The CVSS score for Logjam is (AV:N/AC:M/Au:N/C:N/I:P/A:N).
As noted in that nice dynamic image interpretation, the Impact metric "Confidentiality" is described as "None" (C:N).
But the description at Logjam notes "there is a passive network adversary able to eavesdrop" for Attack 1, and has a video displaying the plaintext of a post to an FBI site. Surely that merits at least a partial confidentiality impact C/P).
And of course if that or the impersonation attacks can be employed against the right user, you can also potentially get full access to information on the server's confidentiality, integrity and availability. But I guess they don't count those follow-on attacks.