September 30, 2016

A few weeks ago I listened to Hanno Böck talk about TLS version intolerance at the Berlin AppSec & Crypto Meetup. He explained how with TLS 1.3 just around the corner there again are growing concerns about faulty TLS stacks found in HTTP servers, load balancers, routers, firewalls, and similar software and devices.

I decided to dig a little deeper and will use this post to explain version intolerance, how version fallbacks work and why they’re insecure, as well as describe the downgrade protection mechanisms available in TLS 1.2 and 1.3. It will end with a look at version negotiation in TLS 1.3 and a proposal that aims to prevent similar problems in the future.

What is version intolerance?

Every time a new TLS version is specified, browsers usually are the fastest to implement and update their deployments. Most major browser vendors have a few people involved in the standardization process to guide the standard and give early feedback about implementation issues.

As soon as the spec is finished, and often far before that feat is done, clients will have been equipped with support for the new TLS protocol version and happily announce this to any server they connect to:

Client: Hi! The highest TLS version I support is 1.2.

Server: Hi! I too support TLS 1.2 so let’s use that to communicate.

[TLS 1.2 connection will be established.]

In this case the highest TLS version supported by the client is 1.2, and so the server picks it because it supports that as well. Let’s see what happens if the client supports 1.2 but the server does not:

Client: Hi! The highest TLS version I support is 1.2.

Server: Hi! I only support TLS 1.1 so let’s use that to communicate.

[TLS 1.1 connection will be established.]

This too is how it should work if a client tries to connect with a protocol version unknown to the server. Should the client insist on any specific version and not agree with the one picked by the server it will have to terminate the connection.

Unfortunately, there are a few servers and more devices out there that implement TLS version negotiation incorrectly. The conversation might go like this:

Client: Hi! The highest TLS version I support is 1.2.

Server: ALERT! I don’t know that version. Handshake failure.

[Connection will be terminated.]

Or:

Client: Hi! The highest TLS version I support is 1.2.

Server: TCP FIN! I don’t know that version.

[Connection will be terminated.]

Or even worse:

Client: Hi! The highest TLS version I support is 1.2.

Server: (I don’t know this version so let’s just not respond.)

[Connection will hang.]

The same can happen with the infamous F5 load balancer that can’t handle ClientHello messages with a length between 256 and 512 bytes. Other devices abort the connection when receiving a large ClientHello split into multiple TLS records. TLS 1.3 might actually cause more problems of this kind due to more extensions and client key shares.

What are version fallbacks?

As browsers usually want to ship new TLS versions as soon as possible, more than a decade ago vendors saw a need to prevent connection failures due to version intolerance. The easy solution was to decrease the advertised version number by one with every failed attempt:

Client: Hi! The highest TLS version I support is 1.2.

Server: ALERT! Handshake failure. (Or FIN. Or hang.)

[TLS version fallback to 1.1.]

Client: Hi! The highest TLS version I support is 1.1.

Server: Hi! I support TLS 1.1 so let’s use that to communicate.

[TLS 1.1 connection will be established.]

A client supporting everything from TLS 1.0 to TLS 1.2 would start trying to establish a 1.2 connection, then a 1.1 connection, and if even that failed a 1.0 connection.

Why are these insecure?

What makes these fallbacks insecure is that the connection can be downgraded by a MITM, by sending alerts or TCP packets to the client, or blocking packets from the server. To the client this is indistinguishable from a network error.

The POODLE attack is one example where an attacker abuses the version fallback to force an SSL 3.0 connection. In response to this browser vendors disabled version fallbacks to SSL 3.0, and then SSL 3.0 entirely, to prevent even up-to-date clients from being exploited. Insecure version fallback in browsers pretty much break the actual version negotiation mechanisms.

Version fallbacks have been disabled since Firefox 37 and Chrome 50. Browser telemetry data showed it was no longer necessary as after years, TLS 1.2 and correct version negotiation was deployed widely enough.

The TLS_FALLBACK_SCSV cipher suite

You might wonder if there’s a secure way to do version fallbacks, and other people did so too. Adam Langley and Bodo Möller proposed a special cipher suite in RFC 7507 that would help a client detect whether the downgrade was initiated by a MITM.

Whenever the client includes TLS_FALLBACK_SCSV {0x56, 0x00} in the list of cipher suites it signals to the server that this is a repeated connection attempt, but this time with a version lower than the highest it supports, because previous attempts failed. If the server supports a higher version than advertised by the client, it MUST abort the connection.

