About four years ago, we first blogged about FLAG_SECURE in Android applications and how it can be used to protect sensitive screens from being captured in screenshots. One of the reasons that prompted our original blog post is the fact that FLAG_SECURE was not intended for security of screenshots but is rather a side effect of DRM.
Since then we have reported the lack of FLAG_SECURE as an issue to multiple Android app vendors. Our most recent blog post was about the fact that Google’s 2FA application (“Google Authenticator”) lacked FLAG_SECURE. We disclosed this specific issue (“Google Authenticator”) because of recent reports of malware attacking such applications using Android’s accessibility services (a11y). Yanick Fratantonio (who had a lot more experience with these services) went through the trouble of putting together a POC, as well as writing a truly excellent blog post about how FLAG_SECURE does NOT protect against attacks using these services.
After reviewing both our research and disclosures published by others, there seem to have been an assumption going around for a while that FLAG_SECURE should protect against such attacks (a11y) but no actual data to back this up. To be clear, FLAG_SECURE should still protect against malware using the MediaProjection API to record video or take screenshots, but it DOES NOT protect against attacks using Android’s accessibility services (a11y).
We amended our earlier posts to make this distinction clear and would like to thank Yanick Fratantonio for bringing this up.
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[…] UPDATE #3 (2020-03-23): Although FLAG_SECURE may protect against malicious apps using the MediaProjection APIs, HOWEVER, as per the comment below from Yanick Fratantonio and his blog post, FLAG_SECURE doesn’t protect against attacks using accessibility services. See our follow-up post here. […]