From a4d3edf3a6ecfde504d02126410d2a65a859b744 Mon Sep 17 00:00:00 2001
From: Stan Ulbrych <stan@python.org>
Date: Tue, 4 Aug 2026 11:19:00 +0200
Subject: [PATCH] [3.12] gh-148169: Fix webbrowser `%action` substitution
 bypass of dash-prefix check (GH-148170) (#148519)

(cherry picked from commit d22922c8a7958353689dc4763dd72da2dea03fff)
---
 Lib/test/test_webbrowser.py                              | 9 +++++++++
 Lib/webbrowser.py                                        | 5 +++--
 .../2026-03-31-09-15-51.gh-issue-148169.EZJzz2.rst       | 2 ++
 3 files changed, 14 insertions(+), 2 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Security/2026-03-31-09-15-51.gh-issue-148169.EZJzz2.rst

diff --git a/Lib/test/test_webbrowser.py b/Lib/test/test_webbrowser.py
index 924607931c3e741..ae161786002569f 100644
--- a/Lib/test/test_webbrowser.py
+++ b/Lib/test/test_webbrowser.py
@@ -103,6 +103,15 @@ 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):
 
diff --git a/Lib/webbrowser.py b/Lib/webbrowser.py
index 00beb7ba29add58..79e00f9f3343d81 100755
--- a/Lib/webbrowser.py
+++ b/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)
diff --git a/Misc/NEWS.d/next/Security/2026-03-31-09-15-51.gh-issue-148169.EZJzz2.rst b/Misc/NEWS.d/next/Security/2026-03-31-09-15-51.gh-issue-148169.EZJzz2.rst
new file mode 100644
index 000000000000000..45cdeebe1b6d64a
--- /dev/null
+++ b/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.
