From 735f25c5e3a0f74438c86468ec4dfbe219d93c91 Mon Sep 17 00:00:00 2001
From: Serhiy Storchaka <storchaka@gmail.com>
Date: Mon, 27 Jan 2025 15:44:00 +0200
Subject: [PATCH] gh-119511: Fix a potential denial of service in imaplib
 (#119514)

The IMAP4 client could consume an arbitrary amount of memory when trying
to connect to a malicious server, because it read a "literal" data with a
single read(size) call, and BufferedReader.read() allocates the bytes
object of the specified size before reading. Now the IMAP4 client reads data
by chunks, therefore the amount of used memory is limited by the
amount of the data actually been sent by the server.

Co-authored-by: Gregory P. Smith <greg@krypto.org>
---
 Lib/imaplib.py                                     | 11 ++++++++++-
 Lib/test/test_imaplib.py                           | 14 ++++++++++++++
 .../2024-05-24-21-00-52.gh-issue-119511.jKrXQ8.rst |  7 +++++++
 3 files changed, 31 insertions(+), 1 deletion(-)
 create mode 100644 Misc/NEWS.d/next/Security/2024-05-24-21-00-52.gh-issue-119511.jKrXQ8.rst

diff --git a/Lib/imaplib.py b/Lib/imaplib.py
index e576c29e67d..db708580a0a 100644
--- a/Lib/imaplib.py
+++ b/Lib/imaplib.py
@@ -52,6 +52,9 @@ AllowedVersions = ('IMAP4REV1', 'IMAP4')        # Most recent first
 # search command can be quite large, so we now use 1M.
 _MAXLINE = 1000000
 
+# Data larger than this will be read in chunks, to prevent extreme
+# overallocation.
+_SAFE_BUF_SIZE = 1 << 20
 
 #       Commands
 
@@ -315,7 +318,13 @@ class IMAP4:
 
     def read(self, size):
         """Read 'size' bytes from remote."""
-        return self.file.read(size)
+        cursize = min(size, _SAFE_BUF_SIZE)
+        data = self.file.read(cursize)
+        while cursize < size and len(data) == cursize:
+            delta = min(cursize, size - cursize)
+            data += self.file.read(delta)
+            cursize += delta
+        return data
 
 
     def readline(self):
diff --git a/Lib/test/test_imaplib.py b/Lib/test/test_imaplib.py
index a6509fc3ba0..2fbf83b264d 100644
--- a/Lib/test/test_imaplib.py
+++ b/Lib/test/test_imaplib.py
@@ -901,6 +901,20 @@ class ThreadedNetworkedTests(unittest.TestCase):
             self.assertRaises(imaplib.IMAP4.error,
                               self.imap_class, *server.server_address)
 
+    def test_truncated_large_literal(self):
+        size = 0
+        class BadHandler(SimpleIMAPHandler):
+            def handle(self):
+                self._send_textline('* OK {%d}' % size)
+                self._send_textline('IMAP4rev1')
+
+        for exponent in range(15, 64):
+            size = 1 << exponent
+            with self.subTest(f"size=2e{size}"):
+                with self.reaped_server(BadHandler) as server:
+                    with self.assertRaises(imaplib.IMAP4.abort):
+                        self.imap_class(*server.server_address)
+
     @threading_helper.reap_threads
     def test_simple_with_statement(self):
         # simplest call
diff --git a/Misc/NEWS.d/next/Security/2024-05-24-21-00-52.gh-issue-119511.jKrXQ8.rst b/Misc/NEWS.d/next/Security/2024-05-24-21-00-52.gh-issue-119511.jKrXQ8.rst
new file mode 100644
index 00000000000..f7b4031120e
--- /dev/null
+++ b/Misc/NEWS.d/next/Security/2024-05-24-21-00-52.gh-issue-119511.jKrXQ8.rst
@@ -0,0 +1,7 @@
+Fix a potential denial of service in the :mod:`imaplib` module. When connecting
+to a malicious server, it could cause an arbitrary amount of memory to be
+allocated. On many systems this is harmless as unused virtual memory is only a
+mapping, but if this hit a virtual address size limit it could lead to a
+:exc:`MemoryError` or other process crash. On unusual systems or builds where
+all allocated memory is touched and backed by actual ram or storage it could've
+consumed resources doing so until similarly crashing.