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

• Substack confirms October 2025 breach that exposes user emails, phone numbers, and metadata
• CEO Chris Best assured that no financial data or credentials were accessed; patched hole and ongoing investigation
• BreachForums thread announces ~700,000 stolen records, despite Substack claiming no evidence of abuse so far

Substack has confirmed that threat actors broke into its systems and stole users’ emails and phone numbers.

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