Couldn't we have - per perhaps shouldn't we have - a "safe domain name" function in Exim that could be used for this and elsewhere where an untrusted domain name enters - it would:


    * remove white space (tab, space, etc)
    * remove non-printing chars
    * remove 'quoting' and 'escaping'
    * make it lower case
    * only allow valid characters for a FQDN

call it something like "safe_fqdn" and then you could do:

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

aren't computers are supposed to be doing the work for us...?


Mike



On 10/17/2016 10:09 PM, Phil Pennock wrote:
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



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