Password Breach Check API

Check whether a password has appeared in known data breaches — as an API over Have I Been Pwned's Pwned Passwords corpus (800+ million unique compromised passwords). It uses k-anonymity: only the first 5 characters of a password's SHA-1 hash are ever sent upstream, so the password itself never leaves in full. Pass a password (hashed in memory, never stored or logged — send it via POST so it never appears in a URL/log) or a SHA-1 hash to learn whether it has been breached and how many times; or fetch a raw k-anonymity range for a 5-character hash prefix and do the matching entirely on your own side for zero password exposure. Screening sign-ups and password resets against breached-password lists is recommended by NIST 800-63b, and this makes it a one-call check. A breach / credential-security resource — distinct from password generators, cryptographic hashing and bcrypt. Open data from Have I Been Pwned (Troy Hunt), CC BY 4.0.