Bug#930375: CVE-2019-12749: DBusServer DBUS_COOKIE_SHA1 authentication bypass

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Bug#930375: CVE-2019-12749: DBusServer DBUS_COOKIE_SHA1 authentication bypass

Simon McVittie-7
Package: libdbus-1-3
Version: 1.0.0-1
Severity: grave
Tags: security fixed-upstream patch
Forwarded: https://gitlab.freedesktop.org/dbus/dbus/issues/269

Joe Vennix of Apple Information Security discovered an implementation flaw
in the DBUS_COOKIE_SHA1 authentication mechanism. A malicious client with
write access to its own home directory could manipulate a ~/.dbus-keyrings
symlink to cause a DBusServer with a different uid to read and write
in unintended locations. In the worst case, this could result in the
DBusServer reusing a cookie that is known to the malicious client, and
treating that cookie as evidence that a subsequent client connection
came from an attacker-chosen uid, allowing authentication bypass.

This vulnerability does not normally affect the standard system
dbus-daemon, which only allows the EXTERNAL authentication mechanism.
In supported branches of dbus it also does not normally affect the standard
session dbus-daemon, for the same reason.

However, this vulnerability can affect third-party users of DBusServer
(such as Upstart in Ubuntu 14.04 LTS), third-party dbus-daemon instances,
standard dbus-daemon instances with non-standard configuration, and the
session bus in older/unsupported dbus branches (such as dbus 1.6.x in
Ubuntu 14.04 LTS).

For buster this has been fixed in libdbus-1-3 1.12.16-1. I'll close this
bug when I have a bug number.

For stretch this has been fixed in upstream release 1.10.28 and I am
discussing with the security team whether it is DSA-worthy, and if so,
whether to upload 1.10.28-0+deb9u1 or a minimal backport.

For experimental this will be fixed by upstream release 1.13.12 when
I've tested it.

If the Debian LTS team want to address this vulnerability
in jessie (which has an EOL dbus branch that we no
longer support upstream), they should backport upstream commit
<https://gitlab.freedesktop.org/dbus/dbus/commit/525c2314c56504fb232f9ec7f25cf7dda4d4a1c4>
and optionally also the build-time test coverage found in
<https://gitlab.freedesktop.org/dbus/dbus/commit/c251e7ea9525c1fc81360bbaf48f86ef6a0ad598>.

Regards,
    smcv

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Bug#930375: CVE-2019-12749: DBusServer DBUS_COOKIE_SHA1 authentication bypass

Simon McVittie-7
Version: 1.12.16-1

On Tue, 11 Jun 2019 at 17:34:40 +0100, Simon McVittie wrote:
> For buster this has been fixed in libdbus-1-3 1.12.16-1. I'll close this
> bug when I have a bug number.