What is a .csr file?

About the format

A .csr file is a certificate signing request (PKCS#10) — the document you send TO a certificate authority to get a certificate issued. It contains the public key, the requested subject (and, in any modern request, the subject-alternative-name list of hostnames), and a self-signature proving you hold the matching private key. The CA verifies your control of the names, then returns a signed certificate; the CSR itself is public material, safe to paste into a CA's order form. The workflow trips people in two places. First, the private key is generated ALONGSIDE the CSR and never leaves your machine — if you lose that key file, the certificate the CA returns is useless and you start over. Second, names not listed in the CSR's SAN section generally won't be in the certificate (CAs ignore the legacy Common Name now), so 'why doesn't my cert cover www?' usually traces back to a CSR that never asked for it. Decoding a CSR before submitting it — checking the SANs and key size — takes seconds and prevents both problems.

How to open a .csr file

Is a .csr file dangerous?

No — it's public request material (the private key stays with you).

Related file types