use the collapsed path in the run_cgi method (closes #19435)

Lib/http/server.py

@@ -926,18 +926,17 @@ class CGIHTTPRequestHandler(SimpleHTTPRequestHandler):
 
     def run_cgi(self):
         """Execute a CGI script."""
-        path = self.path
         dir, rest = self.cgi_info
 
-        i = path.find('/', len(dir) + 1)
+        i = rest.find('/')
         while i >= 0:
-            nextdir = path[:i]
-            nextrest = path[i+1:]
+            nextdir = rest[:i]
+            nextrest = rest[i+1:]
 
             scriptdir = self.translate_path(nextdir)
             if os.path.isdir(scriptdir):
                 dir, rest = nextdir, nextrest
-                i = path.find('/', len(dir) + 1)
+                i = rest.find('/')
             else:
                 break
 

Lib/test/test_httpservers.py

@@ -388,6 +388,11 @@ class CGIHTTPServerTestCase(BaseTestCase):
         else:
             self.pythonexe = sys.executable
 
+        self.nocgi_path = os.path.join(self.parent_dir, 'nocgi.py')
+        with open(self.nocgi_path, 'w') as fp:
+            fp.write(cgi_file1 % self.pythonexe)
+        os.chmod(self.nocgi_path, 0o777)
+
         self.file1_path = os.path.join(self.cgi_dir, 'file1.py')
         with open(self.file1_path, 'w') as file1:
             file1.write(cgi_file1 % self.pythonexe)
@@ -406,6 +411,7 @@ class CGIHTTPServerTestCase(BaseTestCase):
             os.chdir(self.cwd)
             if self.pythonexe != sys.executable:
                 os.remove(self.pythonexe)
+            os.remove(self.nocgi_path)
             os.remove(self.file1_path)
             os.remove(self.file2_path)
             os.rmdir(self.cgi_dir)
@@ -457,6 +463,10 @@ class CGIHTTPServerTestCase(BaseTestCase):
         self.assertEqual((b'Hello World\n', 'text/html', 200), \
              (res.read(), res.getheader('Content-type'), res.status))
 
+    def test_issue19435(self):
+        res = self.request('///////////nocgi.py/../cgi-bin/nothere.sh')
+        self.assertEqual(res.status, 404)
+
     def test_post(self):
         params = urllib.parse.urlencode(
             {'spam' : 1, 'eggs' : 'python', 'bacon' : 123456})

Misc/NEWS

@@ -13,6 +13,8 @@ Core and Builtins
 Library
 -------
 
+- Issue #19435: Fix directory traversal attack on CGIHttpRequestHandler.
+
 - Issue #14984: On POSIX systems, when netrc is called without a filename
   argument (and therefore is reading the user's $HOME/.netrc file), it now
   enforces the same security rules as typical ftp clients: the .netrc file must