.
Coverage of Christophel’s project described the examined device as having a 1 GHz processor and 256 MiB of DRAM—modest specifications by desktop standards, but more than enough for a lightweight game such as Doom. The available reporting does not identify the exact processor, Linux distribution, kernel, board revision or memory manufacturer, so those details should not be inferred. Hackster’s report provides the reported hardware and software context.
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That computing capability explains why the Doom demonstration was plausible. The terminal did not need to be secretly transformed into a conventional PC. It already had a Linux-capable processor, memory, a display and an operating system capable of running additional software once its normal restrictions were bypassed.
Getting past the physical defenses
Payment terminals are designed with a different threat model from ordinary hobbyist electronics. Their enclosures and circuit boards commonly include anti-tamper mechanisms intended to detect unauthorized opening or probing. A tamper event can erase cryptographic material or render the device unusable rather than simply exposing its contents.
According to Hackaday’s account, the Solo had an anti-tamper shield over critical circuitry. Lifting or disturbing it could cause payment-related keys to be erased. Rather than simply removing the shield, Christophel used acupuncture needles to reach two UART test points beneath it.
That detail is memorable, but it should not be mistaken for a general-purpose repair technique. The method described what the researcher did on the examined terminal; it does not establish that the same points, electrical characteristics or procedure apply to every Solo. Probing a payment terminal can destroy the device, trigger key erasure or create legal and financial problems if it is connected to a merchant account or live payment environment.
UART exposed the system behind the payment app
UART is a serial interface frequently used during development, manufacturing and debugging. A properly secured production device may leave such interfaces inaccessible, disabled or authenticated. In this case, a USB-to-UART adapter connected to the exposed test points reportedly provided a Linux terminal as well as access to U-Boot, the device’s bootloader.
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The reporting does not publish the Solo’s UART voltage, baud rate, pinout, connector details, test-point labels or exact bootloader transcript. Those omissions matter: connecting a random serial adapter to unknown points can damage both the target and the adapter. The account demonstrates the security consequence of the interface, not a complete, verified reproduction recipe.
Bootloader access is especially significant because a bootloader operates below the normal payment application. Depending on how it is configured, it may permit changes to boot arguments, recovery operations or alternate software images. The available coverage does not document every command Christophel used or prove that every one of those capabilities was available on every Solo.
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Hackster reported that rebooting the examined terminal resulted in root access without an authentication step. In practical terms, the local serial path led to an administrative shell on the Linux environment.
That is a serious weakness for a production payment device. Root privileges can allow a researcher to inspect files, change system behavior, install or execute software and interfere with the normal payment application. It also changes the trust boundary: the terminal may still contain protected payment components, but the operating system responsible for presenting information and coordinating the user experience can no longer be assumed to be running only authorized code.
The scope must be stated carefully:
- This was a local physical attack path using internal test points.
- The reports do not demonstrate an internet-based or remote exploit.
- The evidence concerns the examined terminal, not every Solo unit, firmware version, regional edition or production batch.
- Root access does not by itself prove that card numbers, PINs or payment cryptographic keys were extracted.
Why Doom was the easy part
Once an embedded Linux system provides an unauthenticated root shell, running a small game is comparatively straightforward. Christophel compiled Doom with workarounds so it could use the terminal’s framebuffer—the part of the Linux graphics stack that allows software to draw directly to the display.
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The technical chain was simple in principle:
- The Solo supplied a processor, memory and Linux environment capable of running the program.
- UART and the boot process exposed the operating system.
- Root privileges removed ordinary restrictions on executing or installing software.
- Doom was built for the target environment.
- Framebuffer-related changes allowed its graphics to appear on the touchscreen.
This was not Doom hidden in the terminal’s factory software, nor necessarily a native port in the strict platform-specific sense. It was Doom running on the terminal’s Linux environment after compilation and display adaptation. The available reports do not identify the precise source port, compiler toolchain, target architecture, libraries, framebuffer device or commands, so those details should not be presented as known.
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The most useful way to understand the finding is to separate payment security into several properties.
| Security property | What the reporting supports |
|---|---|
| Confidentiality | It does not establish that live card data, PIN data or payment keys were read. |
| Integrity | Root access could allow changes to the terminal’s software and user interface; altering the displayed amount was identified as a concern. |
| Availability | A privileged attacker could potentially disable or brick the terminal. |
| Authenticity | The reported shell made it possible to run software outside the intended payment application. |
| Physical security | The anti-tamper design protected critical payment material, but accessible debug points still exposed the application processor’s software environment. |
The distinction between what a customer sees and what a payment system ultimately settles is crucial. A modified display could mislead someone about the amount being requested even if the back-end transaction amount remained unchanged. That is an integrity and trust problem, but it is not proof that the attacker could alter the settled transaction, clone cards or steal payment credentials.
