On 11.02.2019 2:36, Andreas Karlsson wrote:
I will preface this with that I am not a security guy and that also do
not know how the Zstd vompression works, so take any of what I say
with a grain of salt.
On 2/8/19 8:14 AM, Andres Freund wrote:> I think compression is pretty
useful, and I'm not convinced that the
threat model underlying the attacks on SSL really apply to postgres.
I think only because it is usually harder to intercept traffic between
the application server and the database than between the we bbrowser
and the web server.
Imagine the following query which uses the session ID from the cookie
to check if the logged in user has access to a file.
SELECT may_download_file(session_id => $1, path => $2);
When the query with its parameters is compressed the compressed size
will depend on the similarity between the session ID and the requested
path (assuming Zstd works similar to DEFLATE), so by tricking the web
browser into making requests with specifically crafted paths while
monitoring the traffic between the web server and the database the
compressed request size can be use to hone in the session ID and steal
people's login sessions, just like the CRIME attack[1].
So while compression is a very useful feature I am worried that it
also opens application developers to a new set of security
vulnerabilities which they previously were protected from when
compression was removed from SSL.
1. https://en.wikipedia.org/wiki/CRIME
Andreas
Andreas? thank you for clarification. Such kind of attack is really
possible.
But as far as I understand such attack requires injection between
server and database (to be able to analyze traffic between them).
Also such attack is possible only if session_id can be somehow
"guessed". If it is just big random number, then it is very unlikely
that it can be hacked in in this way.
But once again - I am not expert in cryptography.
And this patch is not addressing SSL vulnerabilities when using
compression - I agree, that compression at libpq level is not safer than
SSL level compression.
The goal was to support compression without using SSL. It seems to me
that there are many cases when security is not requires, but reducing
network traffic is desired.
The best example is replication between node in local network.
--
Konstantin Knizhnik
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company
