Re: [exim] safe handling of $tls_sni

On 2016-10-12 at 14:50 +0200, Arkadiusz Miśkiewicz wrote:
> Docs say that $tls_sni has raw data from client:
>
> "Great care should be taken to deal with matters of case, various injection
> attacks in the string (../ or SQL), and ensuring that a valid filename can
> always be referenced; it is important to remember that $tls_sni is arbitrary
> unverified data provided prior to authentication."

Someone read the text I wrote! Woohoo!

(It only took a few years ...)

> What is safest approach to handle $tls_sni when trying
> to expand it to file on filesystem?

Use a cryptographic hash for the filename. Or base64-encode it.
Use symlinks for human-convenience names and any aliases.

Your trade-offs are:
* a cryptographically-skilled attacker might find a collision and ...
get you to issue, to _them_ (and only them) a certificate for a known
system, while on their side they should be looking to validate against
something else. Woo, they just attacked themselves: on your side, you
don't need to care.
* A very long SNI with base64 might look up a very long filename on
disk. Shouldn't be an issue, unless you're mass-hosting on an OS
which only maintains dir hashing for filenames up to a certain length
and need to accept customer-controlled SNI names.
Of course, the systems like that, if memory serves, broke at 32
characters long and a SHA1 hex digest is 40 characters long, so you'd
also want to use ${substr...} to take the first N characters.
* If you have a lot of similar names, sha1 will give you more
readily-distinct values which you can tell apart at a glance.

> ${sha1:${lc:mx.spodhuis.org}}
F0DF49E8B2ACF84D5D290E89F9B673EF44B60E74
> ${str2b64:${lc:mx.spodhuis.org}}
bXguc3BvZGh1aXMub3Jn

So, eg, `/etc/mail/ssl/bXguc3BvZGh1aXMub3Jn.pem` should exist for this
approach, to issue a cert for the name `mx.spodhuis.org`.

> Rule like:
> ${if exists{/etc/mail/ssl/${tls_sni}.pem}{/etc/mail/ssl/${tls_sni}.pem}{/etc/mail/default-cert.pem}

${if exists{/etc/mail/ssl/${str2b64:${lc:tls_sni}}.pem}{/etc/mail/ssl/${str2b64:${lc:tls_sni}}.pem}{/etc/mail/default-cert.pem}
OR
${if exists{/etc/mail/ssl/${sha1:${lc:tls_sni}}.pem}{/etc/mail/ssl/${sha1:${lc:tls_sni}}.pem}{/etc/mail/default-cert.pem}

-Phil