You don’t have to be breached to be compromised. When another company leaks credentials, attackers test those logins against every service they can reach—your GitHub, your Stripe, your Vercel, your email.
Credential stuffing is cheap, automated, and one of the most common ways founders lose accounts. The defense is straightforward once you understand the chain.
What is credential stuffing—in plain English?
Answer: Credential stuffing is when attackers take usernames and passwords leaked from one service and try them on many other services, betting that people reuse passwords.
It’s not “hacking” in any clever sense. The attacker has a list. They have automation. They have all the time in the world. If even 0.1% of attempts succeed, a million-line list yields thousands of working accounts.
Verizon’s 2024 DBIR reports that the use of stolen credentials has appeared in roughly a third of all breaches over the last decade—making it one of the most reliably abused attack paths anywhere.
Where do the credentials come from?
Answer: Public breach dumps, dark-web markets, infostealer logs, and phishing kits—aggregated into ready-to-use lists.
Three main pipelines feed the credential economy:
- Big-name breaches. LinkedIn, Adobe, Dropbox, MyFitnessPal, Collection #1, and the RockYou2024 leak with nearly 10 billion plaintext passwords—each one becomes raw material for stuffing campaigns for years afterward. Your cofounder signed up for one of these with their work email five years ago. The password they used? Probably still in active rotation somewhere.
- Infostealer malware. Trojans like RedLine, Vidar, and Lumma harvest saved browser credentials, cookies, and session tokens from infected machines—often pirated software or a “free” Chrome extension. The output—“stealer logs”—is sold in bulk on Telegram and forums every day. The same machine you use to ship code is the same one you saved your GitHub password into.
- Phishing kits. Real-time credential capture from look-alike login pages:
vercel-login.co,stripe-dashboard.app,github-auth.io.
The resulting “combolists” sell for a few dollars or are shared free. Once you understand the supply, the demand makes sense.
How does a credential stuffing attack actually work?
Answer: Lists in, accounts out—at scale, behind proxies, with bots that look like real users.
A typical campaign looks like this:
- Acquire a combolist (
email:passwordpairs from a recent breach or aggregated dump). - Pick a target. Often an email provider, a SaaS login, a billing portal, or developer tools like GitHub, Vercel, or Stripe.
- Distribute requests through residential proxies so the traffic looks like normal users from real ISPs.
- Solve CAPTCHAs with cheap human-solving services or models.
- Filter the hits. A small percentage will work—those accounts get flagged for follow-up.
- Monetize. Drain the Stripe balance, ransom the GitHub repos, send invoice scams to the founder’s customers, or quietly squat on the account waiting for a payday.
The whole pipeline runs unattended. The attacker’s marginal cost per attempt is essentially zero.
Why is this so effective?
Answer: Because password reuse is the rule, not the exception.
- Independent studies have repeatedly found 65–75% of people reuse passwords across multiple services.
- The average person has 100+ online accounts but only a handful of distinct passwords.
- Even when users vary passwords slightly (
Password1!→Password2!), attacker tooling tries common mutations automatically.
This is why a breach at a fitness app from years ago still produces working logins to corporate email today.
What can attackers do once they have one valid login (to your stuff)?
A single valid credential is rarely the goal—it’s the foothold. For a SaaS founder, here’s what that foothold buys an attacker:
- Email takeover. Read your inbox, change recovery info, lock you out, then use password reset on every other service you’ve ever signed up for.
- GitHub takeover. Pull your private repos, harvest the secrets you forgot to rotate, push malicious commits, ransom the org back to you.
- Stripe takeover. Change the payout bank account, refund every recent charge to “verify cards,” and watch your runway vanish over a long weekend.
- Hosting takeover. Deploy a malicious build to Vercel/Render, redirect your domain, or simply turn everything off until you pay.
- Customer phishing from your real domain. Send invoice scams to your real customer list using your real email. They will believe it. Refunds, chargebacks, and churn follow.
- Stripe account frozen for fraud. Once chargebacks spike, your processor doesn’t ask whether you did it. You lose your payments rail for weeks while you triage.
Across the incidents the Verizon DBIR catalogs each year, valid credentials remain the most common way attackers get in. Not zero-days. Not clever exploits. Just someone else’s leaked password.
How do attackers know your startup is worth targeting?
Answer: Discovery is automated and continuous—the same way defense should be.
Attackers scrape:
- Public domain and subdomain lists from certificate transparency.
- GitHub for organization names, founder/employee emails, and accidentally committed secrets.
- LinkedIn, ProductHunt, and Twitter for cofounder names and email patterns (
firstname@yourstartup.com). - Breach databases indexed by domain (
@yourstartup.com).
Once they have a list of likely team members and a list of leaked passwords for those emails, they have everything they need. A 3-person startup is just as targetable as a 300-person company—often more, because the blast radius of “the founder’s email is compromised” is total.
For the defender’s view of this same surface, see: How to Discover All Your Internet-Facing Assets
How do you break the chain? A founder’s checklist
1) Make password reuse ineffective
- Turn on MFA everywhere: Gmail/Workspace, GitHub, Vercel/Render/Fly, Stripe, your domain registrar, Supabase, OpenAI/Anthropic, your password manager itself. This is a one-evening job that ends 90% of your risk here.
- Prefer passkeys or hardware keys (FIDO2) for the founder account and any “admin” of anything. Phishing-resistant by design.
