From 2552b2e8c9f73b5cdef8842e4e3701bac19c12c6 Mon Sep 17 00:00:00 2001
From: Adi Roiban <adi.roiban@chevah.com>
Date: Mon, 24 Jan 2022 19:09:04 +0000
Subject: [PATCH 3/4] Fix CVE-2022-21716
Initial fix for Twisted version string DoS.
(cherry picked from commit de90dfe1519e996dd150de751c670f8e03daa089)
Update after review.
(cherry picked from commit 9b98116372f5d211ddb9f68916d4ae73bf3c8da7)
Fix typo.
(cherry picked from commit a4523b444760f07e609636264a61a2a07ca0bde5)
---
src/twisted/conch/ssh/transport.py | 9 +++++++++
src/twisted/conch/test/test_transport.py | 21 +++++++++++++++++++++
src/twisted/newsfragments/10284.bugfix | 2 ++
3 files changed, 32 insertions(+)
create mode 100644 src/twisted/newsfragments/10284.bugfix
diff --git a/src/twisted/conch/ssh/transport.py b/src/twisted/conch/ssh/transport.py
index bd76b0a845..6d02df52dd 100644
--- a/src/twisted/conch/ssh/transport.py
+++ b/src/twisted/conch/ssh/transport.py
@@ -677,6 +677,15 @@ class SSHTransportBase(protocol.Protocol):
"""
self.buf = self.buf + data
if not self.gotVersion:
+
+ if len(self.buf) > 4096:
+ self.sendDisconnect(
+ DISCONNECT_CONNECTION_LOST,
+ b"Peer version string longer than 4KB. "
+ b"Preventing a denial of service attack.",
+ )
+ return
+
if self.buf.find(b'\n', self.buf.find(b'SSH-')) == -1:
return
diff --git a/src/twisted/conch/test/test_transport.py b/src/twisted/conch/test/test_transport.py
index 98a3515a75..449dd3f8df 100644
--- a/src/twisted/conch/test/test_transport.py
+++ b/src/twisted/conch/test/test_transport.py
@@ -522,6 +522,27 @@ class BaseSSHTransportTests(BaseSSHTransportBaseCase, TransportTestCase):
r')*$')
self.assertRegex(softwareVersion, softwareVersionRegex)
+ def test_dataReceiveVersionNotSentMemoryDOS(self):
+ """
+ When the peer is not sending its SSH version but keeps sending data,
+ the connection is disconnected after 4KB to prevent buffering too
+ much and running our of memory.
+ """
+ sut = MockTransportBase()
+ sut.makeConnection(self.transport)
+
+ # Data can be received over multiple chunks.
+ sut.dataReceived(b"SSH-2-Server-Identifier")
+ sut.dataReceived(b"1234567890" * 406)
+ sut.dataReceived(b"1235678")
+ self.assertFalse(self.transport.disconnecting)
+
+ # Here we are going over the limit.
+ sut.dataReceived(b"1234567")
+ # Once a lot of data is received without an SSH version string,
+ # the transport is disconnected.
+ self.assertTrue(self.transport.disconnecting)
+ self.assertIn(b"Preventing a denial of service attack", self.transport.value())
def test_sendPacketPlain(self):
"""
diff --git a/src/twisted/newsfragments/10284.bugfix b/src/twisted/newsfragments/10284.bugfix
new file mode 100644
index 0000000000..b2316f1e68
--- /dev/null
+++ b/src/twisted/newsfragments/10284.bugfix
@@ -0,0 +1,2 @@
+twisted.conch.ssh.transport.SSHTransportBase now disconnects the remote peer if the
+SSH version string is not sent in the first 4096 bytes.
--
2.39.2