--- /dev/null
+From 1dc5659aa62848f0aeb5de7bd3839fecc7debefa Mon Sep 17 00:00:00 2001
+From: Simon Tatham <anakin@pobox.com>
+Date: Sat, 19 Jun 2021 15:39:15 +0100
+Subject: [PATCH] New option to reject 'trivial' success of userauth.
+
+Suggested by Manfred Kaiser, who also wrote most of this patch
+(although outlying parts, like documentation and SSH-1 support, are by
+me).
+
+This is a second line of defence against the kind of spoofing attacks
+in which a malicious or compromised SSH server rushes the client
+through the userauth phase of SSH without actually requiring any auth
+inputs (passwords or signatures or whatever), and then at the start of
+the connection phase it presents something like a spoof prompt,
+intended to be taken for part of userauth by the user but in fact with
+some more sinister purpose.
+
+Our existing line of defence against this is the trust sigil system,
+and as far as I know, that's still working. This option allows a bit of
+extra defence in depth: if you don't expect your SSH server to
+trivially accept authentication in the first place, then enabling this
+option will cause PuTTY to disconnect if it unexpectedly does so,
+without the user having to spot the presence or absence of a fiddly
+little sigil anywhere.
+
+Several types of authentication count as 'trivial'. The obvious one is
+the SSH-2 "none" method, which clients always try first so that the
+failure message will tell them what else they can try, and which a
+server can instead accept in order to authenticate you unconditionally.
+But there are two other ways to do it that we know of: one is to run
+keyboard-interactive authentication and send an empty INFO_REQUEST
+packet containing no actual prompts for the user, and another even
+weirder one is to send USERAUTH_SUCCESS in response to the user's
+preliminary *offer* of a public key (instead of sending the usual PK_OK
+to request an actual signature from the key).
+
+This new option detects all of those, by clearing the 'is_trivial_auth'
+flag only when we send some kind of substantive authentication response
+(be it a password, a k-i prompt response, a signature, or a GSSAPI
+token). So even if there's a further path through the userauth maze we
+haven't spotted, that somehow avoids sending anything substantive, this
+strategy should still pick it up.
+
+(cherry picked from commit 5f5c710cf3704737e24ffceb2c918e412a2a674f)
+
+[Retrieved from:
+https://git.tartarus.org/?p=simon/putty.git;a=commit;h=1dc5659aa62848f0aeb5de7bd3839fecc7debefa]
+Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
+---
+ cmdline.c | 8 ++++++++
+ config.c | 4 ++++
+ doc/config.but | 43 +++++++++++++++++++++++++++++++++++++++++++
+ pscp.c | 2 ++
+ psftp.c | 2 ++
+ putty.h | 1 +
+ settings.c | 2 ++
+ ssh.c | 4 +++-
+ ssh1login.c | 14 +++++++++++++-
+ ssh2userauth.c | 20 ++++++++++++++++----
+ sshppl.h | 2 +-
+ unix/uxplink.c | 2 ++
+ windows/winhelp.h | 1 +
+ windows/winplink.c | 2 ++
+ 14 files changed, 100 insertions(+), 7 deletions(-)
+
+diff --git a/cmdline.c b/cmdline.c
+index 7ea5cacc..62d65e19 100644
+--- a/cmdline.c
++++ b/cmdline.c
+@@ -598,6 +598,14 @@ int cmdline_process_param(const char *p, char *value,
+ SAVEABLE(0);
+ conf_set_bool(conf, CONF_tryagent, false);
+ }
++
++ if (!strcmp(p, "-no-trivial-auth")) {
++ RETURN(1);
++ UNAVAILABLE_IN(TOOLTYPE_NONNETWORK);
++ SAVEABLE(0);
++ conf_set_bool(conf, CONF_ssh_no_trivial_userauth, true);
++ }
++
+ if (!strcmp(p, "-share")) {
+ RETURN(1);
+ UNAVAILABLE_IN(TOOLTYPE_NONNETWORK);
+diff --git a/config.c b/config.c
+index ca808511..b8348530 100644
+--- a/config.c
++++ b/config.c
+@@ -2772,6 +2772,10 @@ void setup_config_box(struct controlbox *b, bool midsession,
+ HELPCTX(ssh_auth_bypass),
+ conf_checkbox_handler,
+ I(CONF_ssh_no_userauth));
++ ctrl_checkbox(s, "Disconnect if authentication succeeds trivially",
++ 'n', HELPCTX(ssh_no_trivial_userauth),
++ conf_checkbox_handler,
++ I(CONF_ssh_no_trivial_userauth));
+
+ s = ctrl_getset(b, "Connection/SSH/Auth", "methods",
+ "Authentication methods");
+diff --git a/doc/config.but b/doc/config.but
+index a00ae476..77313282 100644
+--- a/doc/config.but
++++ b/doc/config.but
+@@ -2623,6 +2623,49 @@ interact with them.)
