04 - BruCON 0x0F - Attack Surface and Security Implications of eSIM Technology - Markus Vervier

Summary

Markus Vervier examines the attack surface and security implications of eSIM technology, including its potential uses in security research and red-team operations, as well as the ways it may expose devices to abuse.

How eSIMs work

• An eUICC is a programmable SIM component that can store multiple carrier profiles. Unlike a traditional SIM card, profiles can be provisioned remotely and switched without replacing hardware.
• Consumer eSIM provisioning can begin with a QR code or activation code. The device then contacts a carrier or provisioning server to download a profile.
• Provisioning uses several layers of protection, including GSMA certificates, mutually authenticated TLS, and additional encryption for the profile payload. Vervier said he found no obvious protocol-level weaknesses during his research and considered the profile-isolation design comparatively strong.
• The system is nevertheless complex, with potential weaknesses in the device operating system, provisioning software, carrier websites, profile contents, or eUICC implementations.

Windows attack surface

Vervier demonstrated the Windows Mobile Plans app, which can be used to obtain or manage eSIM connectivity. In the example discussed, the app displayed carrier content in a web view. Because it used a predefined list of operator websites, a vulnerability or compromise on one of those sites could potentially expose users to phishing or other attacks.

He emphasized that this is an attack-surface issue involving the app and its web content, not necessarily a flaw in eSIM technology itself. He also noted that Windows 10 and 11 can allow eSIM installation without administrator privileges. During testing, he encountered crashes in Windows’ Local Profile Assistant service but did not investigate them further.

Test profiles and SIM applets

• Researchers can obtain test eSIM profiles through a provider without becoming a mobile operator. These profiles are useful because their contents and management keys are known.
• With those keys, a researcher can install custom Java applets on an eUICC. The applets can use SIM Toolkit features to interact with the phone, including displaying messages and sending or receiving SMS.
• The applets operate outside the usual Apple or Google app-store controls, although their capabilities are limited by the interfaces and permissions the device allows.
• Vervier demonstrated a SIM Toolkit prompt that resembled a system dialog and could ask for a device passcode. He warned that malicious prompts could be used for phishing or, in some cases, disrupt phone use. Some SIM Toolkit actions, such as launching a browser, may be blocked on modern devices.
• An attacker who can install a profile with known keys may also be able to install applets or issue management commands. This makes test-profile security and key management important.

Out-of-band communications and red-team use

An eSIM can provide connectivity separate from a compromised computer’s usual internet connection. Vervier and his colleague Nicholas developed SMS Shell, a proof of concept that sends commands and responses over SMS. The demonstration showed an agent communicating with a controller even when the computer had no ordinary internet connection. SMS is slow, but can be sufficient for basic command-and-control tasks.

Vervier also described a more concealed possibility: running command-and-control logic inside an eUICC applet. The card can perform cryptographic operations, potentially making its applet contents and traffic difficult for defenders to inspect without access to the relevant management keys. He presented this as a concern for forensic analysis, not as a fully demonstrated implant.

Tools and demonstrations

• SMS Shell: An SMS-based command-and-control proof of concept, described as publicly available on the C GitHub.
• Multirand: A tool for attempting to brute-force profile or eUICC management keys, noted as available on the x41 GitHub.
• Test applet and test profile: Mentioned as planned for release at the time of the talk.
• Demonstrations: Windows Mobile Plans provisioning, custom eSIM profile installation, SIM Toolkit prompts, and SMS Shell operation.

Main takeaway

eSIM provisioning and profile-isolation mechanisms can be robust, but the broader ecosystem creates a wide research and attack surface. The most practical concerns raised were vulnerabilities in supporting software and carrier web content, abuse of SIM Toolkit capabilities, and the use of eSIM connectivity as an out-of-band communications channel.

The speaker noted that only one eSIM profile could be active at a time in the setup discussed.

Main speakers and sources

• Markus Vervier — Presenter and primary source of the research and analysis.
• Nicholas — Colleague credited with work on the SMS Shell proof of concept.
• Audience member — Asked about using separate profiles for control and connectivity. Vervier replied that one profile should generally suffice.