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                                 Security Advisory
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         Subject: Weak Cisco PIX Enable Password Encryption Algorithm

         Authors:
                 mao <mao@oxid.it>
                 Michael Thumann <mthumann@ernw.de>

         Issue date: June, 21, 2002
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1. Summary

	The encryption algorithm used by Cisco PIX Firewall software to	encrypt 
	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 theory
	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.
	Routers 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 encoded.

	For base64 encoding Cisco uses the _crypt_to64() Function of the 
	FreeBSD	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];

         memset(cisco_encoded,0,sizeof(cisco_encoded));
         memset(cleartext,0,sizeof(cleartext));
         strcpy((char*) cleartext,"test");

         MD5Init2(&ctx1);
         MD5Update2(&ctx1,(unsigned char*) cleartext,16);
         MD5Final2(final,&ctx1);

         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
	Bosselaers 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
	recommend 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
	configuration files on TFTP servers, sending them unencrypted via email 
	or using telnet 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 warranty
	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.
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