Abandoned Outlook Add-in Hijacked to Phish 4,000 Users
⚠️ Koi Security found that an abandoned Outlook add-in, AgreeTo, was hijacked to run phishing kits that captured roughly 4,000 Microsoft account credentials. The attacker claimed an orphaned Vercel subdomain referenced in the add-in’s XML manifest and replaced live content with a fake sign-in page while retaining mailbox permissions. Microsoft had validated and signed the original manifest but does not re-review hosted content fetched at runtime. Users should remove AgreeTo and reset affected passwords immediately.