+ This option only affects SSH-2 connections. SSH-1 connections always
+ require an authentication step.
+
++\S{config-ssh-notrivialauth} \q{Disconnect if authentication succeeds
++trivially}
++
++This option causes PuTTY to abandon an SSH session and disconnect from
++the server, if the server accepted authentication without ever having
++asked for any kind of password or signature or token.
++
++This might be used as a security measure. There are some forms of
++attack against an SSH client user which work by terminating the SSH
++authentication stage early, and then doing something in the main part
++of the SSH session which \e{looks} like part of the authentication,
++but isn't really.
++
++For example, instead of demanding a signature from your public key,
++for which PuTTY would ask for your key's passphrase, a compromised or
++malicious server might allow you to log in with no signature or
++password at all, and then print a message that \e{imitates} PuTTY's
++request for your passphrase, in the hope that you would type it in.
++(In fact, the passphrase for your public key should not be sent to any
++server.)
++
++PuTTY's main defence against attacks of this type is the \q{trust
++sigil} system: messages in the PuTTY window that are truly originated
++by PuTTY itself are shown next to a small copy of the PuTTY icon,
++which the server cannot fake when it tries to imitate the same message
++using terminal output.
++
++However, if you think you might be at risk of this kind of thing
++anyway (if you don't watch closely for the trust sigils, or if you
++think you're at extra risk of one of your servers being malicious),
++then you could enable this option as an extra defence. Then, if the
++server tries any of these attacks involving letting you through the
++authentication stage, PuTTY will disconnect from the server before it
++can send a follow-up fake prompt or other type of attack.
++
++On the other hand, some servers \e{legitimately} let you through the
++SSH authentication phase trivially, either because they are genuinely
++public, or because the important authentication step happens during
++the terminal session. (An example might be an SSH server that connects
++you directly to the terminal login prompt of a legacy mainframe.) So
++enabling this option might cause some kinds of session to stop
++working. It's up to you.
++
+ \S{config-ssh-tryagent} \q{Attempt authentication using Pageant}
+
+ If this option is enabled, then PuTTY will look for Pageant (the SSH
+diff --git a/pscp.c b/pscp.c
+index 0bfda92d..a11d1e18 100644
+--- a/pscp.c
++++ b/pscp.c
+@@ -2203,6 +2203,8 @@ static void usage(void)
+ printf(" -i key private key file for user authentication\n");
+ printf(" -noagent disable use of Pageant\n");
+ printf(" -agent enable use of Pageant\n");
++ printf(" -no-trivial-auth\n");
++ printf(" disconnect if SSH authentication succeeds trivially\n");
+ printf(" -hostkey keyid\n");
+ printf(" manually specify a host key (may be repeated)\n");
+ printf(" -batch disable all interactive prompts\n");
+diff --git a/psftp.c b/psftp.c
+index 40cb73f5..c4b5374b 100644
+--- a/psftp.c
++++ b/psftp.c
+@@ -2538,6 +2538,8 @@ static void usage(void)
+ printf(" -i key private key file for user authentication\n");
+ printf(" -noagent disable use of Pageant\n");
+ printf(" -agent enable use of Pageant\n");
++ printf(" -no-trivial-auth\n");
++ printf(" disconnect if SSH authentication succeeds trivially\n");
+ printf(" -hostkey keyid\n");
