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              AUSCERT External Security Bulletin Redistribution
                                    
          ESB-98.169 -- FreeBSD Security Advisory: FreeBSD-SA-98:08
                     IP fragmentation denial of service
                                    
                               05 November 1998

===========================================================================

The FreeBSD Security Team has released the following advisory concerning
a vulnerability in the IP fragment reassembly code that might lead to a
kernel panic. An attacker can create and send a pair of malformed IP
packets which are then reassembled into an invalid UDP datagram that can
cause a server to panic and crash.


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FreeBSD-SA-98:08                                            Security Advisory
                                                                FreeBSD, Inc.

Topic:          IP fragmentation denial of service

Category:       core
Module:         kernel
Announced:      1998-11-04
Affects:        FreeBSD 3.0 and
		FreeBSD-current before the correction date.
Corrected:      FreeBSD-3.0 and FreeBSD-current as of 1998/10/27
FreeBSD only:   Yes

Patches:        ftp://ftp.freebsd.org/pub/FreeBSD/CERT/patches/SA-98:08/

I.   Background    

IP connections are controlled through a series of packets that are
received by the two computers involved in the connection.  
When packets are too large to be sent in a single IP packet (due to
interface hardware limitations for example), they can be fragmented
(unless prohibited by the Don't Fragment flag).
The final destination will reassemble all the fragments of an IP packet
and pass it to higher protocol layers (like TCP or UDP).

II.  Problem Description

There is a bug in the IP fragment reassembly code that might lead
to a kernel panic. An attacker can create and send a pair of
malformed IP packets which are then reassembled into an invalid
UDP datagram. Such an UDP datagram would then cause a server to
panic and crash.


III. Impact

When this bug is exploited the operating system will panic. This results
in a reboot of the system.
This vulnerability has been discussed in public security forums and
exploit programs are circulating to take advantage of this bug.


IV.  Workaround

None.

V.   Solution


    Index: ip_input.c
    ===================================================================
    RCS file: /home/cvsup/freebsd/CVS/src/sys/netinet/ip_input.c,v
    retrieving revision 1.102
    retrieving revision 1.103
    diff -u -u -r1.102 -r1.103
    --- ip_input.c	1998/10/16 03:55:01	1.102
    +++ ip_input.c	1998/10/27 09:11:41	1.103
    @@ -750,7 +750,7 @@
     	 * if they are completely covered, dequeue them.
     	 */
     	for (; q != NULL && ip->ip_off + ip->ip_len > GETIP(q)->ip_off;
    -	     p = q, q = nq) {
    +	     q = nq) {
     		i = (ip->ip_off + ip->ip_len) -
     		    GETIP(q)->ip_off;
     		if (i < GETIP(q)->ip_len) {

=============================================================================
FreeBSD, Inc.

Web Site:                       http://www.freebsd.org/
Confidential contacts:          security-officer@freebsd.org
Security notifications:         security-notifications@freebsd.org
Security public discussion:     freebsd-security@freebsd.org
PGP Key:                ftp://ftp.freebsd.org/pub/FreeBSD/CERT/public_key.asc

Notice: Any patches in this document may not apply cleanly due to
        modifications caused by digital signature or mailer software.
        Please reference the URL listed at the top of this document
        for original copies of all patches if necessary.
=============================================================================


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