Commit 358f513c changed the maximum size of accepted control channel
packets.  This was needed for crypto negotiation (which is needed for a
nice transition to a new default cipher), but exposed a DoS
vulnerability.  The vulnerability was found during the OpenVPN 2.4 code
audit by Quarkslab (commisioned by OSTIF).

To fix the issue, we should not ASSERT() on external input (in this case
the received packet size), but instead gracefully error out and drop the
invalid packet.

Signed-off-by: Steffan Karger <steffan.kar...@fox-it.com>
---
 Changes.rst       | 5 +++++
 src/openvpn/ssl.c | 7 ++++++-
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/Changes.rst b/Changes.rst
index 183e9fa..cc6ca2b 100644
--- a/Changes.rst
+++ b/Changes.rst
@@ -109,6 +109,11 @@ Version 2.3.15
 
 Security fixes
 --------------
+- Fix a pre-authentication denial-of-service attack on both clients and 
servers.
+  By sending a too-large control packet, OpenVPN 2.3.12 and newer can be forced
+  to hit an ASSERT() and stop the process.  If ``--tls-auth`` or 
``--tls-crypt``
+  is used, only attackers that have the ``--tls-auth`` or ``--tls-crypt`` key
+  can mount an attack. (OSTIF/Quarkslab audit finding 5.1, CVE-2017-7478)
 - Fix an authenticated remote DoS vulnerability that could be triggered by
   causing a packet id roll over.  An attack is rather inefficient; a peer
   would need to get us to send at least about 196 GB of data.
diff --git a/src/openvpn/ssl.c b/src/openvpn/ssl.c
index 32d0b6b..c8f093d 100644
--- a/src/openvpn/ssl.c
+++ b/src/openvpn/ssl.c
@@ -3228,7 +3228,12 @@ tls_pre_decrypt (struct tls_multi *multi,
                            /* Save incoming ciphertext packet to reliable 
buffer */
                            struct buffer *in = reliable_get_buf 
(ks->rec_reliable);
                            ASSERT (in);
-                           ASSERT (buf_copy (in, buf));
+                           if (!buf_copy (in, buf))
+                             {
+                               msg (D_MULTI_DROPPED,
+                                    "Incoming control channel packet too big, 
dropping.");
+                               goto error;
+                             }
                            reliable_mark_active_incoming (ks->rec_reliable, 
in, id, op);
                          }
 
-- 
2.7.4


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