use iSECPartners code to validate hostname in certificate

The problem is that if you go to a website whose certificate does not
match its hostname, it should fail.  Try this in a web browser for
https://www.kegel.com/ for example.  Your web browser will say the
certificate is for *.pair.com, not for www.kegel.com, and won't let
you visit it without clicking through a bunch of scary warnings.

However, prior to this commit, https-client was happy to fetch
https://www.kegel.com/ without complaining.  That is bad.  Now, with
this commit, it will properly complain, which is good:

pelletier@chives:~/src/libevent/sample$ ./https-client https://www.kegel.com/
Got 'MatchNotFound' for hostname 'www.kegel.com' and certificate:
/C=US/postalCode=15203/ST=Pennsylvania/L=Pittsburgh/street=Suite 210/street=2403 Sidney Street/O=pair Networks, Inc./OU=Provided by pair Networks, Inc./OU=PairWildcardSSL $250,000/CN=*.pair.com
some request failed - no idea which one though!
error:14090086:SSL routines:SSL3_GET_SERVER_CERTIFICATE:certificate verify failed
ppelletier@chives:~/src/libevent/sample$

It will still succeed for sites with an exactly-matching certificate,
such as https://github.com/ and that is also good!

However, the problem is that the iSECPartners code doesn't handle
wildcards, which means we reject https://ip.appspot.com/ even though
it is perfectly legitimate, because we don't understand the wildcard:

ppelletier@chives:~/src/libevent/sample$ ./https-client https://ip.appspot.com/
Got 'MatchNotFound' for hostname 'ip.appspot.com' and certificate:
/C=US/ST=California/L=Mountain View/O=Google Inc/CN=*.appspot.com
some request failed - no idea which one though!
error:14090086:SSL routines:SSL3_GET_SERVER_CERTIFICATE:certificate verify failed
ppelletier@chives:~/src/libevent/sample$

So, we need to fix this.  In other words, "to be continued..."

sample/https-client.c

@@ -36,6 +36,8 @@
 #include <openssl/err.h>
 #include <openssl/rand.h>
 
+#include "openssl_hostname_validation.h"
+
 static struct event_base *base;
 
