API key definition
An API key is a unique identifier used to authenticate a user, application, or service that wants to access an application programming interface (API). It works like a digital access token — confirming that the requester is authorized to use specific data or features.
Developers use API keys to connect software systems securely. For example, a weather app uses an API key to request forecasts from a data provider, while a payment platform issues keys to verify transactions between merchants and customers. Each key is linked to permissions that define what the app can and cannot do.
See also: API attack, credentials, data breach, cybersecurity, tokenization, application programming interface, encryption
How API keys work
When an app or script calls an API, it includes its key in the request header or query string. The API server checks that key against its internal records:
- If the key is valid and active, access is granted.
- If the key is missing, expired, or revoked, the request is rejected.
Many APIs also log each request along with the key ID to monitor usage or detect anomalies. To reduce risk, providers can restrict keys to certain IP addresses, regions, or specific functions.
Exposed API keys — often found accidentally in public GitHub repositories or unsecured config files — are a major security concern. Attackers can use them to access data, send spam, or exploit paid resources at the owner’s expense.
Why API keys matter
API keys form a first line of defense in controlling access between systems. They help service providers verify traffic, track usage, and detect abuse. However, they’re not a complete security solution — API keys must be stored securely, rotated regularly, and combined with stronger authentication methods like OAuth or digital signatures to prevent misuse.
