[3.14] gh-151695: Fix use-after-free of the curses screen encoding (GH-151696) (GH...

The module-global curses_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 551f8e16f8bb38a1e9c6df259a2a0969493de070)

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

diff --git a/Misc/NEWS.d/next/Library/2026-06-19-07-26-20.gh-issue-151695.IBDlkN.rst b/Misc/NEWS.d/next/Library/2026-06-19-07-26-20.gh-issue-151695.IBDlkN.rst
new file mode 100644 (file)
index 0000000..f44cb6b
--- /dev/null
+++ b/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.

diff --git a/Modules/_cursesmodule.c b/Modules/_cursesmodule.c
index 4f3c4174a20d05bd84d411943a40910fdc1229e0..1f37f3478b57152aeff65f51e67622b9d67fc053 100644 (file)
--- a/Modules/_cursesmodule.c
+++ b/Modules/_cursesmodule.c
@@ -209,7 +209,11 @@ static int curses_initscr_called = FALSE;
 /* Tells whether start_color() has been called to initialise color usage. */
 static int curses_start_color_called = FALSE;
 
-static const char *curses_screen_encoding = NULL;
+/* 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 *curses_screen_encoding = NULL;
 
 /* Utility Checking Procedures */
 
@@ -3518,6 +3522,21 @@ _curses_init_pair_impl(PyObject *module, int pair_number, int fg, int bg)
     Py_RETURN_NONE;
 }
 
+/* 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(curses_screen_encoding);
+    curses_screen_encoding = copy;
+    return 0;
+}
+
 /*[clinic input]
 _curses.initscr
 
@@ -3535,7 +3554,15 @@ _curses_initscr_impl(PyObject *module)
     if (curses_initscr_called) {
         wrefresh(stdscr);
         cursesmodule_state *state = get_cursesmodule_state(module);
-        return PyCursesWindow_New(state, stdscr, NULL, NULL);
+        PyObject *winobj = PyCursesWindow_New(state, stdscr, NULL, NULL);
+        if (winobj == NULL) {
+            return NULL;
+        }
+        if (curses_update_screen_encoding(winobj) < 0) {
+            Py_DECREF(winobj);
+            return NULL;
+        }
+        return winobj;
     }
 
     win = initscr();
@@ -3643,7 +3670,10 @@ _curses_initscr_impl(PyObject *module)
     if (winobj == NULL) {
         return NULL;
     }
-    curses_screen_encoding = ((PyCursesWindowObject *)winobj)->encoding;
+    if (curses_update_screen_encoding(winobj) < 0) {
+        Py_DECREF(winobj);
+        return NULL;
+    }
     return winobj;
 }
 
@@ -5190,6 +5220,8 @@ static void
 cursesmodule_free(void *mod)
 {
     (void)cursesmodule_clear((PyObject *)mod);
+    PyMem_Free(curses_screen_encoding);
+    curses_screen_encoding = NULL;
     curses_module_loaded = 0;  // allow reloading once garbage-collected
 }