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From: Michael Thumann (mthumannernw.de)
Date: Fri Jun 21 2002 - 09:12:52 CDT

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                                     Security Advisory

             Subject: Weak Cisco PIX Enable Password Encryption Algorithm

                     mao <maooxid.it>
                     Michael Thumann <mthumannernw.de>

             Issue date: June, 21, 2002

    1. Summary

             The encryption algorithm used by Cisco PIX Firewall software to
             passwords for "enable" and "passwd" commands is very fast...too fast.
             An off-line password guessing attack could be really effective against
             this kind of passwords.

    2. Systems Affected

             . Cisco PIX Firewalls (all models and all versions)

    3. Details

             . Cisco PIX passwords are limited to a length of 16 Bytes, so in
             there are 255^16 possible passwords, but in real life there are about
             80^16 useful password combinations, take a look at your keyboard to
             verify, even if strong passwords are used.

             Cisco's password encryption is based on base64 encoded MD5 hashes.
             IOS uses 1000 MD5 Update rounds to make password brute forcing
    attacks harder,
             but the PIX firewall uses only one MD5 update and then the digest
    is base64

             For base64 encoding Cisco uses the _crypt_to64() Function of the
             libcrypt library.

             Here's the code to compute PIX password hashes:


             MD5Context ctx1;
             unsigned char final[MD5_SIZE+1];
             unsigned char cleartext [16+1];
             unsigned char cisco_encoded [16+1];

             strcpy((char*) cleartext,"test");

             MD5Update2(&ctx1,(unsigned char*) cleartext,16);

             char* p = (char*) cisco_encoded;
             _crypt_to64(p,*(unsigned long*) (final+0),4); p += 4;
             _crypt_to64(p,*(unsigned long*) (final+4),4); p += 4;
             _crypt_to64(p,*(unsigned long*) (final+8),4); p += 4;
             _crypt_to64(p,*(unsigned long*) (final+12),4); p += 4;


             Due to some weaknesses in the MD5 hash algorithm (den Boer and
             found a so called pseudo-collision) there may be more effective
    attacks in
             the future.

    4. Impact

             . PIX Firewalls are security devices principally used for perimeter
             security. Once gained access to the Firewall by mean of a valid enable
             password an intruder could modify its configuration as wanted.
             In this situation all networks and resources protected by the
             Firewall could be affected.

             . Another important impact is due to the ability of recent version of
             PIX Firewalls softwares (new feature in version 6.2) to sniff traffic.
             The "capture" command could be used by an intruder to perform a
             sniffing of remote traffic based on pre-configured ACLs.

    5. Solution

             . Ask Cisco to use a stronger password scrambler (at least as the one
             ALREADY used on their routers and switches).
             While Cisco doesn't change the encryption algorithm for the PIX we
             using strong passwords with the full length of 16 bytes.

    6. Available Password Crackers

             . Cain & Abel (www.oxid.it)
             from version 2.5 beta13 it includes both crackers for Cisco PIX
    and Routers
             password hashes. The keyrate of those crackers shows the speed and
             feasibility of an off-line password guessing attacks.

             . Too many secrets (www.ernw.de)
             Version 0.9 includes password attacks (brute forcing, dictionary
    and hybrid
             attacks) for Cisco routers and the Cisco PIX firewall

    7. Conclusions

             . The feasibility of an off-line password guessing is something
    that every
             network administrator should consider before leaving PIX
             files on TFTP servers, sending them unencrypted via email or using
             for configuring the PIX.

    8. References

             . Bruce Schneiers 'Applied Cryptography'
             . FreeBSD libcrypt source code

    9. Disclaimer

             The informations in this advisory are provided "AS IS" without
             of any kind. In no event shall the authors be liable for any damages
             whatsoever including direct, indirect, incidental, consequential,
             loss of business profits or special damages due to the misuse of any
             information provided in this advisory.

    Cisco and Cisco PIX are all registered trademarks of the Cisco Corporation.

    ERNW Enno Rey Netzwerke GmbH - Zaehringerstr. 46 - 69115 Heidelberg
    Tel. +49 6221 480390 - Fax +49 6221 419008 - Mobil +49 173 6745903