Credential Stuffing: How Attackers Use Stolen Passwords and How to Stop It

Attackers do not need to crack your password. They already own it, bought from a breach of some other service where one of your users signed up with the same login. In 2020, a wave of credential stuffing hit the Government of Canada and exposed suspicious activity on roughly 48,000 Canada Revenue Agency accounts. The attackers wrote none of their own passwords. They reused yours.

Credential stuffing is an attack method built on one human habit: password reuse. Criminals take username and password pairs leaked from past breaches. They feed those pairs into automated bots. The bots try the stolen logins against your application at scale, thousands of attempts an hour, until a reused password opens a door. The Canadian Centre for Cyber Security sets out the problem plainly in its guidance ITSP.30.035. Your application becomes vulnerable not because someone breached your infrastructure, but because your users reused their credentials elsewhere.

How the attack works

The method is simple, and simplicity is why it scales. An attacker buys a list of leaked credentials from a dark web market. Each entry holds an email and a password from an unrelated breach. The attacker loads the list into bot software. The software spreads attempts across many IP addresses to dodge basic blocking. Some groups go further and pay human operators to mimic normal login behaviour. When a stolen password matches a reused one on your system, the attacker gains a working account. From there they drain funds, steal data, or sell the verified login for a higher price.

Why Canadian organizations keep getting hit

People reuse passwords. Breach data shows the same passwords appear across banking, email, and work accounts. One leaked set opens many services. The CRA incident showed how far the damage reaches when a government login service sits behind reused credentials. Attackers targeted the vendor behind GCKey, fed in stolen pairs, and the service itself held firm while thousands of individual accounts fell. The Cyber Centre confirmed the GCKey service was not compromised. The weak point was never the platform. It was the reused password.

Credential stuffing versus brute force

These two attacks look alike and work differently. A brute force attack guesses passwords, running through combinations until one lands. Credential stuffing guesses nothing. It uses passwords already known to be real, which makes it quieter and far more efficient. A brute force attempt triggers failed-login alarms fast. Credential stuffing, with valid-looking logins from scattered addresses, slips past controls built only to count failures. Your defences need to spot the pattern, not only the volume.

What stops credential stuffing

Multi-factor authentication is your strongest single control. The Cyber Centre calls MFA the most effective protection, and its guidance concludes MFA will stop the vast majority of credential stuffing attacks. A stolen password alone fails when a second factor guards the account. Pair MFA with rate limiting, so a flood of login attempts slows to a crawl. Add bot management to separate human users from automated scripts. Screen new passwords against known breached lists, so a user never sets a password already in a criminal’s hands.

Detection and response

Prevention reduces risk. It never removes it. You need eyes on login activity. Anomaly detection flags a spike in failed logins, a surge from one region, or a single account hit from many addresses. A security analyst reviews these signals and acts before an attacker settles in. When an account falls, your incident response plan takes over: lock the account, force a password reset, and check what the attacker reached. Speed decides how much damage lands. Teams with trained analysts and a rehearsed response contain these events. Teams without them learn the hard way.

Build the skills to defend against it

Technology stops some attacks. Trained people stop the rest. A security analyst who reads login telemetry spots credential stuffing early, and the Mile2 Certified Cybersecurity Analyst program builds those monitoring and detection skills. When an account takeover happens, an incident handler runs the response, and the Certified Incident Handling Engineer track teaches the containment steps one by one. Security leaders set the policy, mandate MFA, and own the standard, work covered by the Certified Information Systems Security Officer certification. To learn the attacker’s playbook, the Certified Professional Ethical Hacker program shows how these attacks run from the other side.

Where to start this week

You do not need a full program to begin. Turn on MFA for every account holding sensitive data, starting with admin and finance logins. Check your login logs for repeated failures from odd locations. Screen your users’ passwords against breached-password lists. Write a short playbook for a suspected account takeover, so your team acts fast instead of improvising. For a Canadian small or mid-sized business, the CCCS Baseline Cyber Security Controls give you a practical checklist without enterprise weight. For a regulated firm, ITSG-33 and PIPEDA set the obligations you build toward. Credential stuffing works because people reuse passwords. Your job is to make one stolen password worthless on its own.