Normally CSR does not need to be secret. CSR is signed by the applicants private key. If an attacker intercepts a CSR, changes email address and sends CSR on CA, CA will reject it, because the signature will not match the content of the CSR. If the attacker signs the changed CSR with attackers private key, the signature will still be invalid, because it ...
This applies over the entire certificate data. Per RFC 5280, section 18.104.22.168:
The signatureValue field contains a digital signature computed upon
the ASN.1 DER encoded tbsCertificate. The ASN.1 DER encoded
tbsCertificate is used as the input to the signature function. This
signature value is encoded as a BIT STRING and included in the