Pedro Fortuna is the Co-Founder and CTO of Jscrambler.
Most discussion on contact tracing apps is centered around the issue of privacy, so one of my roles has been to shed light on the underlying security issues that may make it easier for attackers to tamper with contact tracing apps and potentially breach privacy on a massive scale. With several countries all over the world developing and launching contact tracing apps, it is timely to analyse the differences between these apps and to highlight relevant security issues. Let's take a closer look.
Contact tracing explained
When it comes to COVID-19 apps, there are 2 types of apps that handle sensitive data - symptom tracking and contact tracing. Contact Tracing is the more controversial of the two because of the fear that people have of being monitored. Non-digital contact tracing has existed for a long time. Once someone has been infected, health officials interview the person to check with whom the person had contact recently.
They ask for contact info of people that had contact and interview them as well. This enables tracking contagion and isolating required persons. But the main problem is that this is ad-hoc - people are often interviewed too late in the game and can’t remember everyone who they have been in contact with. Plus, the interviewee may not have their contact info. Another problem is that people are only being isolated after they display symptoms.
Digital contact tracing aims to expand upon this and solve some of these limitations. Most contact tracing apps are based on Bluetooth Low Energy (BLE). Each app instance transmits short-range beacons; so, as people move, their devices pick up other people’s beacon. Each beacon has an ephemeral anonymous identifier that is unique. The ephemeral IDs seen are stored locally in the device. When someone is diagnosed, that person receives an official code from health officials, and by entering that code in the app, that person is officially and voluntarily registered with the infection status.
Then, a list of its own ephemeral IDs used in the last 14 days will be sent to a central server; all other devices periodically download the list of infected people’s IDs and then locally determine if they have been in contact with them recently. For positive contacts, the application then calculates the risk of infection, taking into consideration the estimated proximity and duration of the contact. If the risk is over a certain threshold, the app triggers a notification recommending that the user self-isolates and contacts the national health authority.
Exposed code and reverse-engineering
Due to the fact that they handle very sensitive data, contact tracing apps should offer robust security and be subject to independent security audits. To this end, these apps should follow the general recommendations of the OWASP Mobile Security Testing Guide, specifically the use of Code Signing to reduce the risk of publishing an adulterated version of the application, and also Certificate Pinning, to reduce the risk of Man-in-the-Middle (MITM) attacks. Additional security threats that should be taken into account can be found here.
Of particular note here is the risk posed by exposed code. As outlined by OWASP, this is where an attacker can either directly modify the code, change the contents of memory dynamically, change or replace the system APIs that the application uses or modify the application’s data and resources. Subsequently, this can provide the attacker with a direct method of subverting the intended use of the software for personal or monetary gain. The impact from code modification can be wide ranging in nature, depending upon the nature of the modification itself. But it is especially relevant in decentralized Contact Tracing apps where the data and sensitive algorithms all stay inside the local environment.
OWASP also highlights the risk of attackers reverse-engineering the source code as a gateway for more advanced attacks. In this way, an attacker may exploit reverse engineering to achieve any of the following aims - to reveal information about backend servers, to reveal cryptographic constants and ciphers, to steal intellectual property, to perform attacks against back end systems or to gain intelligence needed to perform subsequent code modification.
Contact app security
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Pedro Fortuna, CTO and Co-Founder, Jscrambler.