Somewhere on the internet, a file containing your email address and a password you used five years ago is sitting in a database alongside billions of other credentials. If you reused that password on even one other service, an attacker may have already tested it — automatically, at scale, and without you ever knowing.
This is credential stuffing, and it has become the single most common way attackers gain access to accounts.
The Mechanics of an Industrialised Attack
Credential stuffing is not guessing. The attacker already has real username-and-password pairs, harvested from previous data breaches or stolen by infostealer malware that quietly copies saved passwords from browsers. These credentials are compiled into structured files known as combo lists and sold on dark web forums and Telegram channels.
The attacker loads a combo list into an automated tool and points it at a target — your bank, your email provider, your favourite streaming service. The tool fires login attempts at the target’s authentication endpoint, rotating through residential proxy networks to disguise the traffic as normal user behaviour. Each credential is tried once, making the attack nearly invisible to basic rate-limiting defences.
The success rate is low — typically between 0.1 and 2 percent — but the scale is enormous. Industry estimates place the global volume at roughly 26 billion credential stuffing attempts per month. At that scale, even a fraction-of-a-percent hit rate translates into thousands of compromised accounts per campaign.
Why It Works: The Password Reuse Problem
Credential stuffing exists because people reuse passwords. According to analysis published alongside the 2025 Verizon Data Breach Investigations Report, only 49 percent of a typical user’s passwords across different services are unique. The other half are duplicates or minor variations — the same password with a different number at the end, or the same base word capitalised differently.
This is exactly what attackers count on. A password leaked from a low-value forum can unlock a high-value bank account if the same credentials were used on both. The original breach may have happened years ago, but if the password was never changed, it remains valid.
Stolen credentials drove 22 percent of all confirmed data breaches in 2025, making credential-based attacks the most common initial access vector for the third consecutive year. High-profile incidents have hit targets ranging from Australian superannuation funds — where members lost a combined half a million dollars — to major retail brands.
The Supply Chain Behind the Attack
Modern credential stuffing runs on a mature supply chain. Infostealer malware families like Lumma and RedLine infect endpoints and silently harvest saved passwords, session cookies, and browser autofill data. This material is packaged into stealer logs and sold to brokers who clean, deduplicate, and categorise the data before reselling it as ready-to-use combo lists.
At the other end of the chain, tools like OpenBullet provide a plug-and-play attack framework. Attackers purchase or download configuration files that define the login flow for specific targets, including how to parse responses and bypass basic security checks. Residential proxy subscriptions ensure the attack traffic comes from ordinary-looking IP addresses.
The entire operation — from acquiring credentials to testing them against targets — can be run with minimal technical skill and at negligible cost. Fresh credentials from recent infostealer campaigns command higher prices because they have higher validity rates, but even aged breach data remains useful against users who never updated their passwords.
What You Can Do
The most effective defence against credential stuffing is also the simplest: never reuse a password. If every account has a unique, randomly generated password, a breach at one service cannot cascade to another. A password manager or a tool like Safe Pass Guru makes this practical — you generate a strong, unique credential for each account and let the tool remember it for you.
Multi-factor authentication adds a second barrier. Even if an attacker has the correct password, they cannot complete the login without the second factor. Hardware security keys and authenticator apps offer the strongest protection; SMS-based codes are better than nothing but can be intercepted through SIM-swapping attacks.
Monitor for breaches proactively. Services like Have I Been Pwned allow you to check whether your email address or passwords have appeared in known data dumps. If they have, change the affected credentials immediately — and any other account where you used the same password.
Finally, treat every password as if it has already been leaked. The volume of stolen credentials in circulation is so large that the question is not whether your data has been exposed, but when. Unique passwords and multi-factor authentication ensure that a single breach stays contained instead of becoming the first domino in a chain of compromised accounts.