From f529b9470752c28ab69c96f31b0dbc10db69b404 Mon Sep 17 00:00:00 2001
From: Stan Ulbrych <stan@python.org>
Date: Mon, 13 Apr 2026 20:02:52 +0100
Subject: [PATCH] gh-148169: Fix webbrowser `%action` substitution bypass of
 dash-prefix check (GH-148170) (cherry picked from commit
 d22922c8a7958353689dc4763dd72da2dea03fff)

Co-authored-by: Stan Ulbrych <stan@python.org>
---
 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 404b3a31a5d2c9..bfbcf112b0b085 100644
--- ./Lib/test/test_webbrowser.py
+++ b/Lib/test/test_webbrowser.py
@@ -119,6 +119,15 @@ def test_open_bad_new_parameter(self):
                        arguments=[URL],
                        kw=dict(new=999))
 
+    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 0e0b5034e5f53d..97aad6eea509eb 100644
--- ./Lib/webbrowser.py
+++ b/Lib/webbrowser.py
@@ -274,7 +274,6 @@ 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:
@@ -288,7 +287,9 @@ def open(self, url, new=0, autoraise=True):
             raise Error("Bad 'new' parameter to open(); "
                         f"expected 0, 1, or 2, got {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 00000000000000..45cdeebe1b6d64
--- /dev/null
+++ ./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.
