[exim] CVE-2018-6789 Exim 4.90 and earlier: buffer overflow

CVE-2018-6789 Exim 4.90 and earlier
===================================

There is a buffer overflow in an utility function, if some pre-conditions
are met. Using a handcrafted message, remote code execution seems to be
possible.

A patch exists already and is being tested.

Currently we're unsure about the severity, we *believe*, an exploit
is difficult. A mitigation isn't known.

Next steps:

* t0:     Distros will get access to our "security" non-public git repo
          (based on the SSH keys known to us)
* t0 +7d: Patch will be published on the official public git repo

t0 will be around 2018-02-08.

Timeline
--------

* 2018-02-05 Report from Meh Chang <meh@???> via exim-security mailing list
* 2018-02-06 Request CVE on https://cveform.mitre.org/ (heiko)
             CVE-2018-6789
* 2018-02-07 Announcement to the public via exim-users, exim-maintainers
             mailing lists and on oss-security mailing list

Updates will follow. Here and on https://exim.org/security/CVE-2018-6789.txt
(Link will start to exist around 11.00 UTC).

    Best regards from Dresden/Germany
    Viele Grüße aus Dresden
    Heiko Schlittermann
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