00480: CVE-2026-4786

Fix webbrowser `%action` substitution bypass of dash-prefix check

Author: StanFromIreland

Lib/test/test_webbrowser.py

@@ -99,6 +99,15 @@ def test_open_new_tab(self):
                    options=[],
                    arguments=[URL])
 
+    def test_reject_action_dash_prefixes(self):
+        browser = self.browser_class(name=CMD_NAME)
+        with self.assertRaises(ValueError):
+            browser.open('%action--incognito')
+        # new=1: action is "--new-window", so "%action" itself expands to
+        # a dash-prefixed flag even with no dash in the original URL.
+        with self.assertRaises(ValueError):
+            browser.open('%action', new=1)
+
 
 class EdgeCommandTest(CommandTestMixin, unittest.TestCase):
 

Lib/webbrowser.py

@@ -268,7 +268,6 @@ def _invoke(self, args, remote, autoraise, url=None):
 
     def open(self, url, new=0, autoraise=True):
         sys.audit("webbrowser.open", url)
-        self._check_url(url)
         if new == 0:
             action = self.remote_action
         elif new == 1:
@@ -282,7 +281,9 @@ def open(self, url, new=0, autoraise=True):
             raise Error("Bad 'new' parameter to open(); " +
                         "expected 0, 1, or 2, got %s" % new)
 
-        args = [arg.replace("%s", url).replace("%action", action)
+        self._check_url(url.replace("%action", action))
+
+        args = [arg.replace("%action", action).replace("%s", url)
                 for arg in self.remote_args]
         args = [arg for arg in args if arg]
         success = self._invoke(args, True, autoraise, url)

Misc/NEWS.d/next/Security/2026-03-31-09-15-51.gh-issue-148169.EZJzz2.rst

@@ -0,0 +1,2 @@
+A bypass in :mod:`webbrowser` allowed URLs prefixed with ``%action`` to pass
+the dash-prefix safety check.