I'd like that - and if we were at it, I'd want a safe $sender_host_address
so we can use RHS expansion without modifying the Makefile :)
On Mon, Oct 17, 2016 at 2:53 PM, Mike Tubby <m...@tubby.org> wrote:
>
> 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:${lc:tls_sni}}.pem}{/etc/mail/ss
>> l/${sha1:${lc:tls_sni}}.pem}{/etc/mail/default-cert.pem}
>>
>> -Phil
>>
>>
>
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--
Brent Jones
br...@brentrjones.com
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