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TLS_FALLBACK_SCSV is a TLS Signaling Cipher Suite Value (SCSV) that can be used to guard against protocol downgrade attacks. The extension can be useful for clients like web browsers, which fall back to a lesser protocol version if attempts to use a higher protocol version fail.

In the attack, the adversary would force a negotiation failure of a higher protocol like TLS 1.2 or 1.1 in hopes the client will retry with TLS 1.0 or SSLv3. Then, the adversary will use defects in the down level protocols to carry out other attacks. Table 1 below lists some of the losses that could occur when traversing down protocol versions.

Protocols, Downgrades and Features
Desired Protocol Downgraded Protocol Loss on Downgrade
TLS 1.2 TLS 1.1 AEAD cipher suites (CCM, GCM, etc)
TLS 1.1 TLS 1.0 Perfect Forward Secrecy (PFS)
TLS 1.0 SSL v3 Too many to list[1]

[1] See Differences Between SSLv2, SSLv3, and TLS and This POODLE Bites: Exploiting The SSL 3.0 Fallback.

The SSL_MODE_SEND_FALLBACK_SCSV extension can be used to remediate the POODLE bug by ensuring clients don't fall back to SSLv3 if the client performs fallbacks. However, the extension does not fix the underlying padding oracle. Rather, it just avoids the defective protocol version.

SCSV means TLS_FALLBACK_SCSV shows up as a cipher suite in the ClientHello. The spurious cipher is added to the cipher suites if the SSL_MODE_SEND_FALLBACK_SCSV option is present.

Note Well: if the client does not perform fallbacks, then the TLS_FALLBACK_SCSV extension is not needed.

Reason for the Extension[edit]

Browsers and other similar software attempt to support every server created on {God|Allah|Brahman|...}'s green earth. If the browser attempts to connect to 1990s era server and the connection fails, then the browser will fallback to a lesser protocol. It falls back to a lesser protocol by initiating a new connection with the down level protocol.

Its curious why the browsers chose to compromise the security of the dominant/standard use cases for a trivial minority. The W3C clearly states two design principals: Secure By Design and Priority of Constituencies. When a browser falls back to insecure protocols when the user asks for a secure scheme, they violate both principals.

The browsers could have provided modern TLS support with 15 or 20 reasonable ciphers (or let the users choose their vanity ciphers); and then require a plug-in to achieve the existing insecure behavior. Browsers seem more than happy to require plug-ins for the most basic functionality, like opening a homepage (try to get Chrome, Firefox or Opera to open your homepage on a new tab).

Even more befuddling, the SSL_MODE_SEND_FALLBACK_SCSV is not needed now that SSLv3 is insecure due to the padding oracle (as if Loern Weith was not clear that SSLv3 was insecure in 2006). In fact, the down level software that is part of the problem likely won't be updated to understand the TLS extension anyway.

Avoiding the Extension[edit]

As stated earlier, SSL_MODE_SEND_FALLBACK_SCSV is not needed if the client does not perform fallbacks. The easiest way to avoid use of the SSL_MODE_SEND_FALLBACK_SCSV is to always specify the protocols you are willing to accept. The detail is you always send the highest protocol version with the ClientHello.

For example, suppose you want to accept TLS 1.0 through TLS 1.2. In this case, you send a ClientHello with that information, and you don't include SSL_MODE_SEND_FALLBACK_SCSV. Below is a portion of the code to do so (its likely similar to the code you have been using for years):

const SSL_METHOD* method = SSLv23_method();
if(method == NULL)) handleFailure();

SSL_CTX* ctx = SSL_CTX_new(method);
if(ctx == NULL) handleFailure();

const long flags = SSL_OP_NO_SSLv2 | SSL_OP_NO_SSLv3 | SSL_OP_NO_COMPRESSION;
SSL_CTX_set_options(ctx, flags);

In the code above, the SSLv23_method enables all protocol version - from SSLv2 through TLS 1.2. The method is then used to create a context object. After the context object is created, weak/wounded/broken protocols and options are removed by setting SSL_OP_NO_SSLv2, SSL_OP_NO_SSLv3 and SSL_OP_NO_COMPRESSION.

The ClientHello will set a minimum protocol version of TLS 1.0 and a maximum protocol version of TLS 1.2 in the ClientHello. The TLS protocol ensures TLS 1.2 is used if available; and TLS 1.1 is used is TLS 1.2 is not available; and TLS 1.0 is used if TLS 1.2 and 1.1 are not available. During key exchange, the ClientHello is MAC'd and used (in part) to derive the premaster_secret, so tampering with protocol versions will be detected.

When TLS 1.3 is added to OpenSSL, it will include TLS 1.3. You won't have to change your code.

Using the Extension[edit]

If you need to use the extension, then the following is an example of how to use it:

const SSL_METHOD* method = TLSv1_2_method();
if(method == NULL)) handleFailure();

SSL_CTX* ctx = SSL_CTX_new(method);
if(ctx == NULL) handleFailure();


Some folks on the OpenSSL mailing list recommend using TLS_FALLBACK_SCSV and SSL_MODE_SEND_FALLBACK_SCSV whenever available. See, for example, Use of TLS_FALLBACK_SCSV on the OpenSSL mailing list.

Related Discussions[edit]

Below are some discussions that occurred on the OpenSSL and IETF mailing lists and around the web.