 static void
@@ -110,6 +112,61 @@ die_openssl(const char *func)
 	exit(1);
 }
 
+/* See http://archives.seul.org/libevent/users/Jan-2013/msg00039.html */
+static int cert_verify_callback(X509_STORE_CTX *x509_ctx, void *arg)
+{
+	char cert_str[256];
+	const char *host = (const char *) arg;
+	const char *res_str = "X509_verify_cert failed";
+	HostnameValidationResult res = Error;
+
+	/* This is the function that OpenSSL would call if we hadn't called
+	 * SSL_CTX_set_cert_verify_callback().  Therefore, we are "wrapping"
+	 * the default functionality, rather than replacing it. */
+	int ok_so_far = X509_verify_cert(x509_ctx);
+
+	X509 *server_cert = X509_STORE_CTX_get_current_cert(x509_ctx);
+
+	if (ok_so_far) {
+		res = validate_hostname(host, server_cert);
+
+		switch (res) {
+		case MatchFound:
+			res_str = "MatchFound";
+			break;
+		case MatchNotFound:
+			res_str = "MatchNotFound";
+			break;
+		case NoSANPresent:
+			res_str = "NoSANPresent";
+			break;
+		case MalformedCertificate:
+			res_str = "MalformedCertificate";
+			break;
+		case Error:
+			res_str = "Error";
+			break;
+		default:
+			res_str = "WTF!";
+			break;
+		}
+	}
+
+	X509_NAME_oneline(X509_get_subject_name (server_cert),
+			  cert_str, sizeof (cert_str));
+
+	if (res == MatchFound) {
+		printf("https server '%s' has this certificate, "
+		       "which looks good to me:\n%s\n",
+		       host, cert_str);
+		return 1;
+	} else {
+		printf("Got '%s' for hostname '%s' and certificate:\n%s\n",
+		       res_str, host, cert_str);
+		return 0;
+	}
+}
+
 int
 main(int argc, char **argv)
 {
@@ -188,7 +245,30 @@ main(int argc, char **argv)
 					       "/etc/ssl/certs/ca-certificates.crt",
 					       NULL))
 		die_openssl("SSL_CTX_load_verify_locations");
+	/* Ask OpenSSL to verify the server certificate.  Note that this
+	 * does NOT include verifying that the hostname is correct.
+	 * So, by itself, this means anyone with any legitimate
+	 * CA-issued certificate for any website, can impersonate any
+	 * other website in the world.  This is not good.  See "The
+	 * Most Dangerous Code in the World" article at
+	 * https://crypto.stanford.edu/~dabo/pubs/abstracts/ssl-client-bugs.html
+	 */
 	SSL_CTX_set_verify(ssl_ctx, SSL_VERIFY_PEER, NULL);
+	/* This is how we solve the problem mentioned in the previous
+	 * comment.  We "wrap" OpenSSL's validation routine in our
+	 * own routine, which also validates the hostname by calling
+	 * the code provided by iSECPartners.  Note that even though
+	 * the "Everything You've Always Wanted to Know About
+	 * Certificate Validation With OpenSSL (But Were Afraid to
+	 * Ask)" paper from iSECPartners says very explicitly not to
+	 * call SSL_CTX_set_cert_verify_callback (at the bottom of
+	 * page 2), what we're doing here is safe because our
+	 * cert_verify_callback() calls X509_verify_cert(), which is
+	 * OpenSSL's built-in routine which would have been called if
+	 * we hadn't set the callback.  Therefore, we're just
+	 * "wrapping" OpenSSL's routine, not replacing it. */
+	SSL_CTX_set_cert_verify_callback (ssl_ctx, cert_verify_callback,
+					  (void *) host);
 
 	// Create event base
 	base = event_base_new();

sample/include.am

@@ -19,9 +19,12 @@ sample_le_proxy_LDADD = libevent.la libevent_openssl.la ${OPENSSL_LIBS} ${OPENSS
 sample_le_proxy_INCLUDES = $(OPENSSL_INCS)
 
 SAMPLES += sample/https-client
-sample_https_client_SOURCES = sample/https-client.c
+sample_https_client_SOURCES = \
+	sample/https-client.c \
+	sample/openssl_hostname_validation.c
 sample_https_client_LDADD = libevent.la libevent_openssl.la ${OPENSSL_LIBS} ${OPENSSL_LIBADD}
 sample_https_client_INCLUDES = $(OPENSSL_INCS)
+noinst_HEADERS += sample/openssl_hostname_validation.h
 endif
 
 noinst_PROGRAMS += $(SAMPLES)

sample/openssl_hostname_validation.c

@@ -0,0 +1,167 @@
+/* Obtained from: https://github.com/iSECPartners/ssl-conservatory */
+
+/*
+Copyright (C) 2012, iSEC Partners.
+
+Permission is hereby granted, free of charge, to any person obtaining a copy of
+this software and associated documentation files (the "Software"), to deal in
+the Software without restriction, including without limitation the rights to
+use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies
+of the Software, and to permit persons to whom the Software is furnished to do
+so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
+ */
+
+/*
+ * 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 <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;
+}

sample/openssl_hostname_validation.h

@@ -0,0 +1,56 @@
+/* Obtained from: https://github.com/iSECPartners/ssl-conservatory */
+
+/*
+Copyright (C) 2012, iSEC Partners.
+
+Permission is hereby granted, free of charge, to any person obtaining a copy of
+this software and associated documentation files (the "Software"), to deal in
+the Software without restriction, including without limitation the rights to
+use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies
+of the Software, and to permit persons to whom the Software is furnished to do
+so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
+ */
+
+/*
+ * 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
+ *
+ */
+
+typedef enum {
+        MatchFound,
+        MatchNotFound,
+        NoSANPresent,
+        MalformedCertificate,
+        Error
+} HostnameValidationResult;
+
+/**
+* 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);