What is a Data Leak

Learn what a data leak is and how to prevent one.

IBM’s 2025 report found that the average data breach costs $4.44 million. But many of those breaches started as data leaks – accidental exposures that nobody noticed until attackers did.

A misconfigured database, a password pushed to a public repo, an employee emailing the wrong file. These aren’t attacks. They’re mistakes that hand attackers exactly what they need.

The difference between a leak that costs nothing and one that costs millions is how fast you find it.

This guide covers what data leaks are, how they happen, and how to prevent them.

What Is a Data Leak?

A data leak happens when sensitive information gets exposed without anyone intending it to. No attacker breaks in. Someone on your side made a mistake.

The key word is “unintentional.” A developer pushes API keys to a public GitHub repo. A database admin leaves a cloud instance open to the internet. An employee emails a customer spreadsheet to the wrong address. Nobody meant to expose the data. But it’s exposed.

The Verizon 2025 DBIR found that human error is involved in the majority of security incidents. Data leaks are the most common result of those errors.

How Is a Data Leak Different From a Data Breach?

The difference matters for how you respond.

A data leak is accidental. Someone on your team made a mistake. A cloud bucket was left public. An email went to the wrong person. There’s no attacker involved – at least not yet.

A data breach is deliberate. An attacker broke into your systems and stole data. They used stolen credentials or exploited a vulnerability.

The gray area: Many breaches start as leaks. A developer accidentally exposes database credentials on GitHub. An attacker finds them hours later, logs in, and steals customer data. Was it a leak or a breach? It was both – the leak created the opening, and the attacker exploited it.

For notification purposes, the distinction matters. If data was only exposed but not accessed by anyone unauthorized, some jurisdictions don’t require notification. Once an attacker accesses the leaked data, it becomes a breach with full notification obligations.

How Do Data Leaks Happen?

Data leaks come from mistakes, not attacks. Here are the most common causes.

Misconfigured cloud storage. This is the #1 cause of large-scale data leaks. Public S3 buckets on AWS and databases with no authentication exposed. Automated scanners find these within hours of misconfiguration. The Microsoft leak in 2022 exposed business transaction data on 65,000 entities through a single misconfigured endpoint.

Credentials in code repositories. Developers push API keys and database passwords to public GitHub repos. Automated bots scan GitHub constantly for these commits. By the time the developer notices and removes the file, the credentials have already been harvested.

Accidental sharing. An employee emails a file to the wrong person. Someone shares a Google Drive folder with “anyone with the link.” A report gets uploaded to the wrong Slack channel. These seem minor, but if the data includes customer PII or credentials, it’s a leak.

Third-party vendor exposure. Your vendors have access to your data. When they have weak security practices, your data leaks through their systems. The Verizon 2025 DBIR tracks supply chain involvement in a growing percentage of incidents.

Improper data disposal. Old laptops sold without wiping drives. Servers decommissioned with data still on them. Physical documents thrown away instead of shredded. These low-tech leaks still happen.

Why Are Data Leaks Dangerous?

The leaked data itself is the problem. What attackers do with it makes it worse.

Leaked credentials lead to breaches. When employee passwords leak, attackers use credential stuffing to test them across hundreds of services automatically. One leaked password can compromise multiple accounts if the employee reused it.

Leaked PII leads to identity theft. Names paired with Social Security numbers or financial details give criminals everything they need to open fraudulent accounts or file fake tax returns.

Social engineering gets easier. When attackers know your internal structure and your employees’ personal details, they craft more convincing phishing emails. Business email compromise becomes easier when attackers have insider knowledge.

Regulatory penalties apply. Data leaks that expose personal information trigger the same compliance obligations as breaches. GDPR and HIPAA don’t distinguish between accidental leaks and deliberate breaches when it comes to penalties.

Reputation damage is real. Customers don’t care whether their data was leaked accidentally or stolen deliberately. The consequences to your reputation are the same either way.

Can Leaked Data Be Unleaked?

No. Once data is out, you can’t put it back.

Leaked data gets copied within minutes. It circulates across dark web forums and criminal marketplaces. Even if you remove it from the original source, copies exist everywhere. You have no way to track or delete them all.

This is why prevention and fast detection matter more than response. If you catch a misconfigured database within an hour, the exposure is limited. If it sits open for six months, the data has been scraped, sold, and redistributed many times over.

For leaked credentials specifically, you can force password resets and invalidate sessions. That doesn’t “unleak” the passwords, but it makes them useless to attackers. Speed is everything.

What Should You Do If Your Password Appears in a Data Leak?

If your phone or browser warns that your password “appeared in a data leak,” take it seriously. Apple, Google, and most browsers now check your saved passwords against known breach databases. When they find a match, you get an alert.

Here’s what to do:

Change the password immediately. Don’t reuse the old one anywhere. Use your password manager to generate a random replacement. If you used the same password on other sites (be honest), change those too.

