answers
why sign-in breaks in in-app browsers
One mechanism causes all of these. Instagram, TikTok, Facebook and the rest open your link in a browser they embed themselves.
Google refuses to run sign-in in an embedded browser, and has since 30 September 2021. Your visitor sees a refusal, a blank screen, or nothing at all.
Each page below covers one app or one error, with the sources quoted. We say when something is unverified rather than filling the gap.
google oauth · in-app browsers
Error 403: disallowed_useragent
Google blocks OAuth in every embedded browser, so sign-in fails inside Instagram and TikTok. What the error means, what does not fix it, and what does.
written 25 Aug 2026
instagram · sign in with google
Instagram google login not working
Sign in with Google fails inside Instagram's in-app browser, usually as a white screen with no error. Why it happens, and the two ways out.
written 26 Aug 2026
facebook · messenger · oauth
Facebook in-app browser OAuth
Two separate failures hit OAuth inside Facebook and Messenger: Google's embedded-browser block, and a popup that cannot open in a single-tab browser.
written 26 Aug 2026
tiktok · in-app browser
TikTok browser sign in
TikTok opens every link in its own browser, and Google refuses to sign anyone in there. TikTok is also the one app with no open-in-browser button.
written 26 Aug 2026
check your own page
Paste your signup or login page. We read it once and name every sign-in button that cannot complete inside an in-app browser.
we read the page once and tell you what breaks. free, no account, nothing stored about you.
We have read the signup pages of well-known products the same way. That is the report.