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

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Message-ID: <20190928003428.GI16334@jumper.schlittermann.de>
Date: Sat, 28 Sep 2019 02:34:28 +0200
From: Heiko Schlittermann <hs@...marc.schlittermann.de>
To: oss-security <oss-security@...ts.openwall.com>
Cc: exim-users@...m.org, exim-maintainers@...m.org
Subject: Exim CVE-2019-16928 RCE using a heap-based buffer overflow

CVE ID:     CVE-2019-16928
Date:       2019-09-27 (CVE assigned)
Version(s): from 4.92 up to and including 4.92.2
Reporter:   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
===

We plan to publish a new security release (*will* be 4.92.3) of Exim
during the next 48 hours, ideally before monday 8.00 UTC. (We're still
running regression tests.) We'll send another notification as soon as
the new release is available.

Distros may have already picked the patch mentioned below and may have
already released a fixed version. Please check your distribution's
changelogs.

If you can't wait, please use use our git repository http://git.exim.org/exim.git,
checkout the branch exim-4.92.2+fixes and use the commit 478effbfd9c3cc5a627fc671d4bf94d13670d65f

A direct link to the commit is:
https://git.exim.org/exim.git/patch/478effbfd9c3cc5a627fc671d4bf94d13670d65f

which basically does:

--- a/src/src/string.c
+++ b/src/src/string.c
@@ -1132,7 +1132,7 @@ store_reset(g->s + (g->size = g->ptr + 1));
 Arguments:
   g            the growable-string
   p            current end of data
-  count                amount to grow by
+  count                amount to grow by, offset from p
 */

 static void
@@ -1590,7 +1590,7 @@ while (*fp)
        }
       else if (g->ptr >= lim - width)
        {
-       gstring_grow(g, g->ptr, width - (lim - g->ptr));
+       gstring_grow(g, g->ptr, width);
        lim = g->size - 1;
        gp = CS g->s + g->ptr;
        }

We thank you for using Exim.

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