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openssl_hostname_validation.c
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144 lines (121 loc) · 4.54 KB
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/*
* Helper functions to perform basic hostname validation using OpenSSL.
*
* Please read "everything-you-wanted-to-know-about-openssl.pdf" before
* attempting to use this code. This whitepaper describes how the code works,
* how it should be used, and what its limitations are.
*
* Author: Alban Diquet
* License: See LICENSE
*
*/
#include <strings.h>
#include <openssl/x509v3.h>
#include <openssl/ssl.h>
#include "openssl_hostname_validation.h"
#define HOSTNAME_MAX_SIZE 255
/**
* Tries to find a match for hostname in the certificate's Common Name field.
*
* Returns MatchFound if a match was found.
* Returns MatchNotFound if no matches were found.
* Returns MalformedCertificate if the Common Name had a NUL character embedded in it.
* Returns Error if the Common Name could not be extracted.
*/
static HostnameValidationResult matches_common_name(const char *hostname, const X509 *server_cert) {
int common_name_loc = -1;
X509_NAME_ENTRY *common_name_entry = NULL;
ASN1_STRING *common_name_asn1 = NULL;
char *common_name_str = NULL;
// Find the position of the CN field in the Subject field of the certificate
common_name_loc = X509_NAME_get_index_by_NID(X509_get_subject_name((X509 *) server_cert), NID_commonName, -1);
if (common_name_loc < 0) {
return Error;
}
// Extract the CN field
common_name_entry = X509_NAME_get_entry(X509_get_subject_name((X509 *) server_cert), common_name_loc);
if (common_name_entry == NULL) {
return Error;
}
// Convert the CN field to a C string
common_name_asn1 = X509_NAME_ENTRY_get_data(common_name_entry);
if (common_name_asn1 == NULL) {
return Error;
}
common_name_str = (char *) ASN1_STRING_data(common_name_asn1);
// Make sure there isn't an embedded NUL character in the CN
if (ASN1_STRING_length(common_name_asn1) != strlen(common_name_str)) {
return MalformedCertificate;
}
// Compare expected hostname with the CN
if (strcasecmp(hostname, common_name_str) == 0) {
return MatchFound;
}
else {
return MatchNotFound;
}
}
/**
* Tries to find a match for hostname in the certificate's Subject Alternative Name extension.
*
* Returns MatchFound if a match was found.
* Returns MatchNotFound if no matches were found.
* Returns MalformedCertificate if any of the hostnames had a NUL character embedded in it.
* Returns NoSANPresent if the SAN extension was not present in the certificate.
*/
static HostnameValidationResult matches_subject_alternative_name(const char *hostname, const X509 *server_cert) {
HostnameValidationResult result = MatchNotFound;
int i;
int san_names_nb = -1;
STACK_OF(GENERAL_NAME) *san_names = NULL;
// Try to extract the names within the SAN extension from the certificate
san_names = X509_get_ext_d2i((X509 *) server_cert, NID_subject_alt_name, NULL, NULL);
if (san_names == NULL) {
return NoSANPresent;
}
san_names_nb = sk_GENERAL_NAME_num(san_names);
// Check each name within the extension
for (i=0; i<san_names_nb; i++) {
const GENERAL_NAME *current_name = sk_GENERAL_NAME_value(san_names, i);
if (current_name->type == GEN_DNS) {
// Current name is a DNS name, let's check it
char *dns_name = (char *) ASN1_STRING_data(current_name->d.dNSName);
// Make sure there isn't an embedded NUL character in the DNS name
if (ASN1_STRING_length(current_name->d.dNSName) != strlen(dns_name)) {
result = MalformedCertificate;
break;
}
else { // Compare expected hostname with the DNS name
if (strcasecmp(hostname, dns_name) == 0) {
result = MatchFound;
break;
}
}
}
}
sk_GENERAL_NAME_pop_free(san_names, GENERAL_NAME_free);
return result;
}
/**
* Validates the server's identity by looking for the expected hostname in the
* server's certificate. As described in RFC 6125, it first tries to find a match
* in the Subject Alternative Name extension. If the extension is not present in
* the certificate, it checks the Common Name instead.
*
* Returns MatchFound if a match was found.
* Returns MatchNotFound if no matches were found.
* Returns MalformedCertificate if any of the hostnames had a NUL character embedded in it.
* Returns Error if there was an error.
*/
HostnameValidationResult validate_hostname(const char *hostname, const X509 *server_cert) {
HostnameValidationResult result;
if((hostname == NULL) || (server_cert == NULL))
return Error;
// First try the Subject Alternative Names extension
result = matches_subject_alternative_name(hostname, server_cert);
if (result == NoSANPresent) {
// Extension was not found: try the Common Name
result = matches_common_name(hostname, server_cert);
}
return result;
}