CSR parser
Decode a PEM or DER PKCS#10 certificate signing request into subject, public key and requested extensions (including subjectAltName, when present in an extensionRequest attribute). Reports the request, not a decision about whether it should be signed.
CSR parser reads a certificate signing request in PEM or DER form and prints its version, subject name, public key and every extension it asks for. Any subjectAltName lives in the extensionRequest attribute and is decoded from there. No cryptographic check happens, so a CSR with a broken signature decodes as cleanly as a valid one.
Questions
What does it show me?
The version, the subject name rendered as CN, O, C and so on, a description of the public key, and any extensions requested inside the PKCS#9 extensionRequest attribute. It reads PEM or raw DER PKCS#10, and it reports the request rather than any opinion about whether it should be signed.
Where are the subject alternative names?
Under requested extensions, when they are there. A CSR almost never carries SANs on the request body itself; they arrive inside an extensionRequest attribute, which this tool walks and decodes. If the line says requested extensions: none, the CSR does not ask for any.
Does it verify the CSR signature?
No. A PKCS#10 request is self signed by the key it contains, and this tool does not check that signature or perform any cryptographic verification at all. It decodes the DER structure and prints what is in it, which means a request with a broken signature parses exactly like a good one.
What error do I get if I hand it a certificate?
If the structure does not look like a request it stops with "not a CertificationRequest SEQUENCE" and your file name. Certificates go to x509-parse and revocation lists go to crl-parse. This tool reads the DER structure rather than the PEM label, so a mislabelled file is judged by its shape.
Does my CSR or key get uploaded?
No. Parsing happens in a Web Worker in this tab with the site own DER decoder, and nothing is transmitted. A CSR carries only a public key, so it is not secret the way a private key is, but the same guarantee applies to everything else you drop on this site.