Microsoft offers an application for Android called “Microsoft Authenticator” which is used to setup two-factor authentication (2FA). This application operates in two modes – one allows to generate standard OTP codes like many other apps (Google Authenticator, Authy, etc). The second mode allows a user to register with Azure’s Multi-Factor Authentication (MFA) service, and allows users to authenticate by tapping a prompt on their phone instead of entering an OTP code (similar to Google Prompt).
However, it looks like that the application still allows screenshots to be taken. This is true for setting up the OTP with a manual seed, viewing generated OTP codes, and entering a username + password when setting up Azure MFA. The implication is that if a user’s device ends up running a rogue app, that app can capture the initial OTP seed (if entered manually), the initial username and password for MFA, as well as all generated OTP codes as they are shown by the app, and thus break two factor authentication.
To replicate, try the following:
- Open the application.
- Setup a new OTP code by adding “other account” + a random seed, and view codes. Alternative, try to sign-up for MFA with a personal account.
- Press Power + Volume Down at any sensitive screen and observe a screenshot being taken.
The underlying reason is because the app is not using “FLAG_SECURE” for such screens (more information on FLAG_SECURE can be found in our earlier blog post). By contrast, many Android apps with higher security requirements use it.
We filed a bug report with the vendor (Microsoft) and here is their response:
Our team assessed the issue, and this does not meet the bar for servicing. We have informed the product team about this issue. MSRC is closing the case.
As for CVE, since there is no fix going for this, we will not be assigning any CVE for this issue.
The vendor has fixed the issue in v6.2002.2038, released on March 25th, 2020. Screen capture is disabled by default and requires an opt-in by the user via settings as per the screenshot below. Users are encouraged to update to the latest version.
- Azure Multi-Factor Authentication – see docs here
- Google Play Link to the app – see here
- Google Prompt for Android – see here
- MSRC Case # 46793
- Our earlier blogpost about FLAG_SECURE on Android – see here