Danger: a real brand never does it
A brand sends the offer details as a .zip or .rar with a password: beware
The offer details or a contract to download in a protected archive, an .exe or an .scr, "install our software": the surest sign of a fake YouTube sponsorship offer.
What it looks like
You will find the offer details and the contract in the attached archive Offer_Campaign.rar (password: 2026). Please open it before our call.
Why it is a sign
Password-protected archives slip past antivirus software and mail filters. Inside, a program dressed up as a contract or an offer document steals your browser sessions in seconds, two-step verification included. That is how most stolen channels are stolen, according to YouTube's help and Google's security team. A program to install "to track the campaign" serves the same purpose.
What a real brand does
A real brand sends the offer details as a PDF or a link to an online document, with no password on an archive, and never asks you to install anything.
What to do
Do not open the archive, not even "to have a look". If you already opened it, change your Google account password from another device and sign out every device. Ask for the offer details as a PDF, through a link you can check.