From: Seth Michael Larson <seth@python.org>
Date: Fri, 20 Mar 2026 14:47:13 +0000 (-0500)
Subject: gh-143930: Reject leading dashes in webbrowser URLs
X-Git-Tag: v3.15.0a8~234
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=82a24a4442312bdcfc4c799885e8b3e00990f02b;p=thirdparty%2FPython%2Fcpython.git

gh-143930: Reject leading dashes in webbrowser URLs
---

diff --git a/Lib/test/test_webbrowser.py b/Lib/test/test_webbrowser.py
index 20d347168b3a..d5bb1400d271 100644
--- a/Lib/test/test_webbrowser.py
+++ b/Lib/test/test_webbrowser.py
@@ -67,6 +67,11 @@ class GenericBrowserCommandTest(CommandTestMixin, unittest.TestCase):
                    options=[],
                    arguments=[URL])
 
+    def test_reject_dash_prefixes(self):
+        browser = self.browser_class(name=CMD_NAME)
+        with self.assertRaises(ValueError):
+            browser.open(f"--key=val {URL}")
+
 
 class BackgroundBrowserCommandTest(CommandTestMixin, unittest.TestCase):
 
diff --git a/Lib/webbrowser.py b/Lib/webbrowser.py
index f2e2394089d5..9ead2990e818 100644
--- a/Lib/webbrowser.py
+++ b/Lib/webbrowser.py
@@ -163,6 +163,12 @@ class BaseBrowser:
     def open_new_tab(self, url):
         return self.open(url, 2)
 
+    @staticmethod
+    def _check_url(url):
+        """Ensures that the URL is safe to pass to subprocesses as a parameter"""
+        if url and url.lstrip().startswith("-"):
+            raise ValueError(f"Invalid URL: {url}")
+
 
 class GenericBrowser(BaseBrowser):
     """Class for all browsers started with a command
@@ -180,6 +186,7 @@ class GenericBrowser(BaseBrowser):
 
     def open(self, url, new=0, autoraise=True):
         sys.audit("webbrowser.open", url)
+        self._check_url(url)
         cmdline = [self.name] + [arg.replace("%s", url)
                                  for arg in self.args]
         try:
@@ -200,6 +207,7 @@ class BackgroundBrowser(GenericBrowser):
         cmdline = [self.name] + [arg.replace("%s", url)
                                  for arg in self.args]
         sys.audit("webbrowser.open", url)
+        self._check_url(url)
         try:
             if sys.platform[:3] == 'win':
                 p = subprocess.Popen(cmdline)
@@ -266,6 +274,7 @@ class UnixBrowser(BaseBrowser):
 
     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:
@@ -357,6 +366,7 @@ class Konqueror(BaseBrowser):
 
     def open(self, url, new=0, autoraise=True):
         sys.audit("webbrowser.open", url)
+        self._check_url(url)
         # XXX Currently I know no way to prevent KFM from opening a new win.
         if new == 2:
             action = "newTab"
@@ -588,6 +598,7 @@ if sys.platform[:3] == "win":
     class WindowsDefault(BaseBrowser):
         def open(self, url, new=0, autoraise=True):
             sys.audit("webbrowser.open", url)
+            self._check_url(url)
             try:
                 os.startfile(url)
             except OSError:
@@ -608,6 +619,7 @@ if sys.platform == 'darwin':
 
         def open(self, url, new=0, autoraise=True):
             sys.audit("webbrowser.open", url)
+            self._check_url(url)
             url = url.replace('"', '%22')
             if self.name == 'default':
                 proto, _sep, _rest = url.partition(":")
@@ -664,6 +676,7 @@ if sys.platform == "ios":
     class IOSBrowser(BaseBrowser):
         def open(self, url, new=0, autoraise=True):
             sys.audit("webbrowser.open", url)
+            self._check_url(url)
             # If ctypes isn't available, we can't open a browser
             if objc is None:
                 return False
diff --git a/Misc/NEWS.d/next/Security/2026-01-16-12-04-49.gh-issue-143930.zYC5x3.rst b/Misc/NEWS.d/next/Security/2026-01-16-12-04-49.gh-issue-143930.zYC5x3.rst
new file mode 100644
index 000000000000..0f27eae99a0d
--- /dev/null
+++ b/Misc/NEWS.d/next/Security/2026-01-16-12-04-49.gh-issue-143930.zYC5x3.rst
@@ -0,0 +1 @@
+Reject leading dashes in URLs passed to :func:`webbrowser.open`