Hackaday reported that payment-processing keys were erased when the anti-tamper shield was lifted and later said patches made those keys inaccessible from the Linux environment. That suggests the terminal’s design separated at least some sensitive payment material from the general-purpose Linux system. The anti-tamper mechanism therefore did not necessarily “fail” outright: it protected critical secrets, while the debug and software boundary around the application processor remained too permissive.
SumUp reportedly patched part of the problem
Christophel reportedly disclosed the findings to SumUp through responsible disclosure. Later coverage said SumUp issued patches intended to prevent payment-processing keys from being accessible from the Linux environment. That is meaningful remediation, but it should not be described as a comprehensively verified fix without an official advisory or complete firmware history.
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The available reports do not definitively identify:
- the affected firmware versions or serial ranges;
- whether every Solo hardware revision was vulnerable;
- whether the debug console was disabled, authenticated or physically redesigned;
- whether merchants had to update or replace terminals;
- whether a public CVE or formal security advisory was issued; or
- whether the original access path remains possible on current 2026 firmware.
SumUp’s developer changelog shows that Solo software capabilities and versions have changed over time, which is another reason not to treat a 2023 report as a current, universal exploit. The evidence supports “the examined device had a serious local weakness and patches reportedly addressed at least part of it,” not “every Solo can still be rooted.”
What the project does—and does not—prove
It proves that the examined SumUp Solo had an accessible UART path leading to Linux and, reportedly, an unauthenticated root shell. It also proves the device had enough computing and graphics capability to run Doom after suitable software changes.
It does not prove that:
- the terminal could be rooted remotely over the internet;
- all SumUp Solo terminals shared the same vulnerability;
- payment keys or customer card data were extracted;
- the settled transaction amount could be changed;
- the issue remains exploitable today; or
- the exact reported procedure can be reproduced from the published coverage alone.
How to learn the same lessons safely
A payment terminal is a poor practice target. Even a personally owned used unit may retain sensitive configuration, require account provisioning or fall under contractual and regulatory restrictions. A rooted terminal should never be returned to production use without an authorized secure re-provisioning process.
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For a safer version of the project, use an inexpensive Linux single-board computer, an intentionally unlocked development board with U-Boot, an old router, a handheld Linux device or another disposable platform with documented hardware. These alternatives can teach the same concepts: identifying a serial console, understanding a bootloader, cross-compiling a program and drawing through a framebuffer.
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A responsible embedded-lab checklist includes:
- Work only on hardware you own or have explicit permission to test.
- Keep the device offline and disconnected from payment accounts and real transactions.
- Verify ground and signal voltage before attaching any UART equipment.
- Use a USB-UART adapter whose logic levels are documented; never assume an unknown target is 5 V-safe.
- Photograph and document hardware before making changes.
- Do not attempt to extract payment credentials or defeat tamper protections.
- Preserve evidence and disclose genuine vulnerabilities to the manufacturer responsibly.
A 3.3 V-capable USB-UART adapter, a suitable logic analyzer, antistatic tools and a recoverable Linux board are sensible categories for this kind of learning. None should be advertised as electrically compatible with the Solo based on the available reporting, because the terminal’s UART specifications were not published in the coverage.
The real lesson behind “Doom runs everywhere”
The joke works because Doom has become a shorthand for proving that an unexpected device is a computer. Here, however, the game obscures the more consequential result. The researcher did not merely discover unused processing power in a payment terminal; he crossed from exposed hardware debugging into privileged control of the Linux software environment.
Payment security is layered. Tamper detection and key isolation can protect the most valuable cryptographic material, yet an accessible unauthenticated debug path can still undermine software authenticity, transaction presentation and customer trust. Strong protection in one layer does not compensate for a weak boundary in another.
As of 2026, this remains best understood as a historical March 2023 report about a particular examined SumUp Solo—not a verified general exploit for current terminals. The durable takeaway is less “Doom runs on payment readers” than “production embedded devices need their development interfaces secured as carefully as their secrets.”
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