gh-119452: Fix OOM vulnerability in http.server

serhiy-storchaka · serhiy-storchaka · commit 0f35918585d0 · 2025-11-18T15:49:48.000+02:00
The CGI server on Windows could consume the amount of memory specified
in the Content-Length header of the request even if the client does not
send such much data. Now it reads the POST request body by chunks,
therefore the momory consumption is proportional to the amont of sent
data.
diff --git a/Lib/http/server.py b/Lib/http/server.py
@@ -134,6 +134,10 @@
 
 DEFAULT_ERROR_CONTENT_TYPE = "text/html;charset=utf-8"
 
+# Data larger than this will be read in chunks, to prevent extreme
+# overallocation.
+SAFE_BUF_SIZE = 1 << 20
+
 class HTTPServer(socketserver.TCPServer):
 
     allow_reuse_address = True    # Seems to make sense in testing environment
@@ -1284,7 +1288,13 @@ def run_cgi(self):
                                  env = env
                                  )
             if self.command.lower() == "post" and nbytes > 0:
-                data = self.rfile.read(nbytes)
+                cursize = 0
+                data = self.rfile.read(min(nbytes, SAFE_BUF_SIZE))
+                while (len(data) < nbytes and len(data) != cursize and
+                       select.select([self.rfile._sock], [], [], 0)[0]):
+                    cursize = len(data)
+                    delta = min(cursize, nbytes - cursize)
+                    data += self.rfile.read(delta)
             else:
                 data = None
             # throw away additional data [see bug #427345]
diff --git a/Lib/test/test_httpservers.py b/Lib/test/test_httpservers.py
@@ -913,6 +913,20 @@ def test_path_without_leading_slash(self):
 print("</pre>")
 """
 
+cgi_file7 = """\
+#!%s
+import os
+import sys
+
+print("Content-type: text/plain")
+print()
+
+content_length = int(os.environ["CONTENT_LENGTH"])
+body = sys.stdin.buffer.read(content_length)
+
+print(f"{content_length} {len(body)}")
+"""
+
 
 @unittest.skipIf(hasattr(os, 'geteuid') and os.geteuid() == 0,
         "This test can't be run reliably as root (issue #13308).")
@@ -952,6 +966,8 @@ def setUp(self):
         self.file3_path = None
         self.file4_path = None
         self.file5_path = None
+        self.file6_path = None
+        self.file7_path = None
 
         # The shebang line should be pure ASCII: use symlink if possible.
         # See issue #7668.
@@ -1006,6 +1022,11 @@ def setUp(self):
             file6.write(cgi_file6 % self.pythonexe)
         os.chmod(self.file6_path, 0o777)
 
+        self.file7_path = os.path.join(self.cgi_dir, 'file7.py')
+        with open(self.file7_path, 'w', encoding='utf-8') as file7:
+            file7.write(cgi_file7 % self.pythonexe)
+        os.chmod(self.file7_path, 0o777)
+
         os.chdir(self.parent_dir)
 
     def tearDown(self):
@@ -1028,6 +1049,8 @@ def tearDown(self):
                 os.remove(self.file5_path)
             if self.file6_path:
                 os.remove(self.file6_path)
+            if self.file7_path:
+                os.remove(self.file7_path)
             os.rmdir(self.cgi_child_dir)
             os.rmdir(self.cgi_dir)
             os.rmdir(self.cgi_dir_in_sub_dir)
@@ -1100,6 +1123,21 @@ def test_post(self):
 
         self.assertEqual(res.read(), b'1, python, 123456' + self.linesep)
 
+    def test_large_content_length(self):
+        for w in range(15, 25):
+            size = 1 << w
+            body = b'X' * size
+            headers = {'Content-Length' : str(size)}
+            res = self.request('/cgi-bin/file7.py', 'POST', body, headers)
+            self.assertEqual(res.read(), b'%d %d' % (size, size) + self.linesep)
+
+    def test_large_content_length_truncated(self):
+        for w in range(18, 65):
+            size = 1 << w
+            headers = {'Content-Length' : str(size)}
+            res = self.request('/cgi-bin/file1.py', 'POST', b'x', headers)
+            self.assertEqual(res.read(), b'Hello World' + self.linesep)
+
     def test_invaliduri(self):
         res = self.request('/cgi-bin/invalid')
         res.read()
diff --git a/Misc/NEWS.d/next/Security/2024-05-23-11-44-41.gh-issue-119452.PRfsSv.rst b/Misc/NEWS.d/next/Security/2024-05-23-11-44-41.gh-issue-119452.PRfsSv.rst
@@ -0,0 +1,3 @@
+Fix OOM vulnerability in :mod:`http.server`, when handling the POST request
+in the CGI server on Windows could cause consuming an arbitrary amount of
+memory.

Original file line number	Diff line number	Diff line change
`@@ -0,0 +1,3 @@`
	`1`	+Fix OOM vulnerability in :mod:`http.server`, when handling the POST request
	`2`	`+in the CGI server on Windows could cause consuming an arbitrary amount of`
	`3`	`+memory.`