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FreeBSD-SA-00:70                                           Security Advisory

                                                                FreeBSD, Inc.



Topic:          ppp "deny_incoming" does not correctly deny incoming packets



Category:       core

Module:         ppp

Announced:      2000-11-14

Credits:        Robin Melville 

Affects:        FreeBSD 3.5, 3.5.1, 4.1, 4.1.1

                FreeBSD 3.5.1-STABLE and 4.1.1-STABLE prior to the

                correction date.

Corrected:      2000-10-30 (FreeBSD 4.1.1-STABLE)

                2000-10-30 (FreeBSD 3.5.1-STABLE)

FreeBSD only:   Yes



I.   Background



The ppp(8) utility includes network address translation functionality

for translating between public and private IP address ranges.  It uses

the libalias library to perform translation services.



II.  Problem Description



The "nat deny_incoming" command is documented as "refusing all

incoming connections" and is commonly used as a simple "firewall" to

prevent outside users from connecting to services on the internal

network.  However the behaviour of the ppp code was changed in the 4.x

and 3.x branches prior to the release of FreeBSD 4.1 and 3.5 (on

2000-06-05 and 2000-06-03 respectively) to allow passing of packets

which are not understood, such as IPSEC packets and other IP protocol

traffic not explicitly recognised by the code as being an "incoming

connection attempt".  While this was arguably incorrect behaviour in

itself, the code also incorrectly allowed through ALL incoming

traffic, effectively turning "deny_incoming" into a no-op.



Thus, users who are using the deny_incoming functionality in the

expectation that it provides a "deny by default" firewall which only

allows through packets known to be part of an existing NAT session,

are in fact allowing other types of unsolicited IP traffic into their

internal network.



The behaviour of ppp was corrected to only allow incoming packets

which are known to be part of a valid NAT session, which gives the

desired packet filtering behaviour in the general case.  Outgoing IP

traffic which is not understood by libalias (such as an outgoing IPSEC

packet part of a VPN) will cause a NAT session to be established which

will allow incoming packets with the corresponding source and

destination IP addresses and protocol number to pass, but all others

to be denied.



This behaviour may be sufficient for the security needs of many users,

although users with advanced filtering or security policy requirements

are advised to use a more configurable packet filter such as those

provided by ipfw(8) or ipf(8) which can meet their needs.



The following released versions of FreeBSD are the only releases

vulnerable to this problem: 3.5, 3.5.1, 4.1, 4.1.1.  It was fixed in

the 4.1.1-STABLE branch prior to the release of FreeBSD 4.2-RELEASE.



III. Impact



Remote users can cause incoming traffic which is not part of an

existing NAT session to pass the NAT gateway, which may constitute a

breach of security policy.



IV.  Workaround



Use a true packet filter such as ipfw(8) or ipf(8) on the PPP gateway

to deny incoming traffic according to the desired security policy.



V.   Solution



One of the following:



1) Upgrade your vulnerable FreeBSD system to 4.1.1-STABLE or

3.5.1-STABLE after the respective correction dates.



2) Apply the patch below and recompile the relevant files:



Either save this advisory to a file, or download the patch and

detached PGP signature from the following locations, and verify the

signature using your PGP utility.



http://ftp.freebsd.org/pub/FreeBSD/CERT/patches/SA-00:70/ppp.patch

http://ftp.freebsd.org/pub/FreeBSD/CERT/patches/SA-00:70/ppp.patch.asc



Execute the following commands as root:



# cd /usr/src/usr.sbin/ppp

# patch -p < /path/to/patch_or_advisory

# make depend && make all install



Patch for vulnerable systems:



Index: nat_cmd.c

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

RCS file: /mnt/ncvs/src/usr.sbin/ppp/nat_cmd.c,v

retrieving revision 1.49

retrieving revision 1.50

diff -u -r1.49 -r1.50

- --- nat_cmd.c 2000/07/11 22:11:31 1.49

+++ nat_cmd.c 2000/10/30 18:02:01 1.50

@@ -421,7 +421,11 @@

       break;



     case PKT_ALIAS_IGNORED:

- -      if (log_IsKept(LogTCPIP)) {

+      if (PacketAliasSetMode(0, 0) & PKT_ALIAS_DENY_INCOMING) {

+        log_Printf(LogTCPIP, "NAT engine denied data:\n");

+        m_freem(bp);

+        bp = NULL;

+      } else if (log_IsKept(LogTCPIP)) {

         log_Printf(LogTCPIP, "NAT engine ignored data:\n");

         PacketCheck(bundle, MBUF_CTOP(bp), bp->m_len, NULL, NULL, NULL);

       }

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