https-client.c

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Author : Patrick Pelletier
Date : 2013-02-27 17:16:27
Hash : 64d9f161
Message : 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

/*
  This is an example of how to hook up evhttp with bufferevent_ssl

  It just GETs an https URL given on the command-line and prints the response
  body to stdout.

  Actually, it also accepts plain http URLs to make it easy to compare http vs
  https code paths.

  Loosely based on le-proxy.c.
 */

#include <stdio.h>
#include <assert.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>

#ifdef WIN32
#include <winsock2.h>
#include <ws2tcpip.h>
#else
#include <sys/socket.h>
#include <netinet/in.h>
#endif

#include <event2/bufferevent_ssl.h>
#include <event2/bufferevent.h>
#include <event2/buffer.h>
#include <event2/listener.h>
#include <event2/util.h>
#include <event2/http.h>
#include <event2/http_struct.h>

#include <openssl/ssl.h>
#include <openssl/err.h>
#include <openssl/rand.h>

#include "openssl_hostname_validation.h"

static struct event_base *base;

static void
http_request_done(struct evhttp_request *req, void *ctx)
{
	char buffer[256];
	int nread;

	if (req == NULL) {
		/* If req is NULL, it means an error occurred, but
		 * sadly we are mostly left guessing what the error
		 * might have been.  We'll do our best... */
		struct bufferevent *bev = (struct bufferevent *) ctx;
		unsigned long oslerr;
		int printed_err = 0;
		int errcode = EVUTIL_SOCKET_ERROR();
		fprintf(stderr, "some request failed - no idea which one though!\n");
		/* Print out the OpenSSL error queue that libevent
		 * squirreled away for us, if any. */
		while ((oslerr = bufferevent_get_openssl_error(bev))) {
			ERR_error_string_n(oslerr, buffer, sizeof(buffer));
			fprintf(stderr, "%s\n", buffer);
			printed_err = 1;
		}
		/* If the OpenSSL error queue was empty, maybe it was a
		 * socket error; let's try printing that. */
		if (! printed_err)
			fprintf(stderr, "socket error = %s (%d)\n",
				evutil_socket_error_to_string(errcode),
				errcode);
		return;
	}

	fprintf(stderr, "Response line: %d %s\n",
		req->response_code, req->response_code_line);

	while ((nread = evbuffer_remove(req->input_buffer, buffer, sizeof(buffer)))
	       > 0) {
		/* These are just arbitrary chunks of 256 bytes.
		 * They are not lines, so we can't treat them as such. */
		fwrite(buffer, nread, 1, stdout);
	}
}

static void
syntax(void)
{
	fputs("Syntax:\n", stderr);
	fputs("   https-client <https-url>\n", stderr);
	fputs("Example:\n", stderr);
	fputs("   https-client https://ip.appspot.com/\n", stderr);

	exit(1);
}

static void
die(const char *msg)
{
	fputs(msg, stderr);
	exit(1);
}

static void
die_openssl(const char *func)
{
	fprintf (stderr, "%s failed:\n", func);

	/* This is the OpenSSL function that prints the contents of the
	 * error stack to the specified file handle. */
	ERR_print_errors_fp (stderr);

	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)
{
	int r;

	struct evhttp_uri *http_uri;
	const char *url, *scheme, *host, *path, *query;
	char uri[256];
	int port;

	SSL_CTX *ssl_ctx;
	SSL *ssl;
	struct bufferevent *bev;
	struct evhttp_connection *evcon;
	struct evhttp_request *req;

	if (argc != 2)
		syntax();

	url = argv[1];
	http_uri = evhttp_uri_parse(url);
	if (http_uri == NULL) {
		die("malformed url");
	}

	scheme = evhttp_uri_get_scheme(http_uri);
	if (scheme == NULL || (strcasecmp(scheme, "https") != 0 &&
	                       strcasecmp(scheme, "http") != 0)) {
		die("url must be http or https");
	}

	host = evhttp_uri_get_host(http_uri);
	if (host == NULL) {
		die("url must have a host");
	}

	port = evhttp_uri_get_port(http_uri);
	if (port == -1) {
		port = (strcasecmp(scheme, "http") == 0) ? 80 : 443;
	}

	path = evhttp_uri_get_path(http_uri);
	if (path == NULL) {
		path = "/";
	}

	query = evhttp_uri_get_query(http_uri);
	if (query == NULL) {
		snprintf(uri, sizeof(uri) - 1, "%s", path);
	} else {
		snprintf(uri, sizeof(uri) - 1, "%s?%s", path, query);
	}
	uri[sizeof(uri) - 1] = '\0';

	// Initialize OpenSSL
	SSL_library_init();
	ERR_load_crypto_strings();
	SSL_load_error_strings();
	OpenSSL_add_all_algorithms();

	/* This isn't strictly necessary... OpenSSL performs RAND_poll
	 * automatically on first use of random number generator. */
	r = RAND_poll();
	if (r == 0) {
		die_openssl("RAND_poll");
	}

	/* Create a new OpenSSL context */
	ssl_ctx = SSL_CTX_new(SSLv23_method());
	if (!ssl_ctx)
		die_openssl("SSL_CTX_new");

	/* Attempt to use the system's trusted root certificates.
	 * (This path is only valid for Debian-based systems.) */
	if (1 != SSL_CTX_load_verify_locations(ssl_ctx,
					       "/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();
	if (!base) {
		perror("event_base_new()");
		return 1;
	}

	// Create OpenSSL bufferevent and stack evhttp on top of it
	ssl = SSL_new(ssl_ctx);
	if (ssl == NULL) {
		die_openssl("SSL_new()");
	}

	if (strcasecmp(scheme, "http") == 0) {
		bev = bufferevent_socket_new(base, -1, BEV_OPT_CLOSE_ON_FREE);
	} else {
		bev = bufferevent_openssl_socket_new(base, -1, ssl,
			BUFFEREVENT_SSL_CONNECTING,
			BEV_OPT_CLOSE_ON_FREE|BEV_OPT_DEFER_CALLBACKS);
	}

	if (bev == NULL) {
		fprintf(stderr, "bufferevent_openssl_socket_new() failed\n");
		return 1;
	}

	bufferevent_openssl_set_allow_dirty_shutdown(bev, 1);

	// For simplicity, we let DNS resolution block. Everything else should be
	// asynchronous though.
	evcon = evhttp_connection_base_bufferevent_new(base, NULL, bev,
		host, port);
	if (evcon == NULL) {
		fprintf(stderr, "evhttp_connection_base_bufferevent_new() failed\n");
		return 1;
	}

	// Fire off the request
	req = evhttp_request_new(http_request_done, bev);
	if (req == NULL) {
		fprintf(stderr, "evhttp_request_new() failed\n");
		return 1;
	}

	evhttp_add_header(req->output_headers, "Host", host);
	evhttp_add_header(req->output_headers, "Connection", "close");

	r = evhttp_make_request(evcon, req, EVHTTP_REQ_GET, uri);
	if (r != 0) {
		fprintf(stderr, "evhttp_make_request() failed\n");
		return 1;
	}

	event_base_dispatch(base);

	evhttp_connection_free(evcon);
	event_base_free(base);

	return 0;
}

kc3-lang/libevent/sample/https-client.c

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kc3-lang/libevent /sample/https-client.c