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  1. #1
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    TLS downgrade attack

    The Logjam (and Another) Vulnerability against Diffie-Hellman Key Exchange

    Schneier on Security [blog]
    May 21, 2015

    ".... The Logjam attack allows a man-in-the-middle attacker to downgrade vulnerable TLS connections to 512-bit export-grade cryptography. This allows the attacker to read and modify any data passed over the connection. ...

    The attack affects any server that supports DHE_EXPORT ciphers, and affects all modern web browsers....

    Much more interesting is the other vulnerability that the researchers found: Millions of HTTPS, SSH, and VPN servers all use the same prime numbers for Diffie-Hellman key exchange. ... After this first step, an attacker can quickly break individual connections.... estimate that an academic team can break a 768-bit prime and that a nation-state can break a 1024-bit prime.... A second prime would allow passive decryption of connections to 66% of VPN servers and 26% of SSH servers. A close reading of published NSA leaks shows that the agency's attacks on VPNs are consistent with having achieved such a break. ...

    Again, the NSA has put surveillance ahead of security. It never bothered to tell us that many of the "secure" encryption systems we were using were not secure. And we don't know what other national intelligence agencies independently discovered and used this attack.

    The good news is now that we know reusing prime numbers is a bad idea, we can stop doing it. ..."

    https://www.schneier.com/blog/archiv...gjam_and_.html

  2. #2
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    Latest versions of Chrome, Firefox, Safari and Android browsers are vulnerable, but Internet Explorer* is not: Security Attack Called Logjam makes Browsers Vulnerable

    (* with Windows Update KB3061518 dated 05/12/2015 installed)
    Last edited by BruceR; 2015-05-24 at 16:21. Reason: added update link

  3. #3
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    I am feeling irony here.

  4. #4
    Super Moderator satrow's Avatar
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    Not sure about Chrome but Firefox and Pale Moon can both be tweaked to block this vulnerability, see the comments on the URL in #2.

  5. #5
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    Quote Originally Posted by BruceR View Post
    Latest versions of Chrome, Firefox, Safari and Android browsers are vulnerable, but latest Internet Explorer is not: Security Attack Called Logjam makes Browsers Vulnerable
    Thanks Bruce. A comment on that link you provided led here. Do this for Mozilla engines and it should cease being vulnerable. PaleMoon users can do it on their own from their menu.
    http://forums.mozillazine.org/viewto...5963#p14165963

    Addendum link for those running servers:
    http://mohammadoweis.hubpages.com/hu...-LOGJAM-attack
    Last edited by Fascist Nation; 2015-05-24 at 14:55.

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  7. #6
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    Good article from Matthew Green at Johns Hopkins on Logjam and Diffie-Hellman...

    http://blog.cryptographyengineering....ek-logjam.html

    He was part of the team that completed the recent audit of TrueCrypt.

  8. #7
    Super Moderator satrow's Avatar
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    It might be better to get the Pale Moon details from the dev: https://forum.palemoon.org/viewtopic.php?f=1&t=8311

  9. #8
    jwoods
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    This site has some interesting figures on browser market share...

    https://www.netmarketshare.com/brows...=2&qpcustomd=0

    Most of the attention on fixes for the client side has been on the large market share browsers.

    I wonder how many in the "Proprietary or Undetectable" category will have a fix forthcoming.

  10. #9
    Super Moderator satrow's Avatar
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    Little point in wondering, if people don't upgrade their browsers to a version published specifically patched, or able to be patched, since this problem came to light, they'll remain vulnerable.

    Don't forget, this is actually about outdated servers that are allowing weak and authentication. If all servers were patched, clients wouldn't be vulnerable.

  11. #10
    jwoods
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    According to openssl.org, there are still over a half million servers that are still vulnerable to FREAK, so the pace of fixes on the server side can be glacial.

  12. #11
    Super Moderator satrow's Avatar
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    If you checkout some of the recent topics about users being unable to connect to servers because of authentication, you'll find that in some cases action was pretty swift, just a few days.

    Victim blaming or placing the burden on the victims isn't something I condone; in every instance, users bumping into these issues, which aren't difficult to check for, should always fire off emails to the server host and company owning the 'secure' site.

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  14. #12
    jwoods
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    Unfortunately, it is often left to the victims to clean up the mess on the client side to stay secure.

    A topic in itself.

  15. #13
    2 Star Lounger 1PW's Avatar
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    Derived from other posts/sites:

    Test your browser: https://weakdh.org/

    Test a server: https://community.qualys.com/message/29345#29345

    Firefox based browser Add-on till Firefox 39.0 is released: https://addons.mozilla.org/en-US/firefox/addon/disable-dhe/

    HTH

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    Trev (2015-05-25)

  17. #14
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    Firefox doesn't need an add-on, you can do it yourself in about 2 minutes - first link in post #5.

    cheers, Paul

  18. #15
    2 Star Lounger 1PW's Avatar
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    Quote Originally Posted by Paul T View Post
    Firefox doesn't need an add-on, you can do it yourself in about 2 minutes - first link in post #5.

    cheers, Paul
    I quite agree Paul, and that procedure is well suited for the digerati.

    The Add-on route, through a secure appearing URL (https://addons.mozilla.org), is for less experienced clients who become quite uncomfortable just after reading the next screen after going to about:config with its dire warning.

    The Add-on route becomes the most attractive when considering deployment to a large client base or huge corporations.

    Let's agree that it's good to have more than one viable solution. Yes?

    Best wishes Paul.

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