From 861bded4e1f0b9c1947a0b3f1b64cfb4e92c539a Mon Sep 17 00:00:00 2001
From: Serhiy Storchaka <storchaka@gmail.com>
Date: Mon, 1 Dec 2025 14:59:09 +0200
Subject: [PATCH] [3.14] gh-119452: Fix a potential virtual memory allocation
 denial of service in http.server (GH-119455)

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,
so that the memory consumption is proportional to the amount of sent
data.
(cherry picked from commit 29c657a1f231c0908796e0c9ff6967e15ab20d9b)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
---
 Lib/http/server.py                            | 15 +++++++-
 Lib/test/test_httpservers.py                  | 38 +++++++++++++++++++
 ...-05-23-11-44-41.gh-issue-119452.PRfsSv.rst |  5 +++
 3 files changed, 57 insertions(+), 1 deletion(-)
 create mode 100644 Misc/NEWS.d/next/Security/2024-05-23-11-44-41.gh-issue-119452.PRfsSv.rst

diff --git a/Lib/http/server.py b/Lib/http/server.py
index 0f2f3f6eec3bda..d2661e00619b9c 100644
--- a/Lib/http/server.py
+++ b/Lib/http/server.py
@@ -127,6 +127,10 @@
 
 DEFAULT_ERROR_CONTENT_TYPE = "text/html;charset=utf-8"
 
+# Data larger than this will be read in chunks, to prevent extreme
+# overallocation.
+_MIN_READ_BUF_SIZE = 1 << 20
+
 class HTTPServer(socketserver.TCPServer):
 
     allow_reuse_address = 1    # Seems to make sense in testing environment
@@ -1218,7 +1222,16 @@ 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, _MIN_READ_BUF_SIZE))
+                while (len(data) < nbytes and len(data) != cursize and
+                       select.select([self.rfile._sock], [], [], 0)[0]):
+                    cursize = len(data)
+                    # This is a geometric increase in read size (never more
+                    # than doubling our the current length of data per loop
+                    # iteration).
+                    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
index 88d06fe04fb726..507522983a2e25 100644
--- a/Lib/test/test_httpservers.py
+++ b/Lib/test/test_httpservers.py
@@ -695,6 +695,20 @@ def test_html_escape_filename(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).")
@@ -728,6 +742,8 @@ 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.
@@ -782,6 +798,11 @@ 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):
@@ -803,6 +824,8 @@ 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)
@@ -875,6 +898,21 @@ 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
new file mode 100644
index 00000000000000..98956627f2b30d
--- /dev/null
+++ b/Misc/NEWS.d/next/Security/2024-05-23-11-44-41.gh-issue-119452.PRfsSv.rst
@@ -0,0 +1,5 @@
+Fix a potential memory denial of service in the :mod:`http.server` module.
+When a malicious user is connected to the CGI server on Windows, it could cause
+an arbitrary amount of memory to be allocated.
+This could have led to symptoms including a :exc:`MemoryError`, swapping, out
+of memory (OOM) killed processes or containers, or even system crashes.