[3.13] gh-151695: Fix use-after-free of the curses screen encoding (GH-151696) (GH-151706) (GH-151723)

The module-global screen_encoding stored a borrowed pointer to the
encoding owned by the window returned by the first initscr() call.  That
window can be deallocated while unctrl() and ungetch(), which have no window
of their own, still use the pointer to encode non-ASCII characters.

Keep a private copy of the encoding instead.
(cherry picked from commit 551f8e1)
(cherry picked from commit 7b55e9a)

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: serhiy-storchaka

Misc/NEWS.d/next/Library/2026-06-19-07-26-20.gh-issue-151695.IBDlkN.rst

@@ -0,0 +1,4 @@
+Fix a use-after-free in the :mod:`curses` module.  The encoding of the initial
+screen, used by :func:`curses.unctrl` and :func:`curses.ungetch` to encode
+non-ASCII characters, is now kept as a private copy instead of a borrowed
+pointer to a window object that may be deallocated.

Modules/_cursesmodule.c

@@ -179,6 +179,10 @@ static int initialised = FALSE;
 /* Tells whether start_color() has been called to initialise color usage. */
 static int initialisedcolors = FALSE;
 
+/* Encoding of the initial screen, used by module-level functions that have
+   no window object to take it from (e.g. unctrl(), ungetch()).  This is a
+   private copy: the window object that initscr() returns may be deallocated
+   while these functions are still in use. */
 static char *screen_encoding = NULL;
 
 /* Utility Macros */
@@ -3274,6 +3278,21 @@ _curses_init_pair_impl(PyObject *module, int pair_number, int fg, int bg)
 
 static PyObject *ModDict;
 
+/* Refresh the private copy of the screen encoding from a freshly created
+   stdscr window object.  Returns 0 on success, -1 with an exception set. */
+static int
+curses_update_screen_encoding(PyObject *winobj)
+{
+    char *copy = _PyMem_Strdup(((PyCursesWindowObject *)winobj)->encoding);
+    if (copy == NULL) {
+        PyErr_NoMemory();
+        return -1;
+    }
+    PyMem_Free(screen_encoding);
+    screen_encoding = copy;
+    return 0;
+}
+
 /*[clinic input]
 _curses.initscr
 
@@ -3287,11 +3306,18 @@ _curses_initscr_impl(PyObject *module)
 /*[clinic end generated code: output=619fb68443810b7b input=514f4bce1821f6b5]*/
 {
     WINDOW *win;
-    PyCursesWindowObject *winobj;
 
     if (initialised) {
         wrefresh(stdscr);
-        return (PyObject *)PyCursesWindow_New(stdscr, NULL, NULL);
+        PyObject *winobj = PyCursesWindow_New(stdscr, NULL, NULL);
+        if (winobj == NULL) {
+            return NULL;
+        }
+        if (curses_update_screen_encoding(winobj) < 0) {
+            Py_DECREF(winobj);
+            return NULL;
+        }
+        return winobj;
     }
 
     win = initscr();
@@ -3383,9 +3409,15 @@ _curses_initscr_impl(PyObject *module)
     SetDictInt("LINES", LINES);
     SetDictInt("COLS", COLS);
 
-    winobj = (PyCursesWindowObject *)PyCursesWindow_New(win, NULL, NULL);
-    screen_encoding = winobj->encoding;
-    return (PyObject *)winobj;
+    PyObject *winobj = PyCursesWindow_New(win, NULL, NULL);
+    if (winobj == NULL) {
+        return NULL;
+    }
+    if (curses_update_screen_encoding(winobj) < 0) {
+        Py_DECREF(winobj);
+        return NULL;
+    }
+    return winobj;
 }
 
 /*[clinic input]