upstream commit

Expand ssh_config's StrictModes option with two new
settings:

StrictModes=accept-new will automatically accept hitherto-unseen keys
but will refuse connections for changed or invalid hostkeys.

StrictModes=off is the same as StrictModes=no

Motivation:

StrictModes=no combines two behaviours for host key processing:
automatically learning new hostkeys and continuing to connect to hosts
with invalid/changed hostkeys. The latter behaviour is quite dangerous
since it removes most of the protections the SSH protocol is supposed to
provide.

Quite a few users want to automatically learn hostkeys however, so
this makes that feature available with less danger.

At some point in the future, StrictModes=no will change to be a synonym
for accept-new, with its current behaviour remaining available via
StrictModes=off.

bz#2400, suggested by Michael Samuel; ok markus

Upstream-ID: 0f55502bf75fc93a74fb9853264a8276b9680b64

1 files changed, 18 insertions, 12 deletions

diff --git a/sshconnect.c b/sshconnect.c
index 4013ec7db..2842d9e59 100644
--- a/sshconnect.c
+++ b/sshconnect.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: sshconnect.c,v 1.284 2017/09/01 05:53:56 djm Exp $ */
+/* $OpenBSD: sshconnect.c,v 1.285 2017/09/03 23:33:13 djm Exp $ */
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
  * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@@ -891,7 +891,8 @@ check_host_key(char *hostname, struct sockaddr *hostaddr, u_short port,
                 if (readonly || want_cert)
                         goto fail;
                 /* The host is new. */
-                if (options.strict_host_key_checking == 1) {
+                if (options.strict_host_key_checking ==
+                    SSH_STRICT_HOSTKEY_YES) {
                         /*
                          * User has requested strict host key checking.  We
                          * will not add the host key automatically.  The only
@@ -900,7 +901,8 @@ check_host_key(char *hostname, struct sockaddr *hostaddr, u_short port,
                         error("No %s host key is known for %.200s and you "
                             "have requested strict checking.", type, host);
                         goto fail;
-                } else if (options.strict_host_key_checking == 2) {
+                } else if (options.strict_host_key_checking ==
+                    SSH_STRICT_HOSTKEY_ASK) {
                         char msg1[1024], msg2[1024];
 
                         if (show_other_keys(host_hostkeys, host_key))
@@ -944,8 +946,8 @@ check_host_key(char *hostname, struct sockaddr *hostaddr, u_short port,
                         hostkey_trusted = 1; /* user explicitly confirmed */
                 }
                 /*
-                 * If not in strict mode, add the key automatically to the
-                 * local known_hosts file.
+                 * If in "new" or "off" strict mode, add the key automatically
+                 * to the local known_hosts file.
                  */
                 if (options.check_host_ip && ip_status == HOST_NEW) {
                         snprintf(hostline, sizeof(hostline), "%s,%s", host, ip);
@@ -987,7 +989,8 @@ check_host_key(char *hostname, struct sockaddr *hostaddr, u_short port,
                  * If strict host key checking is in use, the user will have
                  * to edit the key manually and we can only abort.
                  */
-                if (options.strict_host_key_checking) {
+                if (options.strict_host_key_checking !=
+                    SSH_STRICT_HOSTKEY_OFF) {
                         error("%s host key for %.200s was revoked and you have "
                             "requested strict checking.", type, host);
                         goto fail;
@@ -1040,7 +1043,8 @@ check_host_key(char *hostname, struct sockaddr *hostaddr, u_short port,
                  * If strict host key checking is in use, the user will have
                  * to edit the key manually and we can only abort.
                  */
-                if (options.strict_host_key_checking) {
+                if (options.strict_host_key_checking !=
+                    SSH_STRICT_HOSTKEY_OFF) {
                         error("%s host key for %.200s has changed and you have "
                             "requested strict checking.", type, host);
                         goto fail;
@@ -1127,15 +1131,17 @@ check_host_key(char *hostname, struct sockaddr *hostaddr, u_short port,
                             "\nMatching host key in %s:%lu",
                             host_found->file, host_found->line);
                 }
-                if (options.strict_host_key_checking == 1) {
-                        logit("%s", msg);
-                        error("Exiting, you have requested strict checking.");
-                        goto fail;
-                } else if (options.strict_host_key_checking == 2) {
+                if (options.strict_host_key_checking ==
+                    SSH_STRICT_HOSTKEY_ASK) {
                         strlcat(msg, "\nAre you sure you want "
                             "to continue connecting (yes/no)? ", sizeof(msg));
                         if (!confirm(msg))
                                 goto fail;
+                } else if (options.strict_host_key_checking !=
+                    SSH_STRICT_HOSTKEY_OFF) {
+                        logit("%s", msg);
+                        error("Exiting, you have requested strict checking.");
+                        goto fail;
                 } else {
                         logit("%s", msg);
                 }