TUCoPS :: Unix :: General :: unix5958.htm

COMMAND
	Multiple vulnerabilities in MIT Kerberos 5 releases
SYSTEMS AFFECTED
	MIT Kerberos 5 releases prior to release 1.2.5.
PROBLEM
	In MIT Kerberos security advisory page at:
	
		http://web.mit.edu/kerberos/www/advisories/index.html
	
	
	--snip--
	
	* A remote user can crash the KDC.
	
	* A user authenticated in a remote realm may be able to claim to be
	 other non-local users to an application server.
	
	* It may be possible for a user to gain access to the KDC system and
	 database.
	
	--snip--
	
	Problem 1: KDC null pointer dereferences
	________________________________________
	
	
	Thanks to greg pryzby <GregPryzby@aol.com> for reporting this
	problem.
	
	Certain protocol requests, compliant with the protocol encoding scheme
	but indicative of a client system most likely configured incorrectly,
	can crash a KDC with a null pointer dereference. We do not believe any
	exploit to gain access to the KDC or otherwise alter its behavior is
	possible on systems without storage mapped at address zero. We have not
	explored the effects of this on a system with mapped memory at address
	zero.
	
	The fallback and retransmit algorithm used in the MIT krb5 library will
	cause an application not receiving a reply from a KDC to try other KDCs
	in the same realm; it will iterate through this list a few times, or
	until it gets a response. Thus, one client may take down multiple KDCs.
	
	We believe this vulnerability is limited to the TGS-REQ exchange, that
	is, cases where the user has already authenticated to the KDC or one
	with which it shares inter-realm keys. So (ignoring cases of well-known
	passwords) there is an audit trail of sorts, even if it has to be dug
	out of a core file, and it is not a simple, scriptable attack against
	KDCs in general.
	
	Problem 2: realm transit checks
	_______________________________
	
	
	Thanks to Joseph Sokol-Margolis <seph@mit.edu> and Gerald Britton
	<gbritton@alum.mit.edu> for finding this problem.
	
	Realms with shared keys can impersonate people in other non-local
	realms in certain cases. It may be exploitable in various ways if
	non-local principal names are on critical ACLs.
	
	This vulnerability affects both the KDC and Kerberos application
	servers.
	
	This problem was fixed in the 1.2.3 release. That release also added a
	flag to the KDC config file that can be set to refuse untrusted
	cross-realm authentication, in case application servers cannot be
	updated quickly enough. This is not recommended as a long-term
	solution, because the current model we use says that the application
	server is responsible for doing this validation, which allows (for
	example) a service on a specific machine (perhaps one set up for
	software testing) to be configured to know about authentication paths
	known to the maintainer of the service, even if the maintainer of the
	KDC does not trust these paths for general use within the realm.
	Enforcing this limitation in the KDC takes this option away from the
	maintainers of individual machines.
	
	Problem 3: format strings
	_________________________
	
	
	Thanks to E. Larry Lidz <ellidz@eridu.uchicago.edu> for discovering
	this problem.
	
	Older versions of the MIT KDC used strings containing Kerberos
	principal names as printf-style format strings in logging routines.
	
	At least some cases do not require successful authentication, so this
	can be used as a remote, anonymous attack.
	
	It is easy to crash the KDC with this exploit. We do not know of any
	exploits to gain access to the host system, but we do not rule out the
	possibility.
	
	Problem 4: bounds checking on data sizes
	________________________________________
	
	
	Thanks to CERT for bringing this to our attention.
	
	Some of our code does not do bounds checking on lengths before
	allocating storage. On some systems, attempting to allocate large
	negative amounts of storage can crash the program. Thus, some bogus
	packets may crash the KDC or an application server using Kerberos. We
	do not believe this can be exploited to gain access to the host system.
SOLUTION
	IT recommends updating to 1.2.7 if possible
	
	 Workarounds:
	 ============
	
	 1
	 =
	
	 - Start your KDC from inittab or a loop in a shell script. (The
	 inittab approach may not work well if the KDC is crashed too often
	 in a short span of time.)
	
	 2
	 =
	
	 - Delete or change inter-realm keys so inter-realm authentication is
	 disabled.
	
	 - Remove all non-local principals from all critical ACLs in services
	 using old MIT Kerberos code to validate the realm transit path
	
	 3
	 =
	
	See under problem 1. ***However, these do not address the host access
	possibility.***
	
	 4
	 = 
	
	 - start KDC in a loop in a script, or from inittab
	
	 - do likewise for any server processes that need to handle multiple
	 client connections