sample/openssl_hostname_validation.c

   1 /* Obtained from: https://github.com/iSECPartners/ssl-conservatory */
   2
   3 /*
   4 Copyright (C) 2012, iSEC Partners.
   5
   6 Permission is hereby granted, free of charge, to any person obtaining a copy of
   7 this software and associated documentation files (the "Software"), to deal in
   8 the Software without restriction, including without limitation the rights to
   9 use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies
  10 of the Software, and to permit persons to whom the Software is furnished to do
  11 so, subject to the following conditions:
  12
  13 The above copyright notice and this permission notice shall be included in all
  14 copies or substantial portions of the Software.
  15
  16 THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  17 IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  18 FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
  19 AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
  20 LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
  21 OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
  22 SOFTWARE.
  23  */
  24
  25 /*
  26  * Helper functions to perform basic hostname validation using OpenSSL.
  27  *
  28  * Please read "everything-you-wanted-to-know-about-openssl.pdf" before
  29  * attempting to use this code. This whitepaper describes how the code works,
  30  * how it should be used, and what its limitations are.
  31  *
  32  * Author:  Alban Diquet
  33  * License: See LICENSE
  34  *
  35  */
  36
  37 // Get rid of OSX 10.7 and greater deprecation warnings.
  38 #if defined(__APPLE__) && defined(__clang__)
  39 #pragma clang diagnostic ignored "-Wdeprecated-declarations"
  40 #endif
  41
  42 #include <openssl/x509v3.h>
  43 #include <openssl/ssl.h>
  44 #include <string.h>
  45
  46 #include "openssl_hostname_validation.h"
  47 #include "hostcheck.h"
  48
  49 #define HOSTNAME_MAX_SIZE 255
  50
  51 /**
  52 * Tries to find a match for hostname in the certificate's Common Name field.
  53 *
  54 * Returns MatchFound if a match was found.
  55 * Returns MatchNotFound if no matches were found.
  56 * Returns MalformedCertificate if the Common Name had a NUL character embedded in it.
  57 * Returns Error if the Common Name could not be extracted.
  58 */
  59 static HostnameValidationResult matches_common_name(const char *hostname, const X509 *server_cert) {
  60         int common_name_loc = -1;
  61         X509_NAME_ENTRY *common_name_entry = NULL;
  62         ASN1_STRING *common_name_asn1 = NULL;
  63         char *common_name_str = NULL;
  64
  65         // Find the position of the CN field in the Subject field of the certificate
  66         common_name_loc = X509_NAME_get_index_by_NID(X509_get_subject_name((X509 *) server_cert), NID_commonName, -1);
  67         if (common_name_loc < 0) {
  68                 return Error;
  69         }
  70
  71         // Extract the CN field
  72         common_name_entry = X509_NAME_get_entry(X509_get_subject_name((X509 *) server_cert), common_name_loc);
  73         if (common_name_entry == NULL) {
  74                 return Error;
  75         }
  76
  77         // Convert the CN field to a C string
  78         common_name_asn1 = X509_NAME_ENTRY_get_data(common_name_entry);
  79         if (common_name_asn1 == NULL) {
  80                 return Error;
  81         }
  82         common_name_str = (char *) ASN1_STRING_data(common_name_asn1);
  83
  84         // Make sure there isn't an embedded NUL character in the CN
  85         if ((size_t)ASN1_STRING_length(common_name_asn1) != strlen(common_name_str)) {
  86                 return MalformedCertificate;
  87         }
  88
  89         // Compare expected hostname with the CN
  90         if (Curl_cert_hostcheck(common_name_str, hostname) == CURL_HOST_MATCH) {
  91                 return MatchFound;
  92         }
  93         else {
  94                 return MatchNotFound;
  95         }
  96 }
  97
  98
  99 /**
 100 * Tries to find a match for hostname in the certificate's Subject Alternative Name extension.
 101 *
 102 * Returns MatchFound if a match was found.
 103 * Returns MatchNotFound if no matches were found.
 104 * Returns MalformedCertificate if any of the hostnames had a NUL character embedded in it.
 105 * Returns NoSANPresent if the SAN extension was not present in the certificate.
 106 */
 107 static HostnameValidationResult matches_subject_alternative_name(const char *hostname, const X509 *server_cert) {
 108         HostnameValidationResult result = MatchNotFound;
 109         int i;
 110         int san_names_nb = -1;
 111         STACK_OF(GENERAL_NAME) *san_names = NULL;
 112
 113         // Try to extract the names within the SAN extension from the certificate
 114         san_names = X509_get_ext_d2i((X509 *) server_cert, NID_subject_alt_name, NULL, NULL);
 115         if (san_names == NULL) {
 116                 return NoSANPresent;
 117         }
 118         san_names_nb = sk_GENERAL_NAME_num(san_names);
 119
 120         // Check each name within the extension
 121         for (i=0; i<san_names_nb; i++) {
 122                 const GENERAL_NAME *current_name = sk_GENERAL_NAME_value(san_names, i);
 123
 124                 if (current_name->type == GEN_DNS) {
 125                         // Current name is a DNS name, let's check it
 126                         char *dns_name = (char *) ASN1_STRING_data(current_name->d.dNSName);
 127
 128                         // Make sure there isn't an embedded NUL character in the DNS name
 129                         if ((size_t)ASN1_STRING_length(current_name->d.dNSName) != strlen(dns_name)) {
 130                                 result = MalformedCertificate;
 131                                 break;
 132                         }
 133                         else { // Compare expected hostname with the DNS name
 134                                 if (Curl_cert_hostcheck(dns_name, hostname)
 135                                     == CURL_HOST_MATCH) {
 136                                         result = MatchFound;
 137                                         break;
 138                                 }
 139                         }
 140                 }
 141         }
 142         sk_GENERAL_NAME_pop_free(san_names, GENERAL_NAME_free);
 143
 144         return result;
 145 }
 146
 147
 148 /**
 149 * Validates the server's identity by looking for the expected hostname in the
 150 * server's certificate. As described in RFC 6125, it first tries to find a match
 151 * in the Subject Alternative Name extension. If the extension is not present in
 152 * the certificate, it checks the Common Name instead.
 153 *
 154 * Returns MatchFound if a match was found.
 155 * Returns MatchNotFound if no matches were found.
 156 * Returns MalformedCertificate if any of the hostnames had a NUL character embedded in it.
 157 * Returns Error if there was an error.
 158 */
 159 HostnameValidationResult validate_hostname(const char *hostname, const X509 *server_cert) {
 160         HostnameValidationResult result;
 161
 162         if((hostname == NULL) || (server_cert == NULL))
 163                 return Error;
 164
 165         // First try the Subject Alternative Names extension
 166         result = matches_subject_alternative_name(hostname, server_cert);
 167         if (result == NoSANPresent) {
 168                 // Extension was not found: try the Common Name
 169                 result = matches_common_name(hostname, server_cert);
 170         }
 171
 172         return result;
 173 }