PostgreSQL - Weak DH group

Hello everyone,

I sent few days ago to the security DL a mail reporting a vulnerability in
how Postgres is requesting DH params to be used later for encryption
algorithms. So, due to there is no problem sharing with this group, here is
what I sent:

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Hi folks,

We are working with Postgres 9.3.14 and executing nmap we
found that it is using “weak DH group” (nmap –script ssl-dh-params). Weak =
1024 bits.

See nmap output (1)

We don’t know if other versions are affected or not. The
environment used is a RHEL 6 x86_6, OpenSSL version 1.0.2i with FIPS module.

This issue is similar to what this post explains about using weak DH
parameters: http://www.usefuljs.net/2016/09/29/imperfect-forward-secrecy/

Following with the code, it seems that PostgreSQL has
missed the keyLength OpenSSL parameter, and it delivers into a weak crypto
configuration.. Affected Code:

https://git.postgresql.org/gitweb/?p=postgresql.git;a=
blob;f=src/backend/libpq/be-secure-openssl.c;h=
8d8f12952a4a4f14a15f8647b96935e13d68fb39;hb=48d50840d53eb62842c0d9b54eab9c
d7c9a3a46d

(Thanks to Damian in order to found the affected code)

(1) nmap output:

# nmap –script ssl-dh-params -p 5432 <ip>

Starting Nmap 7.25BETA2 ( https://nmap.org )

Nmap scan report for <ip>

Host is up (0.00035s latency).

PORT STATE SERVICE

5432/tcp open postgresql

| ssl-dh-params:

| VULNERABLE:

| Diffie-Hellman Key Exchange Insufficient Group Strength

| State: VULNERABLE

| Transport Layer Security (TLS) services that use Diffie-Hellman
groups

| of insufficient strength, especially those using one of a few
commonly

| shared groups, may be susceptible to passive eavesdropping attacks.

| Check results:

| WEAK DH GROUP 1

| Cipher Suite: TLS_DHE_RSA_WITH_AES_128_GCM_SHA256

| Modulus Type: Safe prime

| Modulus Source: Unknown/Custom-generated

| Modulus Length: 1024

| Generator Length: 8

| Public Key Length: 1024

| References:

|_ https://weakdh.org

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Thanks in advance

Nicolas Guini