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 #if (OPENSSL_VERSION_NUMBER < 0x10100000L) || \
52 	(defined(LIBRESSL_VERSION_NUMBER) && LIBRESSL_VERSION_NUMBER < 0x20700000L)
53 #define ASN1_STRING_get0_data ASN1_STRING_data
54 #endif
55 
56 /**
57 * Tries to find a match for hostname in the certificate's Common Name field.
58 *
59 * Returns MatchFound if a match was found.
60 * Returns MatchNotFound if no matches were found.
61 * Returns MalformedCertificate if the Common Name had a NUL character embedded in it.
62 * Returns Error if the Common Name could not be extracted.
63 */
matches_common_name(const char * hostname,const X509 * server_cert)64 static HostnameValidationResult matches_common_name(const char *hostname, const X509 *server_cert) {
65         int common_name_loc = -1;
66         X509_NAME_ENTRY *common_name_entry = NULL;
67         ASN1_STRING *common_name_asn1 = NULL;
68         const char *common_name_str = NULL;
69 
70         // Find the position of the CN field in the Subject field of the certificate
71         common_name_loc = X509_NAME_get_index_by_NID(X509_get_subject_name((X509 *) server_cert), NID_commonName, -1);
72         if (common_name_loc < 0) {
73                 return Error;
74         }
75 
76         // Extract the CN field
77         common_name_entry = X509_NAME_get_entry(X509_get_subject_name((X509 *) server_cert), common_name_loc);
78         if (common_name_entry == NULL) {
79                 return Error;
80         }
81 
82         // Convert the CN field to a C string
83         common_name_asn1 = X509_NAME_ENTRY_get_data(common_name_entry);
84         if (common_name_asn1 == NULL) {
85                 return Error;
86         }
87         common_name_str = (char *) ASN1_STRING_get0_data(common_name_asn1);
88 
89         // Make sure there isn't an embedded NUL character in the CN
90         if ((size_t)ASN1_STRING_length(common_name_asn1) != strlen(common_name_str)) {
91                 return MalformedCertificate;
92         }
93 
94         // Compare expected hostname with the CN
95         if (Curl_cert_hostcheck(common_name_str, hostname) == CURL_HOST_MATCH) {
96                 return MatchFound;
97         }
98         else {
99                 return MatchNotFound;
100         }
101 }
102 
103 
104 /**
105 * Tries to find a match for hostname in the certificate's Subject Alternative Name extension.
106 *
107 * Returns MatchFound if a match was found.
108 * Returns MatchNotFound if no matches were found.
109 * Returns MalformedCertificate if any of the hostnames had a NUL character embedded in it.
110 * Returns NoSANPresent if the SAN extension was not present in the certificate.
111 */
matches_subject_alternative_name(const char * hostname,const X509 * server_cert)112 static HostnameValidationResult matches_subject_alternative_name(const char *hostname, const X509 *server_cert) {
113         HostnameValidationResult result = MatchNotFound;
114         int i;
115         int san_names_nb = -1;
116         STACK_OF(GENERAL_NAME) *san_names = NULL;
117 
118         // Try to extract the names within the SAN extension from the certificate
119         san_names = X509_get_ext_d2i((X509 *) server_cert, NID_subject_alt_name, NULL, NULL);
120         if (san_names == NULL) {
121                 return NoSANPresent;
122         }
123         san_names_nb = sk_GENERAL_NAME_num(san_names);
124 
125         // Check each name within the extension
126         for (i=0; i<san_names_nb; i++) {
127                 const GENERAL_NAME *current_name = sk_GENERAL_NAME_value(san_names, i);
128 
129                 if (current_name->type == GEN_DNS) {
130                         // Current name is a DNS name, let's check it
131                         const char *dns_name = (char *) ASN1_STRING_get0_data(current_name->d.dNSName);
132 
133                         // Make sure there isn't an embedded NUL character in the DNS name
134                         if ((size_t)ASN1_STRING_length(current_name->d.dNSName) != strlen(dns_name)) {
135                                 result = MalformedCertificate;
136                                 break;
137                         }
138                         else { // Compare expected hostname with the DNS name
139                                 if (Curl_cert_hostcheck(dns_name, hostname)
140                                     == CURL_HOST_MATCH) {
141                                         result = MatchFound;
142                                         break;
143                                 }
144                         }
145                 }
146         }
147         sk_GENERAL_NAME_pop_free(san_names, GENERAL_NAME_free);
148 
149         return result;
150 }
151 
152 
153 /**
154 * Validates the server's identity by looking for the expected hostname in the
155 * server's certificate. As described in RFC 6125, it first tries to find a match
156 * in the Subject Alternative Name extension. If the extension is not present in
157 * the certificate, it checks the Common Name instead.
158 *
159 * Returns MatchFound if a match was found.
160 * Returns MatchNotFound if no matches were found.
161 * Returns MalformedCertificate if any of the hostnames had a NUL character embedded in it.
162 * Returns Error if there was an error.
163 */
validate_hostname(const char * hostname,const X509 * server_cert)164 HostnameValidationResult validate_hostname(const char *hostname, const X509 *server_cert) {
165         HostnameValidationResult result;
166 
167         if((hostname == NULL) || (server_cert == NULL))
168                 return Error;
169 
170         // First try the Subject Alternative Names extension
171         result = matches_subject_alternative_name(hostname, server_cert);
172         if (result == NoSANPresent) {
173                 // Extension was not found: try the Common Name
174                 result = matches_common_name(hostname, server_cert);
175         }
176 
177         return result;
178 }
179