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authorPhil Pennock <pdp@exim.org>2012-05-06 02:50:57 -0700
committerPhil Pennock <pdp@exim.org>2012-05-06 02:50:57 -0700
commitf0f5a555bee153477d12bcbce90875d46884281c (patch)
tree7d1c217de6f49a7b70c1058afe7eb4680bb39d04 /src
parent5bfb4cdf352ad40304c6bbf0d826569dea761699 (diff)
Disable SSLv2 by default.
Diffstat (limited to 'src')
-rw-r--r--src/README.UPDATING11
-rw-r--r--src/src/tls-openssl.c11
2 files changed, 19 insertions, 3 deletions
diff --git a/src/README.UPDATING b/src/README.UPDATING
index 5b6bea869..12335eab8 100644
--- a/src/README.UPDATING
+++ b/src/README.UPDATING
@@ -39,6 +39,12 @@ Exim version 4.78
the message. No tool has been provided as we believe this is a rare
occurence.
+ * For OpenSSL, SSLv2 is now disabled by default. (GnuTLS does not support
+ SSLv2). RFC 6176 prohibits SSLv2 and some informal surveys suggest no
+ actual usage. You can re-enable with the "openssl_options" Exim option,
+ in the main configuration section. Note that supporting SSLv2 exposes
+ you to ciphersuite downgrade attacks.
+
* With OpenSSL 1.0.1+, Exim now supports TLS 1.1 and TLS 1.2. If built
against 1.0.1a then you will get a warning message and the
"openssl_options" value will not parse "no_tlsv1_1": the value changes
@@ -48,8 +54,9 @@ Exim version 4.78
"openssl_options" gains "no_tlsv1_1", "no_tlsv1_2" and "no_compression".
COMPATIBILITY WARNING: The default value of "openssl_options" is no longer
- "+dont_insert_empty_fragments". We default to unset. That old default was
- grandfathered in from before openssl_options became a configuration option.
+ "+dont_insert_empty_fragments". We default to "+no_sslv2".
+ That old default was grandfathered in from before openssl_options became a
+ configuration option.
Empty fragments are inserted by default through TLS1.0, to partially defend
against certain attacks; TLS1.1+ change the protocol so that this is not
needed. The DIEF SSL option was required for some old releases of mail
diff --git a/src/src/tls-openssl.c b/src/src/tls-openssl.c
index e609670ee..ea32bdb40 100644
--- a/src/src/tls-openssl.c
+++ b/src/src/tls-openssl.c
@@ -481,7 +481,13 @@ list of available digests. */
EVP_add_digest(EVP_sha256());
#endif
-/* Create a context */
+/* Create a context.
+The OpenSSL docs in 1.0.1b have not been updated to clarify TLS variant
+negotiation in the different methods; as far as I can tell, the only
+*_{server,client}_method which allows negotiation is SSLv23, which exists even
+when OpenSSL is built without SSLv2 support.
+By disabling with openssl_options, we can let admins re-enable with the
+existing knob. */
ctx = SSL_CTX_new((host == NULL)?
SSLv23_server_method() : SSLv23_client_method());
@@ -1522,6 +1528,9 @@ BOOL adding, item_parsed;
result = 0L;
/* Prior to 4.78 we or'd in SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS; removed
* from default because it increases BEAST susceptibility. */
+#ifdef SSL_OP_NO_SSLv2
+result |= SSL_OP_NO_SSLv2;
+#endif
if (option_spec == NULL)
{