Breach Database / Substack
Yes — Substack was breached.
- 663,121 accounts affected
- Breach occurred 2025-10-23 · substack.com
- Verified entry in the Have I Been Pwned catalog
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.
What data was exposed
- Email addresses
- Phone numbers
What to do right now
- Be alert for smishing and SIM-swap attempts. Treat unexpected texts and "carrier" calls with suspicion; add a PIN/port-freeze with your mobile carrier.
- Expect convincing phishing emails. Attackers use breached details to write personalized emails. Be suspicious of any message referencing this service.
- Check your other accounts on Have I Been Pwned. Your email address may appear in other breaches you don't know about yet.
- Monitor the apps you use going forward. Clearly watches the breach record for the companies behind your apps and alerts you the moment one appears.
Breach data from Have I Been Pwned. Listing here means the service appears in the public breach record — not that your personal data was affected.