[squid-users] Resolved: Peek-and-splice not working when mixing TLS1.3 servers and TLS1.2 clients

John Sweet-Escott eype69 at gmail.com
Wed Dec 11 18:15:37 UTC 2019

Hi Nikolaus 

This sounds exactly like the symptoms we have encountered. Will build from your patch & test to see if it works in our situation. 


> On 7 Dec 2019, at 13:54, Nikolaus <dc.sqml at ntcomputer.de> wrote:
>  I was able to solve the issue, fixing both squid-side "error:1425F175:SSL routines:ssl_choose_client_version:inappropriate fallback (1/-1/0)" and client-side certificate verification errors when attempting to contact TLS 1.3 server over a TLS 1.3-enabled squid from a TLS 1.2 client. I will first explain what causes the issue before presenting my solution, which involves changes of the squid code base, for anybody who is affected by the same problem.
> I have inspected the squid source code and noticed that TLS peeking works roughly like this:
> 1. The client sends a client_hello, which is parsed by squid using a custom handshake parser.  <-- Uses TLS 1.2
> 2. Squid creates an OpenSSL TLS session for the peeked connection  <-- Uses TLS 1.3!
> 3. Squid forwards the original client_hello handshake message to the server <-- TLS 1.2
> 4. Squid passes the received server_hello response to the OpenSSL session created previously  <-- Response uses TLS 1.2 - Problem!
> Now, the "problem" is that TLS 1.3 defines a set of new protocol downgrade attack prevention mechanisms (can be found e.g. here: https://blog.gypsyengineer.com/en/security/how-does-tls-1-3-protect-against-downgrade-attacks.html). Both OpenSSL and most likely the server implement these. This includes that the server random in the TLS 1.2 server_hello contains an indicator that the server is TL 1.3-capable. The OpenSSL session created by squid detects this, notices that it is TLS 1.3-capable itself, and closes the connection because it assumes a protocol downgrade attack! Little does it know, that our client actually only supports TLS 1.2, so we *want* the lower protocol version.
> My solution includes setting the maximum TLS version of the OpenSSL session to the version received from the client. This proved a little bit difficult, since the way TLS versions are negotiated has also been changed by the TLS 1.3 specification, and the squid handshake parser was not yet able to detect TLS 1.3 correctly - I have therefore also implemented parsing of the SupportedVersions TLS Extension and a preliminary support for sparse version ranges. You can find all these changes at https://github.com/nthuemmel/squid/tree/tls_downgrade_compatibility , which is a fork of squid 4.9. Feel free to compile & test it if you have a transparent peek-and-splice setup and are affected by the "inappropriate fallback" problem.
> I would of course be glad if the fix could be merged into the main squid repository. If you are a dev, please let me know what you think and if I should open a pull request. There are still some TODOs left, because I wasn't sure what the best way is to integrate some of the changes. Notably, there was also a comment which discourages setting a maximum version for the OpenSSL session to improve peek+bump compatibility - I don't have a setup to which this applies, so I don't know how big of an impact this is or if it is still relevant.
> Best Regards
> Nikolaus
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