Public certificate authorities seem to always include the basic constraints extension. Why is this a best practice? What is the risk for a PKI to issuing end-entity certificates that don't have ‘certificate signing’ in the key usage list but lack the basic constraints extension?
I have read parts of https://www.ietf.org/rfc/rfc5280.txt. From what I understand, CA certificates must have the keyCertSign bit and critical basic constraints extension. I still wonder if certificates for TLS or Code Signing must have the basic constraints extension, because the key usage bits already seem to prevent misuse. If it is because of certain legacy implementations, I would like to know which ones.
As experiment, I issued a certificate from an intermediate certificate without the basic constraints extension and with key usage ‘Digital Signature, Key Encipherment (a0)’. For this invalid certificate in the chain, Windows certificate viewer says 'This certificate does not appear to be valid for the selected purpose'. So, I can't find a specific exploit for this certificate in the current Windows implementation, but am not sure if it is vulnerable for other uses.