Substack data breach

Breach date: 2025-10-23. Added to this index: 2026-02-06. Accounts affected: about 663K. Domain: substack.com.

What happened

In October 2025, the publishing platform Substack suffered a data breach that was subsequently circulated more widely in February 2026. The breach exposed 663k account holder records containing email addresses along with publicly visible profile information from Substack accounts, such as publication names and bios. A subset of records also included phone numbers.

Breach data sourced from Have I Been Pwned, used under CC BY 4.0. Additional breach metadata provided by XposedOrNot. DataExposed is an independent service and is not affiliated with or endorsed by either.

What was exposed

Email addresses: The email address you used to register or log in. It is the most common field in breaches because it doubles as your username on most sites.
Phone numbers: Your mobile or landline number, collected at signup for contact, delivery updates, marketing and two-factor codes.

What to do now

Check whether your own address is in this dataset with the free email breach check.
Review the accounts connected to this service and turn on two-factor authentication.
Turn on two-factor authentication on your email account first, because it is the reset path for everything else.
Treat calls and messages that quote real details about you as hostile until you verify them another way.

Frequently asked questions

What data was leaked in the Substack breach?

The Substack breach exposed Email addresses, Phone numbers. About 663K accounts were affected.

Was I affected by the Substack breach?

Run the free email breach check with the address you used on that service. It will tell you whether your address appears in this dataset.

What should I do about the Substack breach?

Treat any message that quotes details from this breach as suspicious, turn on two-factor authentication, and consider removing your details from people-search sites if your address or phone was included.