diff --git a/Lib/idlelib/editor.py b/Lib/idlelib/editor.py
index c76db20c58792d..2327cfbf9b8b77 100644
--- a/Lib/idlelib/editor.py
+++ b/Lib/idlelib/editor.py
@@ -1203,10 +1203,7 @@ def load_extension(self, name):
             self.apply_bindings(keydefs)
             for vevent in keydefs:
                 methodname = vevent.replace("-", "_")
-                while methodname[:1] == '<':
-                    methodname = methodname[1:]
-                while methodname[-1:] == '>':
-                    methodname = methodname[:-1]
+                methodname = methodname.lstrip('<').rstrip('>')
                 methodname = methodname + "_event"
                 if hasattr(ins, methodname):
                     self.text.bind(vevent, getattr(ins, methodname))
@@ -1341,6 +1338,24 @@ def set_indentation_params(self, is_py_src, guess=True):
                 self.usetabs = False
         self.set_tk_tabwidth(self.tabwidth)
 
+    def delete_trail_char_and_space(self, want, chars, tabwidth):
+        chars = chars[:-1]  # remove last character unconditionally
+        current_pos = 0
+        ncharsretained = 0
+        for char in chars:
+            if char == '\t':
+                current_pos = (current_pos // tabwidth + 1) * tabwidth
+            else:
+                current_pos += 1
+            if current_pos > want:
+                break
+            ncharsretained += 1
+        for i in range(ncharsretained, len(chars)):
+            if chars[i] not in " \t":
+                ncharsretained = i + 1
+        chars = chars[:ncharsretained]
+        return chars
+
     def smart_backspace_event(self, event):
         text = self.text
         first, last = self.get_selection_indices()
@@ -1369,13 +1384,10 @@ def smart_backspace_event(self, event):
         assert have > 0
         want = ((have - 1) // self.indentwidth) * self.indentwidth
         # Debug prompt is multilined....
-        ncharsdeleted = 0
-        while True:
-            chars = chars[:-1]
-            ncharsdeleted = ncharsdeleted + 1
-            have = len(chars.expandtabs(tabwidth))
-            if have <= want or chars[-1] not in " \t":
-                break
+        oldchars = chars
+        chars = self.delete_trail_char_and_space(want, chars, tabwidth)
+        ncharsdeleted = len(oldchars) - len(chars)
+        have = len(chars.expandtabs(tabwidth))
         text.undo_block_start()
         text.delete("insert-%dc" % ncharsdeleted, "insert")
         if have < want:
diff --git a/Lib/idlelib/idle_test/test_editor.py b/Lib/idlelib/idle_test/test_editor.py
index 0dfe2f3c58befa..b5ecf36d2eb345 100644
--- a/Lib/idlelib/idle_test/test_editor.py
+++ b/Lib/idlelib/idle_test/test_editor.py
@@ -1,10 +1,11 @@
-"Test editor, coverage 53%."
+"Test editor, coverage approximately 53%."
 
 from idlelib import editor
 import unittest
 from collections import namedtuple
 from test.support import requires
 from tkinter import Tk, Text
+import time
 
 Editor = editor.EditorWindow
 
@@ -237,5 +238,64 @@ def test_rclick(self):
         pass
 
 
+class DeleteWantTest(unittest.TestCase):
+
+    data = [
+        ("abcde" + 10000 * "\t" + 10000 * " ", [
+            (30000, 4, "abcde" + 7499 * "\t"),
+            (41005, 4, "abcde" + 10000 * "\t" + 1001 * " "),
+            (3, 4, "abcde"),
+            (6, 4, "abcde"),
+            (30002, 4, "abcde" + 7499 * "\t"),
+        ]),
+        ("abcde\tabd\t\t", [
+            (7, 4, "abcde\tabd"),
+            (12, 4, "abcde\tabd\t"),
+            (13, 4, "abcde\tabd\t"),
+            (16, 4, "abcde\tabd\t"),
+        ]),
+        ("abcde\tabd\t \ta", [
+            (7, 4, "abcde\tabd"),
+            (12, 4, "abcde\tabd\t"),
+            (13, 4, "abcde\tabd\t "),
+            (16, 4, "abcde\tabd\t \t"),
+        ]),
+        ("    ", [
+            (2, 2, "  "),
+        ]),
+    ]
+
+    def mock_delete_trail_char_and_space(self, want, chars, tabwidth):
+        ncharsdeleted = 0
+        while True:
+            chars = chars[:-1]
+            ncharsdeleted = ncharsdeleted + 1
+            have = len(chars.expandtabs(tabwidth))
+            if have <= want or chars[-1] not in " \t":
+                break
+        return chars
+
+    def do_tests(self):
+        ew = Editor()
+        for dat in self.data:
+            test_str = dat[0]
+            for da in dat[1]:
+                with self.subTest(want=da[0], tabwidth=da[1], input=test_str):
+                    res = ew.delete_trail_char_and_space(da[0], test_str, da[1])
+                    self.assertEqual(res, da[2])
+
+    def test_delete_trail_char_and_space(self):
+        with unittest.mock.patch.object(Editor, '__init__', return_value=None):
+            initial_time_new = time.time()
+            self.do_tests()
+            time_new = time.time() - initial_time_new
+
+            with unittest.mock.patch.object(Editor, 'delete_trail_char_and_space', self.mock_delete_trail_char_and_space):
+                initial_time_old = time.time()
+                self.do_tests()
+                time_old = time.time() - initial_time_old
+
+            self.assertGreaterEqual(time_old / time_new, 10)
+
 if __name__ == '__main__':
     unittest.main(verbosity=2)
diff --git a/Misc/NEWS.d/next/Security/2025-05-29-03-24-18.gh-issue-134873.dziqkQ.rst b/Misc/NEWS.d/next/Security/2025-05-29-03-24-18.gh-issue-134873.dziqkQ.rst
new file mode 100644
index 00000000000000..389b8af30ef048
--- /dev/null
+++ b/Misc/NEWS.d/next/Security/2025-05-29-03-24-18.gh-issue-134873.dziqkQ.rst
@@ -0,0 +1 @@
+Fix a DOS vulnerability in :mod:`idlelib` regarding string slicing.