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TUCoPS :: Unix :: General :: bt801.txt

Postfix 1.1.12 remote DoS / Postfix 1.1.11 bounce scanning






Good morning list,                                     ,--.   ,--.
                                                       \  /-~-\  /
======================================================= )' a a `( ========
1. Posfix 1.1.12 remote DoS (CAN-2003-0540)           .(  ,---.  ),
========================================================`(_o_o_)'=========

There is a remotely exploitable denial of service vulnerability in Postfix
up to and including 1.1.12. The vulnerability does not affect the most
current version, 2.0, due to a major overhaul of the address parsing code.
Releases prior to 1.1.9 are not vulnerable by default, but will be exposed
if append_dot_mydomain is turned off in the configuration file (see
section 3 for more details).

Recent 1.1 releases, having no publicly disclosed security problems, are
still commonly used and shipped in several popular Linux distributions,
including Red Hat 9 or Debian 3.0 (woody) - those distributions both ship
1.1.11.

The vulnerability lies in the address parser code. By supplying a remote
SMTP listener with a malformed envelope address, it is possible to,
depending on the method, either:

  - Cause the queue manager, nqmgr, to lock up permanently, effectively
    stopping any queue processing - all mail traffic supressed. Restarting
    the service has no effect - a specific entry has to be removed from
    the queue to fix the problem. For that reason, a builtin watchdog
    that restarts nqmgr after a period of nonresponsive behavior, is
    not able to cause a recovery from this condition.

    The attack can be performed by forcing the service to queue a mail
    to an address that would generate a bounce - depending on the
    configuration, it can be <nonexistent@local-server-name>, or, if user
    names are being checked, <nonexistent@[127.0.0.1]>. The "mail from" or
    "Errors-To" address should be set to "<.!>" or
    "<.!@local-server-name>". An attempt to parse and rewrite the latter
    address when preparing a bounce will lock up the service.

...or...

  - Lock up a single instance of the smtp listener in a unusable state
    that persists after the client disconnects. By repeating this,
    it is possible to DoS the service (or entire system, depending
    on the configuration) in a very effective manner.

    This can be achieved by providing any valid "MAIL FROM" in a SMTP
    conversation, and then supplying a "RCPT TO" similar to "MAIL FROM"
    in the previous example. If the server is vulnerable, the session
    should freeze at this point.

The latter approach, since it only creates a single stalled process, is a
less intrusive method of testing your systems for this issue remotely.

The attack can be detected by looking for "resolve_clnt_query: null
recipient" in your maillog. It is then necessary to find the problematic
entry in the queue and remove it manually, then restart the service.

It should be noted that it is often possible to attack instances that do
not have port 25 reachable from the Internet - envelope addresses and
certain headers such as Errors-To may very well be preserved when a
message is relayed via another system or service.


==========================================================================
2. Postfix 1.1.11 Bounce scan / DDoS agent issue (CAN-2003-0468)
==========================================================================

There is a remotely exploitable vulnerability in Postfix 1.1.11 (and
earlier versions). Postfix 1.1.12 and 2.0 is NOT affected. The problem was
apparently spotted and fixed in 1.1.12 (note 200221121 in HISTORY file),
although it has been tagged as a change preventing bogus log entries, and
not described as a security issue; there was no public information or
discussion about its implications on security forums, not prompting users
to upgrade. It might be that the significance of this problem was simply
overlooked.

Since the issue has been rediscovered during the analysis of the previous
issue, I decided it's worth mentioning here, especially since 1.1.11 is
shipped all over the place.

The problem enables an attacker to use Postfix 1.1.11 as a DDoS agent or
for bounce scans of other hosts on the Internet, or probing firewalled
internal networks. The problem is triggered by an attempt to deliver to:

  <[server_ip]:service!@local-host-name>

This address will cause Postfix to connect an arbitrary IP at an arbitrary
port and attempt to talk SMTP. The conversation will likely fail before
any user-dependent data is sent to the remote party, which limits the
exposure, but is sufficient to bounce-scan.

The address can be either sent in "RCPT TO" (the attacker would have the
right to relay to this system - which makes it a viable method of
bounce-scanning your ISP/mail account provider), in which case the sender
would then look for bounces stating the problem (SMTP conversation error,
connection timeout or connection refused), or in "MAIL FROM" / Errors-To,
in which case, the attacker can likely perform a queue timing attack to
detect whether a port is open by inserting control messages that are
intended to bounce.

