Postfix: DH-1024 weak key exchange is enabled.

Solution Verified - Updated

Environment

  • Red Hat Enterprise Linux 7
  • Red Hat Enterprise Linux 8
  • Red Hat Enterprise Linux 9
  • Postfix

Issue

  • Key exchange (dh 1024) of lower strength than certificate key.
  • Weak SSL/TLS Key Exchange vulnerability on 25 Port.
  • How to generate DH Parameter file with custom key size ?

Resolution

In postfix configuration, smtpd_tls_dh1024_param_file option defines the dh parameter file.

  • Generate dh parameter with higher bits and mention it in postfix configuration in order to fix this issue.

  • To generate dh parameter file of certain size use below command:

# openssl dhparam -out /etc/postfix/<name-of-file-to-generate> <size of key>

Example:

# openssl dhparam -out /etc/postfix/dh2048.pem 2048

  • Above command will generate dh parameter file with key size 2048.

  • Once the command is completed add below line to postfix configuration file /etc/postfix/main.cf:

smtpd_tls_dh1024_param_file = /etc/postfix/dh2048.pem

Note: Even if dh parameter with higher key size is generated, here option name will remain i.e. smtpd_tls_dh1024_param_file.

  • Restart the postfix 
# systemctl restart postfix

Root Cause

  • Key exchange (dh 1024) had lower strength than recommended size. After increasing key size vulnerability should get fixed.

Diagnostic Steps

  • Run below nmap command to discover open ports and enabled ciphers. Output will be something like this
# nmap --script ssl-enum-* <ip-address>
Starting Nmap 7.92 ( https://nmap.org ) at 2022-05-20 10:10 UTC
Nmap scan report for <ip>
Host is up (0.11s latency).
Not shown: 998 closed tcp ports (conn-refused)
PORT   STATE SERVICE
22/tcp open  ssh
25/tcp open  smtp
| ssl-enum-ciphers: 
|   TLSv1.2: 
|     ciphers: 
|       TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (secp256r1) - A
|       TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384 (secp256r1) - A
|       TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA (secp256r1) - A
|       TLS_DHE_RSA_WITH_AES_256_GCM_SHA384 (dh 1024) - A
|       TLS_DHE_RSA_WITH_AES_256_CBC_SHA256 (dh 1024) - A
|       TLS_DHE_RSA_WITH_AES_256_CBC_SHA (dh 1024) - A
|       TLS_DHE_RSA_WITH_CAMELLIA_256_CBC_SHA (dh 1024) - A
|       TLS_RSA_WITH_AES_256_GCM_SHA384 (rsa 2048) - A
|       TLS_RSA_WITH_AES_256_CBC_SHA256 (rsa 2048) - A
|       TLS_RSA_WITH_AES_256_CBC_SHA (rsa 2048) - A
|       TLS_RSA_WITH_CAMELLIA_256_CBC_SHA (rsa 2048) - A
|       TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 (secp256r1) - A
|       TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256 (secp256r1) - A
|       TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA (secp256r1) - A
|       TLS_DHE_RSA_WITH_AES_128_GCM_SHA256 (dh 1024) - A
|       TLS_DHE_RSA_WITH_AES_128_CBC_SHA256 (dh 1024) - A
|       TLS_DHE_RSA_WITH_AES_128_CBC_SHA (dh 1024) - A
|       TLS_DHE_RSA_WITH_CAMELLIA_128_CBC_SHA (dh 1024) - A
|       TLS_RSA_WITH_AES_128_GCM_SHA256 (rsa 2048) - A
|       TLS_RSA_WITH_AES_128_CBC_SHA256 (rsa 2048) - A
|       TLS_RSA_WITH_AES_128_CBC_SHA (rsa 2048) - A
|       TLS_RSA_WITH_CAMELLIA_128_CBC_SHA (rsa 2048) - A
|     compressors: 
|       NULL
|     cipher preference: server
|     warnings: 
|       Key exchange (dh 1024) of lower strength than certificate key
|_  least strength: A
  • Second last line shows weak dh parameter for postfix service (port 25). Also many of cipher suites are using it as per the above output: TLS_DHE_RSA_WITH_CAMELLIA_128_CBC_SHA (dh 1024) - A
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