What is a .cer file?

About the format

A .cer file holds a single digital certificate — the public identity document of TLS and code signing: who the certificate belongs to (subject), who vouches for it (issuer), the validity window, and the public key. The extension doesn't tell you the encoding: a .cer can be binary DER (starts with byte 0x30) or Base64 PEM (starts with -----BEGIN CERTIFICATE-----); Windows opens both with its certificate viewer on double-click. Important reassurance: a .cer contains NO private key — it's the shareable half, safe to email or publish. The private key travels only in .pfx/.p12 bundles or .key files. Common reasons a .cer lands in your hands: a web-server certificate to install, a corporate root certificate IT wants in your trust store, a code-signing certificate to verify, or an export made while debugging an HTTPS problem. Installing one into 'Trusted Root Certification Authorities' is a real trust decision — anything signed by that certificate will be believed — so do that only for certificates whose origin you're sure of.

How to open a .cer file

Is a .cer file dangerous?

The file itself can't run. The risk is TRUSTING a bad certificate (installing it as a root) — that's a decision, not an accident.

Related file types