Security Bulletin: Vulnerability in Diffie-Hellman ciphers affects IBM GPFS for Windows (CVE-2015-4000)

Summary

The Logjam Attack on TLS connections using the Diffie-Hellman (DH) key exchange protocol affects IBM GPFS for Windows V3.5

Vulnerability Details

CVEID: CVE-2015-4000**
DESCRIPTION:** The TLS protocol could allow a remote attacker to obtain sensitive information, caused by the failure to properly convey a DHE_EXPORT ciphersuite choice. An attacker could exploit this vulnerability using man-in-the-middle techniques to force a downgrade to 512-bit export-grade cipher. Successful exploitation could allow an attacker to recover the session key as well as modify the contents of the traffic. This vulnerability is commonly referred to as “Logjam”.
CVSS Base Score: 4.3
CVSS Temporal Score: See https://exchange.xforce.ibmcloud.com/vulnerabilities/103294 for the current score
CVSS Environmental Score*: Undefined
CVSS Vector: (AV:N/AC:M/Au:N/C:P/I:N/A:N)

Affected Products and Versions

OpenSSH for GPFS V3.5 for Windows

Remediation/Fixes

None

Workarounds and Mitigations

On the Windows node running GPFS V3.5, login with Administrator privileges and perform the following steps from an elevated SUA Korn Shell (ksh):

1. Issue the commands:

awk ‘{ if ($5 <= 2000) printf “#”; print }’ /usr/lpp/mmfs/platform/etc/moduli > /tmp/large_moduli
cp /tmp/large_moduli /usr/lpp/mmfs/platform/etc/moduli
cp /tmp/large_moduli /usr/local/etc/moduli

2. Edit the file /usr/local/etc/sshd_config and append the following line at the end. Make sure that all algorithm names are comma separated in a single line, without any intervening spaces or tabs.

KexAlgorithms
[email protected],ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group14-sha1,diffie-hellman-gr
oup-exchange-sha1,diffie-hellman-group-exchange-sha256

3. Edit the file /usr/local/etc/ssh_config and append the following line at the end. Make sure that all algorithm names are comma separated in a single line, without any intervening spaces or tabs.

KexAlgorithms
[email protected],ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group14-sha1

4. To generate any missing keys, issue:

/usr/lpp/mmfs/platform/bin/ssh-keygen -A

5. To verify the configuration files, issue:

/usr/lpp/mmfs/platform/sbin/sshd -T

Make sure that there are no error messages and that kexalgorithms matches as specified in (2).

6. Restart the sshd daemon or reboot the node so that the modified configuration takes effect.