oss-security - Re: Exim CVE-2019-16928 RCE using a heap-based buffer overflow

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Message-ID: <20190928232024.GK16334@jumper.schlittermann.de>
Date: Sun, 29 Sep 2019 01:20:24 +0200
From: Heiko Schlittermann <hs@...marc.schlittermann.de>
To: oss-security@...ts.openwall.com, exim-users@...m.org,
	exim-maintainers@...m.org, exim-announce@...m.org
Subject: Re: Exim CVE-2019-16928 RCE using a heap-based buffer
 overflow

** Exim 4.92.3 released (security release) **

CVE ID:     CVE-2019-16928
Date:       2019-09-27 (CVE assigned)
Version(s): from 4.92 up to and including 4.92.2
Reporter:   QAX-A-TEAM <areuu@...look.com>
Reference:  https://bugs.exim.org/show_bug.cgi?id=2449
Issue:      Heap-based buffer overflow in string_vformat,
            remote code execution seems to be possible

Conditions to be vulnerable
===========================

All versions from (and including) 4.92 up to (and including) 4.92.2 are
vulnerable.

Details
=======

There is a heap-based buffer overflow in string_vformat (string.c).
The currently known exploit uses a extraordinary long EHLO string to
crash the Exim process that is receiving the message. While at this
mode of operation Exim already dropped its privileges, other paths to
reach the vulnerable code may exist.

Mitigation
==========

There is - beside updating the server - no known mitigation.

Fix
===

Download and build the fixed version 4.92.3

    Tarballs: https://ftp.exim.org/pub/exim/exim4/
    Git:      https://github.com/Exim/exim.git (mirror)
                git://git.exim.org/exim.git
              - tag    exim-4.92.3
              - branch exim-4.92.3+fixes

The tagged commit is the officially released version. The +fixes branch
isn't officially maintained, but contains the security fix *and* useful
fixes.

The tarballs, the Git tag, and the Git commits are signed with my GPG
key (same as I used to sign this mail.)

If you can't install the above versions, ask your package maintainer for
a version containing the backported fix. On request and depending on our
resources we will support you in backporting the fix.  (Please note,
the Exim project officially doesn't support versions prior the current
stable version.)

Timeline
=========

- 2019-09-27    Report as Bug 2499
- 2019-09-28    Announcement to exim-maintainers, oss-security
- 2019-09-28    Release 4.92.3, Release-Announcements to
                exim-{announce,users,maintainers}, oss-security

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