- Use a password manager (1Password, Bitwarden) for everything. Every cofounder, every contractor, day one of onboarding.
2) Block known-bad passwords in your own app
- When your users sign up or reset, check submitted passwords against breach corpora (Have I Been Pwned offers a free k-anonymity API; you never upload the full password).
- Reject any password that’s appeared in a known dump—even if it meets your length and complexity rules. This costs you almost nothing and protects your customers from their own past habits.
3) Detect stuffing attempts before they succeed
- Rate-limit logins per IP, per username, and per session. Upstash + a 10-line middleware is enough to start.
- Alert on velocity spikes: 1,000 login attempts across 1,000 different usernames from one proxy range is a stuffing campaign, not a busy day.
- Watch for impossible travel (logins from two countries within minutes) and unusual user agents. Even a daily summary email is better than nothing.
4) Monitor for your domain in breach data
- Set up alerts for
@yourstartup.comappearing in new breach dumps or infostealer logs. - When a hit appears, force password resets on every service that account touches and kill all active sessions.
- Don’t wait until the third-party notification email lands in a shared inbox nobody reads.
5) Reduce the blast radius of any single compromise
- Least privilege on every account. The contractor helping with marketing doesn’t need owner access to your Stripe.
- Session timeouts on sensitive surfaces (admin, billing, GitHub).
- Audit logs on critical actions, with alerts when recovery email, phone, or MFA settings change—on any account that matters.
6) Tighten email so attackers can’t impersonate you
- Configure SPF, DKIM, DMARC on your sending domain (move DMARC to
quarantine, thenrejectonce you’re confident). - Without this, an attacker who owns one teammate’s account can send convincing phishing as anyone else on the team—or worse, send invoice scams to your real customer list that look indistinguishable from your real emails.
Related: Top 5 Security Risks Your Website Is Probably Exposed To
A short scorecard you can run in an afternoon
| Check | Why it matters | Target |
|---|---|---|
| MFA on every founder/admin account (email, GitHub, Stripe, hosting, DNS) | Single biggest reducer of takeover risk | 100% |
| Passkeys/hardware keys on founder email + GitHub | Stops attacks that proxy one-time codes | Set up |
| Breached-password check at your own signup/reset | Protects your customers from their old habits | On |
| Login rate-limit + alert on velocity spike | Detects stuffing before hits accumulate | On |
Domain monitoring for @yourstartup.com in breach data | Find leaks before attackers use them | Set up |
| DMARC policy on your sending domain | Stops impersonation of you to your customers | reject |
A real-world pattern: from someone else’s breach to your worst week
Here’s how this chain usually plays out for a SaaS founder:
- Your cofounder signs up for a developer tool three years ago using their work email and their usual password.
- That tool gets breached. The credentials end up in a combolist that’s resold for $15.
- An attacker filters the list by domain, sees your startup, and tries the password against Google Workspace. It works.
- The attacker reads the inbox for a day. They find your Stripe email, your Vercel email, your GitHub email.
- They reset the Stripe password (the reset code lands in the inbox they now own), change the payout account, and trigger refunds on recent charges to “verify cards.”
- They push a malicious commit to your repo from a hijacked GitHub session and email your 200 paying customers a fake “service update” with a link to a phishing page on your real domain.
- By Monday: Stripe has frozen your account, your customers are demanding refunds you can’t process, your design partners are quietly leaving the Slack, and you’re spending the week explaining instead of shipping.
Total attacker effort: a few hours. Total cost to you: weeks of recovery, churned customers, a frozen payments rail, and the kind of trust damage you can’t refactor your way out of.
The breach wasn’t yours. The consequences are.
FAQs
Is credential stuffing the same as brute force?
No. Brute force tries many passwords against one account. Credential stuffing tries known-valid passwords across many accounts. Stuffing is far more effective and far harder to detect with naive rate-limits.
Does MFA fully stop this?
MFA stops the vast majority of credential stuffing attacks. Phishing-resistant MFA (passkeys, hardware keys) stops nearly all of them, including attacks that proxy one-time codes. For founders, the highest-leverage hour you can spend this month is enabling passkeys on your email, GitHub, and Stripe.
My SaaS uses Google/GitHub login only—are we safe?
Safer, but not safe. If your customer’s Google account is taken over, the attacker logs into your product as them. Your service still has to be ready: log auth events, flag impossible travel, surface session changes to the user.
Should we proactively check our team’s emails against breach databases?
Yes. Continuous breach monitoring for your domain is one of the cheapest, highest-leverage controls a founder can set up.
How often do new breaches happen?
Constantly. Have I Been Pwned indexes new dumps weekly; infostealer logs publish daily. Treat breach monitoring as a continuous control, not an annual check.
Final Thoughts
The most painful incidents in startup life often start with credentials someone else leaked. You can’t prevent third-party breaches, but you can make sure their fallout never reaches your inbox, your repos, or your Stripe.
A short list of controls—MFA + passkeys on the accounts that matter, breached-password screening in your own app, login anomaly alerts, and continuous breach monitoring for your domain—is enough to disarm most of these campaigns before they become your worst week.
Want to know if your or your cofounder’s credentials are already in public breach data—without checking every week yourself?
Try Warin — continuous external and breach monitoring with clear, AI-generated fix guides, built for founders who’d rather be shipping.
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