Reluctantly add server-side support for TLS_FALLBACK_SCSV.

This allows for clients that willingly choose to perform a downgrade and
attempt to establish a second connection at a lower protocol after the
previous attempt unexpectedly failed, to be notified and have the second
connection aborted, if the server does in fact support a higher protocol.

TLS has perfectly good version negotiation and client-side fallback is
dangerous. Despite this, in order to maintain maximum compatability with
broken web servers, most mainstream browsers implement this. Furthermore,
TLS_FALLBACK_SCSV only works if both the client and server support it and
there is effectively no way to tell if this is the case, unless you control
both ends.

Unfortunately, various auditors and vulnerability scanners (including
certain online assessment websites) consider the presence of a not yet
standardised feature to be important for security, even if the clients do
not perform client-side downgrade or the server only supports current TLS
protocols.

Diff is loosely based on OpenSSL with some inspiration from BoringSSL.

Discussed with beck@ and miod@.

ok bcook@

1 files changed, 5 insertions, 2 deletions

diff --git a/src/lib/libssl/ssl.h b/src/lib/libssl/ssl.h
index 626b967f15..73d007400f 100644
--- a/src/lib/libssl/ssl.h
+++ b/src/lib/libssl/ssl.h
@@ -1,4 +1,4 @@
-/* $OpenBSD: ssl.h,v 1.82 2015/02/12 03:45:25 jsing Exp $ */
+/* $OpenBSD: ssl.h,v 1.83 2015/02/22 15:54:27 jsing Exp $ */
 /* Copyright (C) 1995-1998 Eric Young (eay@cryptsoft.com)
  * All rights reserved.
  *
@@ -1363,7 +1363,8 @@ DECLARE_PEM_rw(SSL_SESSION, SSL_SESSION)
 #define SSL_AD_UNRECOGNIZED_NAME        TLS1_AD_UNRECOGNIZED_NAME
 #define SSL_AD_BAD_CERTIFICATE_STATUS_RESPONSE TLS1_AD_BAD_CERTIFICATE_STATUS_RESPONSE
 #define SSL_AD_BAD_CERTIFICATE_HASH_VALUE TLS1_AD_BAD_CERTIFICATE_HASH_VALUE
-#define SSL_AD_UNKNOWN_PSK_IDENTITY     TLS1_AD_UNKNOWN_PSK_IDENTITY /* fatal */
+#define SSL_AD_UNKNOWN_PSK_IDENTITY     TLS1_AD_UNKNOWN_PSK_IDENTITY /* fatal */
+#define SSL_AD_INAPPROPRIATE_FALLBACK   TLS1_AD_INAPPROPRIATE_FALLBACK /* fatal */
 
 #define SSL_ERROR_NONE                  0
 #define SSL_ERROR_SSL                   1
@@ -2174,6 +2175,7 @@ void ERR_load_SSL_strings(void);
 #define SSL_R_HTTPS_PROXY_REQUEST                        155
 #define SSL_R_HTTP_REQUEST                               156
 #define SSL_R_ILLEGAL_PADDING                            283
+#define SSL_R_INAPPROPRIATE_FALLBACK                     373
 #define SSL_R_INCONSISTENT_COMPRESSION                   340
 #define SSL_R_INVALID_CHALLENGE_LENGTH                   158
 #define SSL_R_INVALID_COMMAND                            280
@@ -2319,6 +2321,7 @@ void ERR_load_SSL_strings(void);
 #define SSL_R_TLSV1_ALERT_DECRYPTION_FAILED              1021
 #define SSL_R_TLSV1_ALERT_DECRYPT_ERROR                  1051
 #define SSL_R_TLSV1_ALERT_EXPORT_RESTRICTION             1060
+#define SSL_R_TLSV1_ALERT_INAPPROPRIATE_FALLBACK         1086
 #define SSL_R_TLSV1_ALERT_INSUFFICIENT_SECURITY          1071
 #define SSL_R_TLSV1_ALERT_INTERNAL_ERROR                 1080
 #define SSL_R_TLSV1_ALERT_NO_RENEGOTIATION               1100