Patch inspired by one from Daniel Silverstone in Debian bug #229232:

We now have an option where a remote window title query returns a well-formed
response containing the empty string. This should keep stop any server-side
application that was expecting a response from hanging, while not permitting
the response to be influenced by an attacker.

We also retain the ability to stay schtum. The existing checkbox has thus
grown into a set of radio buttons.

I've changed the default to the "empty string" response, even in the backward-
compatibility mode of loading old settings, which is a change in behaviour;
any users who want the old behaviour back will have to explicitly select it. I
think this is probably the Right Thing. (The only drawback I can think of is
that an attacker could still potentially use the relevant fixed strings for
mischief, but we already have other, similar reports.)

[originally from svn r7043]

config.c

@@ -1363,9 +1363,13 @@ void setup_config_box(struct controlbox *b, int midsession,
 		  HELPCTX(features_retitle),
 		  dlg_stdcheckbox_handler,
 		  I(offsetof(Config,no_remote_wintitle)));
-    ctrl_checkbox(s, "Disable remote window title querying (SECURITY)",
+    ctrl_radiobuttons(s, "Response to remote title query (SECURITY):", 'q', 3,
-		  'q', HELPCTX(features_qtitle), dlg_stdcheckbox_handler,
+		      HELPCTX(features_qtitle),
-		  I(offsetof(Config,no_remote_qtitle)));
+		      dlg_stdradiobutton_handler,
+		      I(offsetof(Config,remote_qtitle_action)),
+		      "None", I(TITLE_NONE),
+		      "Empty string", I(TITLE_EMPTY),
+		      "Window title", I(TITLE_REAL), NULL);
     ctrl_checkbox(s, "Disable destructive backspace on server sending ^?",'b',
 		  HELPCTX(features_dbackspace),
 		  dlg_stdcheckbox_handler, I(offsetof(Config,no_dbackspace)));

doc/config.but

@@ -882,7 +882,7 @@ commands from the server. If you find PuTTY is doing this
 unexpectedly or inconveniently, you can tell PuTTY not to respond to
 those server commands.
 
-\S{config-features-qtitle} Disabling remote \i{window title} querying
+\S{config-features-qtitle} Response to remote \i{window title} querying
 
 \cfg{winhelp-topic}{features.qtitle}
 
@@ -899,8 +899,28 @@ service to have the new window title sent back to the server as if
 typed at the keyboard. This allows an attacker to fake keypresses
 and potentially cause your server-side applications to do things you
 didn't want. Therefore this feature is disabled by default, and we
-recommend you do not turn it on unless you \e{really} know what you
+recommend you do not set it to \q{Window title} unless you \e{really}
-are doing.
+know what you are doing.
+
+There are three settings for this option:
+
+\dt \q{None}
+
+\dd PuTTY makes no response whatsoever to the relevant escape
+sequence. This may upset server-side software that is expecting some
+sort of response.
+
+\dt \q{Empty string}
+
+\dd PuTTY makes a well-formed response, but leaves it blank. Thus,
+server-side software that expects a response is kept happy, but an
+attacker cannot influence the response string. This is probably the
+setting you want if you have no better ideas.
+
+\dt \q{Window title}
+
+\dd PuTTY responds with the actual window title. This is dangerous for
+the reasons described above.
 
 \S{config-features-dbackspace} Disabling \i{destructive backspace}
 

putty.h

@@ -297,6 +297,11 @@ enum {
     LD_ECHO			       /* local echo */
 };
 
+enum {
+    /* Actions on remote window title query */
+    TITLE_NONE, TITLE_EMPTY, TITLE_REAL
+};
+
 enum {
     /* Protocol back ends. (cfg.protocol) */
     PROT_RAW, PROT_TELNET, PROT_RLOGIN, PROT_SSH,
@@ -486,7 +491,7 @@ struct config_tag {
     int no_remote_wintitle;	       /* disable remote retitling */
     int no_dbackspace;		       /* disable destructive backspace */
     int no_remote_charset;	       /* disable remote charset config */
-    int no_remote_qtitle;	       /* disable remote win title query */
+    int remote_qtitle_action;	       /* remote win title query action */
     int app_cursor;
     int app_keypad;
     int nethack_keypad;

