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===========================================================================
              AUSCERT External Security Bulletin Redistribution
                                      
                                      
       ESB-98.093 -- FreeBSD Security Advisory - FreeBSD-SA-98:06.icmp
                                smurf attack
                                11 June 1998

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

The FreeBSD Security Team has released the following advisory concerning
the "smurf" denial of service attack.

The problem discussed in this advisory has previously been discussed in
CERT/CC advisory CA-98.01 which was released as AusCERT ESB-98.001.

The following security bulletin is provided as a service to AusCERT's
members.  As AusCERT did not write this document, AusCERT has had no
control over its content.  As such, the decision to use any or all of this
information is the responsibility of each user or organisation, and should
be done so in accordance with site policies and procedures.

NOTE: This is only the original release of the security bulletin.  It will
not be updated when the original bulletin is.  If downloading at a later
date, it is recommended that the bulletin is retrieved from the original
authors to ensure that the information is still current.

Contact information for FreeBSD is included in the Security Bulletin below.
If you have any questions or need further information, please contact them
directly.

Previous advisories and external security bulletins can be retrieved from:

	http://www.auscert.org.au/Information/advisories.html

If you believe that your system has been compromised, contact AusCERT or
your representative in FIRST (Forum of Incident Response and Security
Teams).

Internet Email: auscert@auscert.org.au
Facsimile:	(07) 3365 7031
Telephone:	(07) 3365 4417 (International: +61 7 3365 4417)
		AusCERT personnel answer during Queensland business hours
		which are GMT+10:00 (AEST).
		On call after hours for emergencies.


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=============================================================================
FreeBSD-SA-98:06                                            Security Advisory
                                                                FreeBSD, Inc.

Topic:          smurf attack

Category:       core
Module:         kernel
Announced:      1998-06-10
Affects:        FreeBSD 2.2.*, FreeBSD-stable and FreeBSD-current 
		before 1998/05/26 suffer from this problem.
Corrected:      FreeBSD-current as of 1998/05/26
		FreeBSD-stable as of 1998/05/26
FreeBSD only:   yes

Patches:        ftp://ftp.freebsd.org/pub/FreeBSD/CERT/patches/SA-98:06/

=============================================================================
IMPORTANT MESSAGE: The FreeBSD security officer now uses the policy
ftp://ftp.freebsd.org/pub/FreeBSD/POLICY.asc for sending out
advisories.
=============================================================================

I.   Background    

     As can be read in CERT advisory CA-98.01.smurf, there exists
     a denial of service attack called "smurfing". This attack sends
     ICMP echo requests to the broadcast address of a network. This
     results in the source address of the ICMP packets being flooded
     with ICMP echo replies. Of course, the source address is
     spoofed.
      

II.  Problem Description

     A solution at the intermediate network being abused to generate
     the ICMP echo replies is to either block ICMP echo requests
     directed to a broadcast address or to configure the hosts on
     that network not to respond to such an ICMP request.  In the
     CERT advisory, the following was reported:

       In FreeBSD 2.2.5 and up, the tcp/ip stack does not respond
       to ICMP echo requests destined for broadcast and multicast
       addresses by default. This behavior can be changed via the
       sysctl command via mib net.inet.icmp.bmcastecho.

     Unfortunately, an error was made with the implementation of
     this functionality and, despite the text in the CERT
     advisory, the net.inet.icmp.bmcastecho sysctl variable default
     is to respond to ICMP packets sent to the networks broadcast
     address. You should explicitly run the command
	sysctl -w net.inet.icmp.bmcastecho=0
     to disable this.

III. Impact

     Your network can suffer performance degradation when a
     large amount of spoofed ICMP is sent to your broadcast address.

IV.  Workaround

     Block ICMP echo requests to broadcast addresses in your kernel
     using ipfw(8).  See CERT advisory CA-98.01.smurf for more
     workarounds.

V.   Solution

     Apply the following patch:

     Patch for 3.0-current, 2.2-stable, 2.2.5 and 2.2.6 systems:

  Index: ip_icmp.c
  ===================================================================
  RCS file: /home/cvsup/freebsd/CVS/src/sys/netinet/ip_icmp.c,v
  retrieving revision 1.29
  retrieving revision 1.30
  diff -u -r1.29 -r1.30
  --- ip_icmp.c	1997/08/25 16:29:27	1.29
  +++ ip_icmp.c	1998/05/26 11:34:30	1.30
  @@ -375,8 +375,7 @@
   
   	case ICMP_ECHO:
   		if (!icmpbmcastecho
  -		    && (m->m_flags & (M_MCAST | M_BCAST)) != 0
  -		    && IN_MULTICAST(ntohl(ip->ip_dst.s_addr))) {
  +		    && (m->m_flags & (M_MCAST | M_BCAST)) != 0) {
   			icmpstat.icps_bmcastecho++;
   			break;
   		}
  @@ -385,8 +384,7 @@
   
   	case ICMP_TSTAMP:
   		if (!icmpbmcastecho
  -		    && (m->m_flags & (M_MCAST | M_BCAST)) != 0
  -		    && IN_MULTICAST(ntohl(ip->ip_dst.s_addr))) {
  +		    && (m->m_flags & (M_MCAST | M_BCAST)) != 0) {
   			icmpstat.icps_bmcasttstamp++;
   			break;
   		}


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