What is a data leak?
Author: Domeny.ForSale editorial team Published
In brief: A data leak occurs when information reaches unauthorised people or is made available to them. It may result from an attack, human error or inadequate system security—it does not necessarily mean that data has been published online. The consequences depend primarily on the type of information exposed: an email address leak calls for different action than the exposure of a password or a PESEL number, Poland’s national identification number.
When does a data leak occur?
Examples of data leaks include someone stealing a customer database, an employee sending a document to the wrong recipient, or a folder containing confidential files being publicly accessible without a login. Data can be copied while still remaining in the company’s system. The fact that no files have disappeared is therefore no indication that the data is secure.
A leak may involve personal data, meaning information that can identify a person, but also trade secrets, program code or API keys—credentials that allow programs to use services. Not every incident is therefore a privacy issue; sometimes the threat is access to infrastructure or the loss of confidentiality of documentation.
In the context of the GDPR, the broader term is personal data breach. This covers not only unauthorised disclosure or access, but also, among other things, the loss, destruction or alteration of data. The word “leak” alone does not describe every possible breach.
How does information get exposed?
The cause may be a software vulnerability, but it can just as easily be an ordinary mistake. Typical scenarios include:
- Account takeover: an attacker obtains a password, for example through phishing—impersonating a trusted sender to trick someone into disclosing information.
- Incorrect permissions: a file, backup or database is accessible to more people than it should be.
- Sending error: a message containing data goes to the wrong recipient or exposes recipients’ addresses in the “To” field.
- Malware: a program steals files, passwords or active session data that may allow someone to access an account without logging in again.
- Actions by someone with access: an employee or contractor copies information beyond the scope of their authorisation.
A leak at a service provider can affect a user who has made no mistake themselves. Securing your own device is therefore necessary, but it does not replace choosing services carefully and limiting the amount of data you share.
What determines the consequences of a leak?
The most important thing is to establish exactly what was exposed and what other information it can be combined with. An email address alone is usually not enough to log in to an account. Combined with a purchase history, however, it can make it easier to craft a convincing message supposedly sent by a shop.
| Exposed information | Main threat | First action |
|---|---|---|
| Email address or phone number | Targeted scams and unwanted contact | Verify the sender through an independent channel |
| Password | Account takeover and login attempts on other services | Change the password everywhere you reused it |
| Payment card details | Unauthorised payments | Contact your bank |
| PESEL number or identity document details | Attempts to impersonate the person whose data was exposed | Consider placing a protective restriction on your PESEL number and check further recommendations |
| Medical records or private correspondence | Privacy violations, blackmail | Preserve evidence and secure access to your accounts |
For example, a shop reports a leak of email addresses and order histories, but not passwords or payment details. The most immediate risk may be a fraudulent request for an additional payment on a specific order. Such a notice does not automatically mean you need to replace your card, but it does mean you should verify payment requests directly with the shop.
What should you do after being notified of a leak?
First, confirm that the notice is genuine. Open the service provider’s app or type its website address yourself, rather than following a link in a suspicious message. Fake data leak warnings can also be used to trick people into disclosing data.
- Establish the scope of the incident. Check what data was exposed, whether your account is affected and what action the service provider recommends. An unclear notice is a reason to ask questions, not proof that everything has been leaked.
- Secure the accounts at risk. If a password was exposed, set a new, unique one. Change it on any other services where you used it as well; give particular priority to your email account, which can be used to reset other passwords.
- Check access. Sign out unfamiliar sessions or all active sessions, verify your account recovery details and enable multi-factor authentication—an additional step to confirm a login.
- Match your response to the data involved. If financial information is affected, contact your bank through an official channel. Check the rules and scope of protection associated with placing a restriction on your PESEL number on a public administration website or in the mObywatel app.
- Keep records. Save the notice, dates, suspicious messages and transaction information. In cases of fraud, these will help you report the matter to your bank or the police.
Do not enter your password, PESEL number or card details into a random “data leak checker”. The absence of an email address from a database of known incidents also does not guarantee that the data has never been exposed.
How can you reduce risk and respond within a company?
Users can limit the consequences of a future leak by using unique passwords stored in a password manager, adding an extra login verification step and deleting accounts they no longer need. These measures will not prevent every disclosure of information, but they make it harder to use one incident to take over accounts on other services.
A company should restrict unauthorised access, preserve evidence of the incident and establish its scope. Employees should promptly notify the person responsible for security rather than deleting logs or evidence themselves. Assessing the incident may also require the involvement of the data protection officer, if one has been appointed.
The obligations to report a breach to the supervisory authority and notify affected people depend on the circumstances and the risk assessment. This is general information, not individual legal advice; current requirements and deadlines should be confirmed in the GDPR and materials from UODO, Poland’s Personal Data Protection Office.
If you have just received a warning, start by establishing the scope of the exposed data from the official notice. This information will help you choose urgent actions rather than make random changes.
See also: What is CERT Polska?
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