+ printf(" manually specify a host key (may be repeated)\n");
+ printf(" -batch disable all interactive prompts\n");
+diff --git a/putty.h b/putty.h
+index ef6f7a37..7789f217 100644
+--- a/putty.h
++++ b/putty.h
+@@ -1428,6 +1428,7 @@ NORETURN void cleanup_exit(int);
+ X(INT, NONE, sshprot) \
+ X(BOOL, NONE, ssh2_des_cbc) /* "des-cbc" unrecommended SSH-2 cipher */ \
+ X(BOOL, NONE, ssh_no_userauth) /* bypass "ssh-userauth" (SSH-2 only) */ \
++ X(BOOL, NONE, ssh_no_trivial_userauth) /* disable trivial types of auth */ \
+ X(BOOL, NONE, ssh_show_banner) /* show USERAUTH_BANNERs (SSH-2 only) */ \
+ X(BOOL, NONE, try_tis_auth) \
+ X(BOOL, NONE, try_ki_auth) \
+diff --git a/settings.c b/settings.c
+index 547af0dc..32a53c54 100644
+--- a/settings.c
++++ b/settings.c
+@@ -609,6 +609,7 @@ void save_open_settings(settings_w *sesskey, Conf *conf)
+ #endif
+ write_setting_s(sesskey, "RekeyBytes", conf_get_str(conf, CONF_ssh_rekey_data));
+ write_setting_b(sesskey, "SshNoAuth", conf_get_bool(conf, CONF_ssh_no_userauth));
++ write_setting_b(sesskey, "SshNoTrivialAuth", conf_get_bool(conf, CONF_ssh_no_trivial_userauth));
+ write_setting_b(sesskey, "SshBanner", conf_get_bool(conf, CONF_ssh_show_banner));
+ write_setting_b(sesskey, "AuthTIS", conf_get_bool(conf, CONF_try_tis_auth));
+ write_setting_b(sesskey, "AuthKI", conf_get_bool(conf, CONF_try_ki_auth));
+@@ -1025,6 +1026,7 @@ void load_open_settings(settings_r *sesskey, Conf *conf)
+ gpps(sesskey, "LogHost", "", conf, CONF_loghost);
+ gppb(sesskey, "SSH2DES", false, conf, CONF_ssh2_des_cbc);
+ gppb(sesskey, "SshNoAuth", false, conf, CONF_ssh_no_userauth);
++ gppb(sesskey, "SshNoTrivialAuth", false, conf, CONF_ssh_no_trivial_userauth);
+ gppb(sesskey, "SshBanner", true, conf, CONF_ssh_show_banner);
+ gppb(sesskey, "AuthTIS", false, conf, CONF_try_tis_auth);
+ gppb(sesskey, "AuthKI", true, conf, CONF_try_ki_auth);
+diff --git a/ssh.c b/ssh.c
+index 00a516ea..91e7e869 100644
+--- a/ssh.c
++++ b/ssh.c
+@@ -254,7 +254,9 @@ static void ssh_got_ssh_version(struct ssh_version_receiver *rcv,
+ connection_layer, ssh->savedhost, ssh->fullhostname,
+ conf_get_filename(ssh->conf, CONF_keyfile),
+ conf_get_bool(ssh->conf, CONF_ssh_show_banner),
+- conf_get_bool(ssh->conf, CONF_tryagent), username,
++ conf_get_bool(ssh->conf, CONF_tryagent),
++ conf_get_bool(ssh->conf, CONF_ssh_no_trivial_userauth),
++ username,
+ conf_get_bool(ssh->conf, CONF_change_username),
+ conf_get_bool(ssh->conf, CONF_try_ki_auth),
+ #ifndef NO_GSSAPI
+diff --git a/ssh1login.c b/ssh1login.c
+index 8486cbf0..519838d7 100644
+--- a/ssh1login.c
++++ b/ssh1login.c
+@@ -27,7 +27,7 @@ struct ssh1_login_state {
+
+ char *savedhost;
+ int savedport;
+- bool try_agent_auth;
++ bool try_agent_auth, is_trivial_auth;
+
+ int remote_protoflags;
+ int local_protoflags;
+@@ -105,6 +105,8 @@ PacketProtocolLayer *ssh1_login_new(
+ s->savedhost = dupstr(host);
+ s->savedport = port;
+ s->successor_layer = successor_layer;
++ s->is_trivial_auth = true;
++
+ return &s->ppl;
+ }
+
+@@ -645,6 +647,7 @@ static void ssh1_login_process_queue(PacketProtocolLayer *ppl)
+ s->ppl.bpp, SSH1_CMSG_AUTH_RSA_RESPONSE);
+ put_data(pkt, ret + 5, 16);
+ pq_push(s->ppl.out_pq, pkt);
++ s->is_trivial_auth = false;
+ crMaybeWaitUntilV(
+ (pktin = ssh1_login_pop(s))
+ != NULL);
+@@ -814,6 +817,7 @@ static void ssh1_login_process_queue(PacketProtocolLayer *ppl)
+ s->ppl.bpp, SSH1_CMSG_AUTH_RSA_RESPONSE);
+ put_data(pkt, buffer, 16);
+ pq_push(s->ppl.out_pq, pkt);
++ s->is_trivial_auth = false;