When a port is open, SMTP greeting timeout occurs after a longer while,
pausing queue processing. When a port is closed, the entry is immediately
marked as deferred and queue processing continues.

It is also possible to use this problem to stage a DDoS attack, by making
a number of Postfix hosts around the world attempt to connect services on
a particular machine over and over again, until each queue entry finally
expires and is discarded or delivered to postmaster.


==========================================================================
3. Vendor status / fix and workardound information
==========================================================================

Wietse Venema has been contacted on July 27 regarding the first issue,
confirmed the problem described in #1 and released a patch to address it.
The information was then passed down to vendor-sec.

Below is a detailed fix and workaround info from the author:

  To find out your Postfix version, use the command "postconf
  mail_version".  Versions prior to 1.1 show a date instead of a
  version number (e.g., Postfix-20010228-pl08). Versions 1.1 and
  later may show a date in addition to the version number (e.g.,
  2.0.14-20030717).

  Postfix versions 2.0 and later:

    Not vulnerable, because the trivial-rewrite code was completely
    restructured. The current Postfix version is 2.0.13.

    A not vulnerable Postfix version can protect vulnerable Postfix
    systems as described in the workarounds section below.

  Postfix versions 1.1.9 .. 1.1.12:

    These are vulnerable, and are fixed by upgrading to version
    1.1.13 which will be made available via http://www.postfix.org/
    and via individual vendors, or by applying the patch below.
    The workarounds section below has instructions for sites that
    cannot upgrade Postfix immediately.

  Postfix versions prior to 1.1.9:

    These become vulnerable only when the append_dot_mydomain
    feature is set to "no" (you can verify this with the command
    "postconf append_dot_mydomain"). Use the command "postconf -e
    append_dot_mydomain=yes" to update the setting if necessary.

    Sites that must use "append_dot_mydomain=no" should either
    upgrade to a fixed Postfix version, or should apply the one-line
    patch at the end of this text. This patch has been tested with
    Postfix versions back to 19991231.

  Workarounds for Postfix versions 1.1.9 - 1.1.12:

    Verify that the append_dot_mydomain feature is set to "yes" by
    using the command "postconf append_dot_mydomain". Use the
    command "postconf -e append_dot_mydomain=yes" to update the
    setting if necessary.

    Sites that must use "append_dot_mydomain=no" should either
    upgrade to a fixed Postfix version, or should apply the one-line
    patch at the end of this text.

    Specify "resolve_dequoted_address=no" in main.cf.

    An additional workaround is needed for hosts that must forward
    mail from the Internet to, for example, primary MX hosts or to
    internal hosts.  This is because with resolve_dequoted_address=no,
    Postfix no longer recognizes user@bad.domain@good.domain as a
    mail relaying attempt.  To close this loophole, use a regular
    expression to block sender-specified routing in SMTP recipient
    addresses:

        /etc/postfix/main.cf:
            smtpd_recipient_restrictions =
                permit_mynetworks,
                check_recipient_access regexp:/etc/postfix/recipient_regexp
                ...other restrictions...
                check_relay_domains

        /etc/postfix/recipient_regexp:
            /[%!@].*[%!@]/       550 Sender-specified routing rejected

  Workarounds to protect vulnerable down-stream Postfix systems:

    Reject Errors-To: message headers with multiple routing
    operators:

        /etc/postfix/main.cf:
            header_checks = regexp:/etc/postfix/header_checks

        /etc/postfix/header_checks:
            /^errors-to:.*[%!@].*[%!@]/        reject

    Reject SMTP sender addresses with multiple routing operators:

        /etc/postfix/main.cf:
            smtpd_sender_restrictions =
                check_sender_access regexp:/etc/postfix/sender_regexp
                ...other restrictions...

        /etc/postfix/sender_regexp:
            /[%!@].*[%!@]/       550 Sender-specified routing rejected

diff -cr /tmp/postfix-1.1.12/src/trivial-rewrite/resolve.c src/trivial-rewrite/resolve.c
*** /tmp/postfix-1.1.12/src/trivial-rewrite/resolve.c	Fri Nov 22 12:32:33 2002
--- src/trivial-rewrite/resolve.c	Mon Jul 28 11:36:49 2003
***************
*** 148,153 ****
--- 148,154 ----
  	    if (saved_domain)
  		tok822_free_tree(saved_domain);
  	    saved_domain = domain;
+ 	    domain = 0;
  	}

  	/*

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