[3.11] gh-119511: Fix a potential denial of service in imaplib (GH-119514) (#129357)

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.
(cherry picked from commit 735f25c5e3a0f74438c86468ec4dfbe219d93c91)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
Co-authored-by: Gregory P. Smith <greg@krypto.org>

diff --git a/Lib/imaplib.py b/Lib/imaplib.py
index fa4c0f8f62361a108f0001dc9f6abc0be2de6d01..20b86c35d334d5da35dfb1d64b750fa4be4782ef 100644 (file)
--- 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 bd0fc9c2da1c23efb568a9e7ae24058ae6e98ffa..7665532d01ef9d20b2b60e53d3acc5a506159034 100644 (file)
--- a/Lib/test/test_imaplib.py
+++ b/Lib/test/test_imaplib.py
@@ -915,6 +915,20 @@ 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 (file)
index 0000000..f7b4031
--- /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.