Fix "bcrypt.compareSync returns false for correct password"

Troubleshoot bcrypt password verification failures caused by character encoding or truncation.

The Problem (Error Root Cause)Exception

A user is trying to log in with the correct password, but `bcrypt.compare()` always returns false. This often happens if the password exceeds 72 bytes, or if it was accidentally hashed twice during registration.

Identified via runtime validation & stack traces

The Solution (Step-by-Step Fix)Verified

Bcrypt truncates passwords at 72 bytes. Ensure you are not hashing a hex string that exceeds this limit. Also verify that your registration flow does not hash the password on the frontend AND the backend.

Deterministic, non-destructive resolution

Code Standard: Bad Pattern vs Verified Fix

Live Syntax

Anti-Pattern vs Verified Fix

1// ❌ Bad: Hashing the hash
2const hash = await bcrypt.hash(req.body.password, 10);
3// Later saving `await bcrypt.hash(hash, 10)` by accident
4 
5// ✅ Good: Verify plain text against original hash
6const match = await bcrypt.compare(req.body.password, storedHash);

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Frequently Asked Questions

Q:What is the 72-byte limit?

The bcrypt algorithm only uses the first 72 bytes of the password. Any characters after that are ignored.

Q:How can I manually check a hash?

Use our Password Hash & Verifier tool to test candidate passwords against existing bcrypt hashes.