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");