Password Breach Checker

Name: Password Breach Checker
Author: Jai Mittal

First-party data: Launch Ready Code has scanned 700+ AI-built apps. Average Launch Readiness Score: 44/100. Most common critical failures: missing HTTP security headers (83%), no rate limiting on auth endpoints (71%), exposed API keys or secrets (67%), absent database Row Level Security (58%). — Jai Mittal, Founder & CTO

By Jai Mittal, CTO · Launch Ready Code · August 16, 2026

The vulnerabilities described here align with the OWASP Top 10, the industry-standard list of the most critical web security risks, and MITRE CWE Top 25. Severity uses CVSS v3 scoring from NIST's National Vulnerability Database.

Check whether a password has appeared in known data breaches. We hash it in your browser and send only the first 5 characters of the hash — your actual password never leaves this page.

How does the breach check work?

k-anonymity model — your password is SHA-1 hashed in the browser; only the first 5 hex characters are sent to the API, never the full hash or plaintext
Have I Been Pwned database — checks against 1.1 billion+ compromised passwords compiled from LinkedIn, Adobe, Collection #1, and thousands of other public breaches
Exposure count — shows exactly how many times the password appears across all known breach datasets
Privacy guarantee — the server receives only a 5-character prefix and returns all matching suffixes; matching happens in your browser with zero password transmission
NIST SP 800-63B compliance — the check is the exact mechanism NIST recommends for validating passwords against known-compromised lists at registration

Standards reference: NIST SP 800-63B Section 5.1.1 requires checking new passwords against known breach corpuses. The Have I Been Pwned Pwned Passwords API (Troy Hunt, 2017) contains over 1.1 billion compromised passwords from public breaches. Credential stuffing accounted for 34% of all login traffic analysed in Cloudflare's 2024 Application Security Report.

Breach check

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How Does a Breached Password Put Your App at Risk?

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How does breach checking work, and why is it privacy-safe?

This tool checks whether a password appears in Have I Been Pwned's database of over 850 million compromised passwords, using a technique called k-anonymity. Your password is never sent over the network. Instead, the tool hashes your password locally with SHA-1, sends only the first 5 characters of that hash to the API, receives all matching hash suffixes, and checks for a match entirely in your browser. The full password or its complete hash never leaves your device.

The Have I Been Pwned Passwords dataset is compiled from publicly disclosed data breaches including LinkedIn (2012, 117M records), Adobe (2013, 153M), Collection #1 (2019, 773M), and thousands of smaller breaches. If a password appears in this database, it means attackers actively use it in credential-stuffing attacks against any service that uses the same password.

Why do breached passwords put your SaaS app at risk?

Credential stuffing is now the most common attack vector for account takeovers. Attackers take credentials leaked from one breach (email + password pairs) and automatically try them against thousands of other services. If any of your users have reused a password that appears in a breach database, their account is at risk regardless of how secure your infrastructure is.

NIST SP 800-63B explicitly recommends checking new passwords against breach databases at the time of registration and password change. Apps built with Supabase, Firebase, or Auth.js can integrate HIBP checking via their server-side auth hooks. The check should happen server-side, not only in the browser, to prevent bypassing by disabling JavaScript.

What should you do if a password is breached?

If this tool shows a password has been seen in breaches, the account using it should change passwords immediately. For SaaS builders, if you find that users are registering with known-breached passwords, add server-side HIBP validation to your registration and password-reset flows, display a clear warning message, and require a stronger alternative — without revealing which specific breach the password was found in.

What sources and research back these findings?

OWASP Foundation — OWASP Top 10 Web Application Security Risks (2021), the industry-standard vulnerability taxonomy referenced for all security categories in this article
MITRE Corporation — CWE Top 25 Most Dangerous Software Weaknesses (2024), the weakness classification used to rank and prioritise code-level findings
NIST National Vulnerability Database — NVD CVE severity ratings; all CVSS scores cited here are drawn from published NVD records
Jai Mittal, Founder & CTO, Launch Ready Code — Proprietary data from 700+ AI-built app security audits, 2025–2026. Average Launch Readiness Score: 44/100. Most common critical failures: missing HTTP security headers (83% of scans), no rate limiting on auth endpoints (71%), exposed API keys or secrets (67%), absent database Row Level Security (58%).