CVE-2020-7043 (C, published 2020-02-27T18:15Z)
Description: value: An issue was discovered in openfortivpn 1.11.0 when used with OpenSSL before 1.0.2. tunnel.c mishandles certificate validation because hostname comparisons do not consider '\0' characters, as demonstrated by a good.example.com\x00evil.example.com attack.
Severity: MEDIUM (CVSS v3 base score: 9.1)
Weakness: CWE-295 \u2014 Improper Certificate Validation
The product does not validate, or incorrectly validates, a certificate.
Repository: https://github.com/adrienverge/openfortivpn
Files changed:
Makefile.am
src/openssl_hostname_validation.c
src/openssl_hostname_validation.h
src/tunnel.c
tests/lint/run.sh
Commit message: fix TLS Certificate CommonName NULL Byte Vulnerability

CVE-2020-7043 TLS Certificate CommonName NULL Byte Vulnerability is fixed
with this commit

with #8 hostname validation for the certificate was introduced
but unfortunately strncasecmp() was used to compare the byte array
against the expected hostname. This does not correctly treat a CN
which contains a NULL byte. In order to fix this vulnerability
the reference implementation from iSECPartners has been included
into the code.


Vulnerable code:
		       src/xml.h src/userinput.c src/userinput.h
	char common_name[FIELD_SIZE + 1];
	// Use explicit Common Name check if native validation not available.
	// Note: this will ignore Subject Alternative Name fields.
	if (subj
	    && X509_NAME_get_text_by_NID(subj, NID_commonName, common_name,
	                                 FIELD_SIZE) > 0
	    && strncasecmp(common_name, tunnel->config->gateway_host,
	                   FIELD_SIZE) == 0)
		cert_valid = 1;
./tests/lint/eol-at-eof.sh $(git ls-files) || rc=1
./tests/lint/line_length.py $(git ls-files '*.[ch]') || rc=1
./tests/lint/astyle.sh $(git ls-files '*.[ch]') || rc=1

Fixed code:
		       src/xml.h src/userinput.c src/userinput.h \
		       src/openssl_hostname_validation.c \
		       src/openssl_hostname_validation.h
/* Obtained from: https://github.com/iSECPartners/ssl-conservatory */

/*
 * Helper functions to perform basic hostname validation using OpenSSL.
 *
 * Author:  Alban Diquet
 *
 * 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.
 *
 */
 

#include <strings.h>
#include <openssl/x509v3.h>
#include <openssl/ssl.h>

#define HOSTNAME_MAX_SIZE 255

#ifndef HAVE_X509_CHECK_HOST

#include "openssl_hostname_validation.h"


/**
* 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;
}

#endif
/* Obtained from: https://github.com/iSECPartners/ssl-conservatory */

/*
 * Helper functions to perform basic hostname validation using OpenSSL.
 *
 * Author:  Alban Diquet
 *
 * 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.
 *
 */

#ifndef HAVE_X509_CHECK_HOST
 
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);

#endif
#ifndef HAVE_X509_CHECK_HOST
#include "openssl_hostname_validation.h"
#endif
	// Use validate_hostname form iSECPartners if native validation not available
	// in order to avoid TLS Certificate CommonName NULL Byte Vulnerability
	if (validate_hostname(tunnel->config->gateway_host, cert) == MatchFound)
 		cert_valid = 1;
./tests/lint/eol-at-eof.sh $(git ls-files | grep -v openssl_hostname_validation) || rc=1
./tests/lint/line_length.py $(git ls-files '*.[ch]' | grep -v openssl_hostname_validation) || rc=1
./tests/lint/astyle.sh $(git ls-files '*.[ch]' | grep -v openssl_hostname_validation) || rc=1