Substack data breach confirmed: user phone numbers email addresses all stolen in attack, here’s what we know

• Substack confirms October 2025 breach exposing user emails, phone numbers, and metadata
• CEO Chris Best assured no financial data or credentials were accessed; hole patched and investigation ongoing
• BreachForums thread advertises ~700K stolen records, despite Substack claiming no evidence of abuse so far

Substack has confirmed threat actors broke into its systems and stole user emails and phone numbers.

On social media people are sharing screenshots of a data breach notification letter, sent to affected individuals by Substack CEO Chris Best saying the company found “evidence of a problem with our systems” on February 3. This problem allowed an unidentified and unauthorized third party to “access limited user data without permission, including email addresses, phone numbers, and other internal metadata.”