The drawback here however is that a client even if it implements fallback with a Signaling Cipher Suite Value doesn’t know the highest protocol version supported by the server, and whether it implements a TLS_FALLBACK_SCSV check. Common web servers will likely be updated faster than others, but router or load balancer manufacturers might not deem it important enough to implement and ship updates for.

Signatures in TLS 1.2

It’s been long known to be problematic that signatures in TLS 1.2 don’t cover the list of cipher suites and other messages sent before server authentication. They sign the ephemeral DH parameters sent by the server and include the *Hello.random values as nonces to prevent replay attacks:

h = Hash(ClientHello.random + ServerHello.random + ServerParams)

Signing at least the list of cipher suites would have helped prevent downgrade attacks like FREAK and Logjam. TLS 1.3 will sign all messages before server authentication, even though it makes Transcript Collision Attacks somewhat easier to mount. With SHA-1 not allowed for signatures that will hopefully not become a problem anytime soon.

Downgrade Sentinels in TLS 1.3

With neither the client version nor its cipher suites (for the SCSV) included in the hash signed by the server’s certificate in TLS 1.2, how do you secure TLS 1.3 against downgrades like FREAK and Logjam? Stuff a special value into ServerHello.random .

The TLS WG decided to put static values (sometimes called downgrade sentinels) into the server’s nonce sent with the ServerHello message. TLS 1.3 servers responding to a ClientHello indicating a maximum supported version of TLS 1.2 MUST set the last eight bytes of the nonce to:

0x44 0x4F 0x57 0x4E 0x47 0x52 0x44 0x01

If the client advertises a maximum supported version of TLS 1.1 or below the server SHOULD set the last eight bytes of the nonce to:

0x44 0x4F 0x57 0x4E 0x47 0x52 0x44 0x00

If not connecting with a downgraded version, a client MUST check whether the server nonce ends with any of the two sentinels and in such a case abort the connection. The TLS 1.3 spec here introduces an update to TLS 1.2 that requires servers and clients to update their implementation.

Unfortunately, this downgrade protection relies on a ServerKeyExchange message being sent and is thus of limited value. Static RSA key exchanges are still valid in TLS 1.2, and unless the server admin disables all non-forward-secure cipher suites the protection can be bypassed.

The comeback of insecure fallbacks?

Current measurements show that enabling TLS 1.3 by default would break a significant fraction of TLS handshakes due to version intolerance. According to Ivan Ristić, as of July 2016, 3.2% of servers from the SSL Pulse data set reject TLS 1.3 handshakes.

This a very high number and would affect way too many people. Alas, with TLS 1.3 we have only limited downgrade protection for forward-secure cipher suites. And that is assuming that most servers either support TLS 1.3 or update their 1.2 implementations. TLS_FALLBACK_SCSV , if supported by the server, will help as long as there are no attacks tampering with the list of cipher suites.

The TLS working group has been thinking about how to handle intolerance without bringing back version fallbacks, and there might be light at the end of the tunnel.

Version negotiation with extensions

The next version of the proposed TLS 1.3 spec, draft 16, will introduce a new version negotiation mechanism based on extensions. The current ClientHello.version field will be frozen to TLS 1.2, i.e. {3, 3} , and renamed to legacy_version . Any number greater than that MUST be ignored by servers.

To negotiate a TLS 1.3 connection the protocol now requires the client to send a supported_versions extension. This is a list of versions the client supports, in preference order, with the most preferred version first. Clients MUST send this extension as servers are required to negotiate TLS 1.2 if it’s not present. Any version number unknown to the server MUST be ignored.

This still leaves potential problems with big ClientHello messages or choking on unknown extensions unaddressed, but according to David Benjamin the main problem is ClientHello.version . We will hopefully be able to ship browsers that have TLS 1.3 enabled by default, without bringing back insecure version fallbacks.

However, it’s not unlikely that implementers will screw up even the new version negotiation mechanism and we’ll have similar problems in a few years down the road.

GREASE-ing the future

David Benjamin, following Adam Langley’s advice to have one joint and keep it well oiled, proposed GREASE (Generate Random Extensions And Sustain Extensibility), a mechanism to prevent extensibility failures in the TLS ecosystem.

The heart of the mechanism is to have clients inject “unknown values” into places where capabilities are advertised by the client, and the best match selected by the server. Servers MUST ignore unknown values to allow introducing new capabilities to the ecosystem without breaking interoperability.