If you're like me, you participated in the 2015 Safer Internet Day by completing Google's Security Checkup to get an additional 2GB of quota in your Google Drive plan. (If you have not, you can complete the Security Checkup by February 17th to qualify for the storage increase.) Also if you're like me, when Google told you that you had less secure app authentication enabled, you didn't know what you used that needed less-secure authentication, so you turned it off like a good web citizen.
If you've read this far, I'm assuming you've earned the hat trick of "like me", and you also use Google Talk through the OSX Messages app. The next time you tried to log in to Google Talk via Messages after completing the audit, Google Talk rejected your password.
If you want to continue to use Google Talk via Messages, you'll need to re-enable the "less secure" authentication mechanism in your Google account.
- Go to https://www.google.com/settings/security/lesssecureapps
- Select "Turn on"
- Cringe a little bit
- Log into Google Talk via Messages
Update 2017-07-04: If you use two-factor authentication for your Google account, and your Google Talk account is not appearing as an account in Messages, here's how to fix it:
Generate an app specific password in your Google Account. I called mine "Messages", but you can call it whatever you like.
Open Messages, and from the Messages menu, select "Add Account..."
Select "Other Messages Account..." and press Continue.
Select "Jabber" as the account type, fill in your gmail address as the username.
Use your app-specific 16 character password as the password here.
Uncheck "automatically find server and port", and fill in talk.google.com as the server, 5222 as the port (which will likely switch to 5223 as you sign in, since we're using SSL.) Keep the Use SSL checkbox checked.
This will add a separate account only for Messages, besides your possibly already-existing Google account entry in your macOS accounts preferences.