Be more strict about escape sequences that rename windows or set titles:

ignore any that not valid UTF-8 outright, and for good measure pass the
result through our UTF-8-aware vis(3).

input.c

@@ -1896,8 +1896,10 @@ input_exit_osc(struct input_ctx *ictx)
 	switch (option) {
 	case 0:
 	case 2:
+		if (utf8_isvalid(p)) {
 			screen_set_title(ictx->ctx.s, p);
 			server_status_window(ictx->wp->window);
+		}
 		break;
 	case 4:
 		input_osc_4(ictx->wp, p);
@@ -1909,7 +1911,7 @@ input_exit_osc(struct input_ctx *ictx)
 		input_osc_11(ictx->wp, p);
 		break;
 	case 12:
-		if (*p != '?') /* ? is colour request */
+		if (utf8_isvalid(p) && *p != '?') /* ? is colour request */
 			screen_set_cursor_colour(ictx->ctx.s, p);
 		break;
 	case 52:
@@ -1945,6 +1947,8 @@ input_exit_apc(struct input_ctx *ictx)
 		return;
 	log_debug("%s: \"%s\"", __func__, ictx->input_buf);
 
+	if (!utf8_isvalid(ictx->input_buf))
+		return;
 	screen_set_title(ictx->ctx.s, ictx->input_buf);
 	server_status_window(ictx->wp->window);
 }
@@ -1968,9 +1972,10 @@ input_exit_rename(struct input_ctx *ictx)
 		return;
 	log_debug("%s: \"%s\"", __func__, ictx->input_buf);
 
+	if (!utf8_isvalid(ictx->input_buf))
+		return;
 	window_set_name(ictx->wp->window, ictx->input_buf);
 	options_set_number(ictx->wp->window->options, "automatic-rename", 0);
-
 	server_status_window(ictx->wp->window);
 }
 

screen.c

@@ -21,6 +21,7 @@
 #include <stdlib.h>
 #include <string.h>
 #include <unistd.h>
+#include <vis.h>
 
 #include "tmux.h"
 
@@ -107,7 +108,7 @@ void
 screen_set_title(struct screen *s, const char *title)
 {
 	free(s->title);
-	s->title = xstrdup(title);
+	utf8_stravis(&s->title, title, VIS_OCTAL|VIS_CSTYLE|VIS_TAB|VIS_NL);
 }
 
 /* Resize screen. */

tmux.h

@@ -2318,6 +2318,7 @@ enum utf8_state	 utf8_open(struct utf8_data *, u_char);
 enum utf8_state	 utf8_append(struct utf8_data *, u_char);
 enum utf8_state	 utf8_combine(const struct utf8_data *, wchar_t *);
 enum utf8_state	 utf8_split(wchar_t, struct utf8_data *);
+int		 utf8_isvalid(const char *);
 int		 utf8_strvis(char *, const char *, size_t, int);
 int		 utf8_stravis(char **, const char *, int);
 char		*utf8_sanitize(const char *);

utf8.c

@@ -207,6 +207,31 @@ utf8_stravis(char **dst, const char *src, int flag)
 	return (len);
 }
 
+/* Does this string contain anything that isn't valid UTF-8? */
+int
+utf8_isvalid(const char *s)
+{
+	struct utf8_data	 ud;
+	const char		*end;
+	enum utf8_state		 more;
+	size_t			 i;
+
+	end = s + strlen(s);
+	while (s < end) {
+		if ((more = utf8_open(&ud, *s)) == UTF8_MORE) {
+			while (++s < end && more == UTF8_MORE)
+				more = utf8_append(&ud, *s);
+			if (more == UTF8_DONE)
+				continue;
+			return (0);
+		}
+		if (*s < 0x20 || *s > 0x7e)
+			return (0);
+		s++;
+	}
+	return (1);
+}
+
 /*
  * Sanitize a string, changing any UTF-8 characters to '_'. Caller should free
  * the returned string. Anything not valid printable ASCII or UTF-8 is

window.c

@@ -29,6 +29,7 @@
 #include <time.h>
 #include <unistd.h>
 #include <util.h>
+#include <vis.h>
 
 #include "tmux.h"
 
@@ -408,7 +409,7 @@ void
 window_set_name(struct window *w, const char *new_name)
 {
 	free(w->name);
-	w->name = xstrdup(new_name);
+	utf8_stravis(&w->name, new_name, VIS_OCTAL|VIS_CSTYLE|VIS_TAB|VIS_NL);
 	notify_window("window-renamed", w);
 }