Enable MFA on the affected account. A leaked password is less dangerous if attackers also need a second factor. Use an authenticator app, not SMS.

Check for suspicious activity. Look at recent login history for the account. Unusual locations, devices you don’t recognize, or activity at times you weren’t online are all red flags. If you see any, assume the account was compromised and follow your incident response plan.

Don’t ignore the warning. Many people dismiss these alerts, especially when they pop up frequently. But each one represents a real credential that’s circulating on criminal marketplaces. Attackers use credential stuffing tools to test leaked passwords across hundreds of services automatically.

For organizations, these individual warnings are a signal of a larger problem. If employees’ passwords are showing up in leak databases, your company may already be in attacker crosshairs.

How Do You Check If Your Data Has Been Leaked?

You don’t have to wait for a browser warning to find out.

For individuals: Have I Been Pwned lets you search by email address to see which third-party breaches include your data. It’s free and widely trusted. But it only covers known breach dumps – it won’t catch credentials stolen by infostealer malware or data exposed through unsecured databases.

For organizations: Third-party breach lookups aren’t enough. The fastest-growing source of leaked corporate credentials is infostealer malware, and those credentials appear on criminal marketplaces and Telegram channels, not in third-party breaches. Breachsense’s dark web scan lets you check your corporate domain right now. For ongoing protection, dark web monitoring watches stealer log marketplaces, hacker forums, and ransomware leak sites continuously. It also indexes data from unsecured databases (e.g. Elasticsearch or MongoDB servers left open) and catches stolen session tokens that bypass MFA.

Check your code repositories too. Search GitHub for your company’s domain name. You might find API keys, database connection strings, or internal credentials that developers accidentally committed. Tools like GitGuardian automate this scanning.

Audit your cloud configurations. Use your cloud provider’s security tools (AWS Security Hub, Azure Security Center) to check for publicly accessible resources. A misconfigured S3 bucket won’t show up in credential breach databases, but it’s still a data leak.

What Are Some Real Data Leak Examples?

Data leaks happen to companies of every size. These examples show how different types of leaks play out.

Microsoft (2022) – A misconfigured endpoint exposed business transaction data for over 65,000 entities. The data was accessible to anyone with the URL. No authentication required. Microsoft fixed it after security researchers reported it, but the data had been exposed for an unknown period. This is the classic misconfiguration leak – no attacker needed.

Twitch (2021) – The entire Twitch source code and creator payout data were leaked after a server misconfiguration. Over 125 GB of data appeared on 4chan. The leak included streamer earnings data that generated massive public attention. Twitch attributed it to a “server configuration change.”

Facebook (2021) – Personal data on 533 million Facebook users from 106 countries was posted on a hacking forum. The data included phone numbers and full names. Facebook said the data was scraped through a vulnerability that was patched in 2019, but the leaked dataset continued circulating years later. This illustrates why leaked data can’t be unleaked.

Capital One (2019) – A former AWS employee exploited a misconfigured web application firewall to access Capital One’s data on AWS. The leak exposed data on 106 million credit card applicants, including 140,000 Social Security numbers. Capital One paid $190 million to settle the class-action lawsuit plus $80 million in regulatory fines.

Each of these started with a configuration mistake or vulnerability, not a targeted attack. The attackers came after.

How Do You Prevent Data Leaks?

The root causes are human error and misconfiguration. Target both.

Audit cloud configurations continuously. Don’t assume your cloud setup is secure because it was secure last month. Configurations drift. New resources get deployed with default settings. Automate compliance checking against your provider’s security benchmarks. Tools like AWS Config and Azure Policy flag misconfigurations in real time.

Scan code repositories for secrets. Use automated scanning tools that detect credentials and API keys before they’re committed to public repos. GitGuardian and similar tools catch secrets in real time. Better yet, use environment variables and secrets managers so credentials never touch code in the first place.

Enforce least privilege access. The fewer people who can access sensitive data, the fewer opportunities for accidental exposure. Review permissions quarterly and remove access that’s no longer needed.

Encrypt sensitive data. Even if data is accidentally exposed, encryption makes it useless without the keys. Many notification laws exempt properly encrypted data from reporting requirements.

Train employees on data handling. People make mistakes. Training reduces how often. Focus on practical scenarios: how to share files securely and what to do if you send something to the wrong person. Make it safe to report mistakes without fear of punishment.

Monitor for leaked credentials. Dark web monitoring watches criminal marketplaces for your organization’s exposed passwords. When employee credentials appear in breach dumps or stealer logs, you get an alert so you can force resets before attackers log in.

Have a response plan ready. When a leak happens (and it will), your team needs to know exactly what to do. Who investigates? Who decides on notification? How fast can you contain the exposure? See our data breach response plan for the full framework.

The difference between a minor incident and a major breach is often just detection speed. Book a demo to see how Breachsense monitors the dark web for your organization’s leaked data.