Security Bulletin: Logjam vulnerability in TLS affects IBM CICS Transaction Gateway (CVE-2015-4000)

Summary

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”.

Vulnerability Details

CVE ID: CVE-2015-4000

Description : The TLS protocol 1.2 and earlier, when a DHE_EXPORT ciphersuite is enabled on a server but not on a client, does not properly convey a DHE_EXPORT choice, which allows man-in-the-middle attackers to conduct cipher-downgrade attacks by rewriting a ClientHello with DHE replaced by DHE_EXPORT and then rewriting a ServerHello with DHE_EXPORT replaced by DHE, aka the “Logjam” issue.

CVSS Base Score: 4.3
CVSS Temporal Score: See <https://exchange.xforce.ibmcloud.com/vulnerabilities/103294&gt;
CVSS Environmental Score*: Undefined
CVSS Vector: (AV:N/AC:M/Au:N/C:P/I:N/A:N)

Affected Products and Versions

CICS Transaction Gateway for Multiplatforms v7.2, v8.0, v8.1, v9.0 and v9.1. Inclusion in this list does not imply that all the products are supported. See the IBM Support Lifecycle page for product end of support dates

Workarounds and Mitigations

The use of DH or DHE Ciphers by CICS Transaction Gateway can be prevented by configuring CICS TG to only accept more secure cipher suites, This can be done by listing the acceptable cipher suites using the ciphersuites parameter in the ctg.ini file, or by adding the acceptable cipher suites to the “Use only these ciphers” suites in the “SSL settings” section of the CICS TG configuration tool.
See the CICS TG for Multiplatforms Knowledge Center for more details.