Was I in the Substack breach?
Substack is a publishing and newsletter platform. Its breach happened in October 2025 and exposed 663,121 accounts. It included phone numbers and email addresses. If you had an account there by October 2025, check your address below to see whether it is one of them.
Breach record last updated . Steps read from the government pages at the foot of this page.
What happened?
The platform was breached in October 2025 and in February 2026 the data began to circulate further. Account records held email addresses and public profile details, for example bios and the names of publications. Some records also held phone numbers.
- Organisation
- Substack, a publishing and newsletter platform (substack.com)
- When it happened
- October 2025
- Added to monitored breach databases
- 6 February 2026
- Accounts in the data
- 663,121
- What was exposed
- phone numbers and email addresses
The breach record cites a public report on csoonline.com.
What was exposed, and what does it mean for me?
- Your phone numberphone numbers
Expect texts and calls that use your real details to sound genuine. A phone number is also what a fraudster needs to try to move your service to their own SIM card.
- Your email addressemail addresses
Expect messages that look like they come from this service or another one you use, written to get you to click a link or sign in on a fake page.
What should I do now?
In this order, for what this breach exposed.
1. Protect your phone number
- Ask your mobile carrier to put a PIN or passcode on your account, and ask whether it offers a lock on SIM changes and number transfers.
- Treat texts about deliveries, accounts or payments with care. Do not tap the link: contact the company on a number you already have.
2. Expect phishing
- Be wary of messages that urge you to act fast, ask for personal details, or link to an address that is slightly off.
- If a message might be genuine, do not use its links or phone numbers. Go to the site by typing its address yourself.
Is my address in it?
Enter your email address. We check that one address against monitored breach databases and show which breaches list it and what each exposed.
No account needed. We keep the address you check, when, and how many breach records it was found in, to prevent abuse, until you ask us to delete it. Privacy
Where does this come from?
- The breach record in monitored breach databases: dates, account count and what was exposed. What happened is in our own words.Read 14 October 2026
- Canadian Anti-Fraud Centre: Phishing, including text messagesRead 14 October 2026
- CISA: Recognize and report phishingRead 14 October 2026
- Get Cyber Safe (Government of Canada): Fact sheet on phishingRead 14 October 2026