settings.c

@@ -324,7 +324,7 @@ void save_open_settings(void *sesskey, int do_host, Config *cfg)
     write_setting_i(sesskey, "NoRemoteResize", cfg->no_remote_resize);
     write_setting_i(sesskey, "NoAltScreen", cfg->no_alt_screen);
     write_setting_i(sesskey, "NoRemoteWinTitle", cfg->no_remote_wintitle);
-    write_setting_i(sesskey, "NoRemoteQTitle", cfg->no_remote_qtitle);
+    write_setting_i(sesskey, "RemoteQTitleAction", cfg->remote_qtitle_action);
     write_setting_i(sesskey, "NoDBackspace", cfg->no_dbackspace);
     write_setting_i(sesskey, "NoRemoteCharset", cfg->no_remote_charset);
     write_setting_i(sesskey, "ApplicationCursorKeys", cfg->app_cursor);
@@ -606,7 +606,17 @@ void load_open_settings(void *sesskey, int do_host, Config *cfg)
     gppi(sesskey, "NoRemoteResize", 0, &cfg->no_remote_resize);
     gppi(sesskey, "NoAltScreen", 0, &cfg->no_alt_screen);
     gppi(sesskey, "NoRemoteWinTitle", 0, &cfg->no_remote_wintitle);
-    gppi(sesskey, "NoRemoteQTitle", 1, &cfg->no_remote_qtitle);
+    {
+	/* Backward compatibility */
+	int no_remote_qtitle;
+	gppi(sesskey, "NoRemoteQTitle", 1, &no_remote_qtitle);
+	/* We deliberately interpret the old setting of "no response" as
+	 * "empty string". This changes the behaviour, but hopefully for
+	 * the better; the user can always recover the old behaviour. */
+	gppi(sesskey, "RemoteQTitleAction",
+	     no_remote_qtitle ? TITLE_EMPTY : TITLE_REAL,
+	     &cfg->remote_qtitle_action);
+    }
     gppi(sesskey, "NoDBackspace", 0, &cfg->no_dbackspace);
     gppi(sesskey, "NoRemoteCharset", 0, &cfg->no_remote_charset);
     gppi(sesskey, "ApplicationCursorKeys", 0, &cfg->app_cursor);

terminal.c

@@ -65,6 +65,8 @@
 
 #define has_compat(x) ( ((CL_##x)&term->compatibility_level) != 0 )
 
+char *EMPTY_WINDOW_TITLE = "";
+
 const char sco2ansicolour[] = { 0, 4, 2, 6, 1, 5, 3, 7 };
 
 #define sel_nl_sz  (sizeof(sel_nl)/sizeof(wchar_t))
@@ -3791,8 +3793,11 @@ static void term_out(Terminal *term)
 				break;
 			      case 20:
 				if (term->ldisc &&
-				    !term->cfg.no_remote_qtitle) {
+				    term->cfg.remote_qtitle_action != TITLE_NONE) {
-				    p = get_window_title(term->frontend, TRUE);
+				    if(term->cfg.remote_qtitle_action == TITLE_REAL)
+					p = get_window_title(term->frontend, TRUE);
+				    else
+					p = EMPTY_WINDOW_TITLE;
 				    len = strlen(p);
 				    ldisc_send(term->ldisc, "\033]L", 3, 0);
 				    ldisc_send(term->ldisc, p, len, 0);
@@ -3801,8 +3806,11 @@ static void term_out(Terminal *term)
 				break;
 			      case 21:
 				if (term->ldisc &&
-				    !term->cfg.no_remote_qtitle) {
+				    term->cfg.remote_qtitle_action != TITLE_NONE) {
-				    p = get_window_title(term->frontend,FALSE);
+				    if(term->cfg.remote_qtitle_action == TITLE_REAL)
+					p = get_window_title(term->frontend, FALSE);
+				    else
+					p = EMPTY_WINDOW_TITLE;
 				    len = strlen(p);
 				    ldisc_send(term->ldisc, "\033]l", 3, 0);
 				    ldisc_send(term->ldisc, p, len, 0);