diff --git a/Lib/http/server.py b/Lib/http/server.py
index dab1eb65c4b..6a565394d83 100644
--- a/Lib/http/server.py
+++ b/Lib/http/server.py
@@ -971,7 +971,7 @@ class CGIHTTPRequestHandler(SimpleHTTPRequestHandler):
         (and the next character is a '/' or the end of the string).
 
         """
-        collapsed_path = _url_collapse_path(self.path)
+        collapsed_path = _url_collapse_path(urllib.parse.unquote(self.path))
         dir_sep = collapsed_path.find('/', 1)
         head, tail = collapsed_path[:dir_sep], collapsed_path[dir_sep+1:]
         if head in self.cgi_directories:
diff --git a/Lib/test/test_httpservers.py b/Lib/test/test_httpservers.py
index 4e62963725a..47c67968f76 100644
--- a/Lib/test/test_httpservers.py
+++ b/Lib/test/test_httpservers.py
@@ -464,6 +464,11 @@ class CGIHTTPServerTestCase(BaseTestCase):
                 (res.read(), res.getheader('Content-type'), res.status))
         self.assertEqual(os.environ['SERVER_SOFTWARE'], signature)
 
+    def test_urlquote_decoding_in_cgi_check(self):
+        res = self.request('/cgi-bin%2ffile1.py')
+        self.assertEqual((b'Hello World\n', 'text/html', 200),
+                (res.read(), res.getheader('Content-type'), res.status))
+
 
 class SocketlessRequestHandler(SimpleHTTPRequestHandler):
     def __init__(self):
diff --git a/Misc/NEWS b/Misc/NEWS
index f7dd62c9190..31980cd25d5 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -13,6 +13,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #21766: Prevent a security hole in CGIHTTPServer by URL unquoting paths
+  before checking for a CGI script at that path.
+
 - Fix arbitrary memory access in JSONDecoder.raw_decode with a negative second
   parameter. Bug reported by Guido Vranken.