+
+ mp_free(challenge);
+ mp_free(response);
+@@ -1105,6 +1109,7 @@ static void ssh1_login_process_queue(PacketProtocolLayer *ppl)
+ put_stringz(pkt, prompt_get_result_ref(s->cur_prompt->prompts[0]));
+ pq_push(s->ppl.out_pq, pkt);
+ }
++ s->is_trivial_auth = false;
+ ppl_logevent("Sent password");
+ free_prompts(s->cur_prompt);
+ s->cur_prompt = NULL;
+@@ -1121,6 +1126,13 @@ static void ssh1_login_process_queue(PacketProtocolLayer *ppl)
+ }
+ }
+
++ if (conf_get_bool(s->conf, CONF_ssh_no_trivial_userauth) &&
++ s->is_trivial_auth) {
++ ssh_proto_error(s->ppl.ssh, "Authentication was trivial! "
++ "Abandoning session as specified in configuration.");
++ return;
++ }
++
+ ppl_logevent("Authentication successful");
+
+ if (conf_get_bool(s->conf, CONF_compression)) {
+diff --git a/ssh2userauth.c b/ssh2userauth.c
+index 01243baf..451d5abe 100644
+--- a/ssh2userauth.c
++++ b/ssh2userauth.c
+@@ -28,7 +28,7 @@ struct ssh2_userauth_state {
+
+ PacketProtocolLayer *transport_layer, *successor_layer;
+ Filename *keyfile;
+- bool show_banner, tryagent, change_username;
++ bool show_banner, tryagent, notrivialauth, change_username;
+ char *hostname, *fullhostname;
+ char *default_username;
+ bool try_ki_auth, try_gssapi_auth, try_gssapi_kex_auth, gssapi_fwd;
+@@ -82,6 +82,7 @@ struct ssh2_userauth_state {
+ int len;
+ PktOut *pktout;
+ bool want_user_input;
++ bool is_trivial_auth;
+
+ agent_pending_query *auth_agent_query;
+ bufchain banner;
+@@ -134,7 +135,7 @@ static const PacketProtocolLayerVtable ssh2_userauth_vtable = {
+ PacketProtocolLayer *ssh2_userauth_new(
+ PacketProtocolLayer *successor_layer,
+ const char *hostname, const char *fullhostname,
+- Filename *keyfile, bool show_banner, bool tryagent,
++ Filename *keyfile, bool show_banner, bool tryagent, bool notrivialauth,
+ const char *default_username, bool change_username,
+ bool try_ki_auth, bool try_gssapi_auth, bool try_gssapi_kex_auth,
+ bool gssapi_fwd, struct ssh_connection_shared_gss_state *shgss)
+@@ -149,6 +150,7 @@ PacketProtocolLayer *ssh2_userauth_new(
+ s->keyfile = filename_copy(keyfile);
+ s->show_banner = show_banner;
+ s->tryagent = tryagent;
++ s->notrivialauth = notrivialauth;
+ s->default_username = dupstr(default_username);
+ s->change_username = change_username;
+ s->try_ki_auth = try_ki_auth;
+@@ -157,6 +159,7 @@ PacketProtocolLayer *ssh2_userauth_new(
+ s->gssapi_fwd = gssapi_fwd;
+ s->shgss = shgss;
+ s->last_methods_string = strbuf_new();
++ s->is_trivial_auth = true;
+ bufchain_init(&s->banner);
+ bufchain_sink_init(&s->banner_bs, &s->banner);
+
+@@ -818,6 +821,7 @@ static void ssh2_userauth_process_queue(PacketProtocolLayer *ppl)
+ sigblob);
+ pq_push(s->ppl.out_pq, s->pktout);
+ s->type = AUTH_TYPE_PUBLICKEY;
++ s->is_trivial_auth = false;
+ } else {
+ ppl_logevent("Pageant refused signing request");
+ ppl_printf("Pageant failed to "
+@@ -1038,6 +1042,7 @@ static void ssh2_userauth_process_queue(PacketProtocolLayer *ppl)
+ ssh_key_free(key->key);
+ sfree(key->comment);
+ sfree(key);
++ s->is_trivial_auth = false;
+ }
+
+ #ifndef NO_GSSAPI
+@@ -1169,6 +1174,7 @@ static void ssh2_userauth_process_queue(PacketProtocolLayer *ppl)
+ * no longer says CONTINUE_NEEDED
+ */
+ if (s->gss_sndtok.length != 0) {
++ s->is_trivial_auth = false;
+ s->pktout =
+ ssh_bpp_new_pktout(
+ s->ppl.bpp, SSH2_MSG_USERAUTH_GSSAPI_TOKEN);
+@@ -1288,7 +1294,6 @@ static void ssh2_userauth_process_queue(PacketProtocolLayer *ppl)
+ * Loop while the server continues to send INFO_REQUESTs.
