CIS CYBER ALERT

DATE ISSUED:
05/20/2015

SUBJECT:
Logjam Attack: Vulnerability in TLS

CIS recently became aware of a vulnerability in certain implementations of HTTPS using TLS, which could allow for the disclosure of sensitive information. This vulnerability is caused by a basic design flaw in the way that TLS handles Diffie-Hellman key exchanges and allows an attacker to intercept the HTTPS connection from vulnerable clients or servers by downgrading the RSA key to a weaker, export-grade, 512-bit RSA key. With the implementation of the weak key, an attacker can attempt to break the cipher in order to perform a man-in-the-middle (MITM) attack. This attack vector has been demonstrated against both web and mail servers utilizing vulnerable versions of HTTPS. The associated attack has been nicknamed Logjam, and is similar to the recent FREAK vulnerability reported in March.

To date, Microsoft's Internet Explorer is the only browser that has been patched against Logjam. Other major browsers including Chrome, Firefox, and Safari are expected to be updated soon.

A successful attack may lead to the disclosure of sensitive information and cookie-based authentication credentials which may lead to other attacks.

Recommendations:
We recommend the following actions:

  • Apply appropriate patches/updates to vulnerable systems immediately after appropriate testing, when they become available.
  • For servers, immediately disable support for the RSA_Export cipher suite.
  • Remind users not to visit un-trusted websites or follow links provided by unknown or un-trusted sources.
  • Inform and educate users regarding the threats posed by hypertext links contained in emails or attachments, especially from un-trusted sources.


The researchers who discovered this vulnerability have posted a guide with mitigation advice for many popular servers which may be found at https://weakdh.org/sysadmin.html

REFERENCES:
Ars Technica:
http://arstechnica.com/security/2015/05/https-crippling-attack-threatens-tens-of-thousands-of-web-and-mail-servers/

Weakdh:
https://weakdh.org/logjam.html
https://weakdh.org/sysadmin.html

ZDNet:
http://www.zdnet.com/article/logjam-security-flaw-leaves-tens-of-thousands-of-https-websites-vulnerable/