+ */
+ while (pktin->type == SSH2_MSG_USERAUTH_INFO_REQUEST) {
+-
+ ptrlen name, inst;
+ strbuf *sb;
+
+@@ -1308,6 +1313,7 @@ static void ssh2_userauth_process_queue(PacketProtocolLayer *ppl)
+ */
+ s->num_prompts = get_uint32(pktin);
+ for (uint32_t i = 0; i < s->num_prompts; i++) {
++ s->is_trivial_auth = false;
+ ptrlen prompt = get_string(pktin);
+ bool echo = get_bool(pktin);
+
+@@ -1472,7 +1478,7 @@ static void ssh2_userauth_process_queue(PacketProtocolLayer *ppl)
+ pq_push_front(s->ppl.in_pq, pktin);
+
+ } else if (s->can_passwd) {
+-
++ s->is_trivial_auth = false;
+ /*
+ * Plain old password authentication.
+ */
+@@ -1731,6 +1737,12 @@ static void ssh2_userauth_process_queue(PacketProtocolLayer *ppl)
+ }
+
+ userauth_success:
++ if (s->notrivialauth && s->is_trivial_auth) {
++ ssh_proto_error(s->ppl.ssh, "Authentication was trivial! "
++ "Abandoning session as specified in configuration.");
++ return;
++ }
++
+ /*
+ * We've just received USERAUTH_SUCCESS, and we haven't sent
+ * any packets since. Signal the transport layer to consider
+diff --git a/sshppl.h b/sshppl.h
+index b339d67c..2c1dbf42 100644
+--- a/sshppl.h
++++ b/sshppl.h
+@@ -116,7 +116,7 @@ PacketProtocolLayer *ssh2_transport_new(
+ PacketProtocolLayer *ssh2_userauth_new(
+ PacketProtocolLayer *successor_layer,
+ const char *hostname, const char *fullhostname,
+- Filename *keyfile, bool show_banner, bool tryagent,
++ Filename *keyfile, bool show_banner, bool tryagent, bool notrivialauth,
+ const char *default_username, bool change_username,
+ bool try_ki_auth,
+ bool try_gssapi_auth, bool try_gssapi_kex_auth,
+diff --git a/unix/uxplink.c b/unix/uxplink.c
+index 3e2a9b6b..240783f4 100644
+--- a/unix/uxplink.c
++++ b/unix/uxplink.c
+@@ -527,6 +527,8 @@ static void usage(void)
+ printf(" -i key private key file for user authentication\n");
+ printf(" -noagent disable use of Pageant\n");
+ printf(" -agent enable use of Pageant\n");
++ printf(" -no-trivial-auth\n");
++ printf(" disconnect if SSH authentication succeeds trivially\n");
+ printf(" -noshare disable use of connection sharing\n");
+ printf(" -share enable use of connection sharing\n");
+ printf(" -hostkey keyid\n");
+diff --git a/windows/winhelp.h b/windows/winhelp.h
+index ae5a7a7f..9011df45 100644
+--- a/windows/winhelp.h
++++ b/windows/winhelp.h
+@@ -111,6 +111,7 @@
+ #define WINHELP_CTX_ssh_kex_repeat "config-ssh-kex-rekey"
+ #define WINHELP_CTX_ssh_kex_manual_hostkeys "config-ssh-kex-manual-hostkeys"
+ #define WINHELP_CTX_ssh_auth_bypass "config-ssh-noauth"
++#define WINHELP_CTX_ssh_no_trivial_userauth "config-ssh-notrivialauth"
+ #define WINHELP_CTX_ssh_auth_banner "config-ssh-banner"
+ #define WINHELP_CTX_ssh_auth_privkey "config-ssh-privkey"
+ #define WINHELP_CTX_ssh_auth_agentfwd "config-ssh-agentfwd"
+diff --git a/windows/winplink.c b/windows/winplink.c
+index 9bda0712..58d43e6d 100644
+--- a/windows/winplink.c
++++ b/windows/winplink.c
+@@ -149,6 +149,8 @@ static void usage(void)
+ printf(" -i key private key file for user authentication\n");
+ printf(" -noagent disable use of Pageant\n");
+ printf(" -agent enable use of Pageant\n");
++ printf(" -no-trivial-auth\n");
++ printf(" disconnect if SSH authentication succeeds trivially\n");
+ printf(" -noshare disable use of connection sharing\n");
+ printf(" -share enable use of connection sharing\n");
+ printf(" -hostkey keyid\n");
+--
+2.20.1
+
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