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FreeBSD TCP Sequence Number Vulnerability
FreeBSD TCP Sequence Number Vulnerability Privacy and Legal Notice


L-003: FreeBSD TCP Sequence Number Vulnerability

October 12, 2000 20:00 GMT
PROBLEM:       FreeBSD has a pseudo-random number generator for producing TCP
               sequence numbers. The creation of sequence numbers by the
               generator has been found to be flawed. A malicious user can
               easily guess new sequence numbers.
PLATFORM:      Systems derived from 4.4BSD-Lite2, which includes FreeBSD 3.x,
               4.x and 5.x.
DAMAGE:        The ability to predict TCP sequence numbers can allow a
               malicious user to spoof an IP address. This can allow a
               malicious user to gain access to a system by masquerading as a
               legitimite host. This untrusted system can receive all of
               the system privileges that were afforded to the spoofed system.
               A malicious user could gain access to files, databases, and
               system information. The security of the system and other remote
               systems could be placed in jeopardy.
SOLUTION:      Apply the patch as stated in the advisory. In addition, review
               the advisory for further means of improving security on a
               system that uses address based authentication.

VULNERABILITY The risk is MEDIUM, the vulnerability has been publicly ASSESSMENT: discussed.
[****** Begin FreeBSD Advisory ******] FreeBSD-SA-00:52 Security Advisory FreeBSD, Inc. Topic: TCP uses weak initial sequence numbers Category: core Module: kernel Announced: 2000-10-06 Credits: Hacker Emergency Response Team Affects: FreeBSD 3.x, 4.x and 5.x prior to the correction date Corrected: 2000-09-28 (5.0-CURRENT, 4.1.1-STABLE, 3.5.1-STABLE) FreeBSD only: NO I. Background TCP network connections use an initial sequence number as part of the connection handshaking. According to the TCP protocol, an acknowledgement packet from a remote host with the correct sequence number is trusted to come from the remote system with which an incoming connection is being established, and the connection is established. II. Problem Description It has long been known that an attacker who can guess the initial sequence number which a system will use for the next incoming TCP connection can spoof a TCP connection handshake coming from a machine to which he does not have access, and then send arbitrary data into the resulting TCP connection which will be accepted by the server as coming from the spoofed machine. Systems derived from 4.4BSD-Lite2 including FreeBSD include code which attempts to introduce an element of unpredictability into the initial sequence numbers to prevent sequence number guessing by a remote attacker. However the pseudo-random number generator used is a simple linear congruent generator, and based on observations of a few initial sequence values from legitimate connections with a server, an attacker can guess with high probability the value which will be used for the next connection. In order for this to be successfully exploited, the attacker must also satisfy the following conditions: a) be able to initiate several consecutive TCP connections to an open port on the server in a short space of time (immediately followed by the attack itself). Quiescent servers (those which are not receiving connections from other systems at the time of attack) are therefore most vulnerable to the attack. b) be able to prevent the spoofed client machine from responding to the packets sent to it from the server, by making use of an address which is offline or by executing a denial of service attack against it to prevent it from responding. c) make use of an application-level protocol on the server which authenticates or grants trust solely based on the IP address of the client, not any higher-level authentication mechanisms such as a password or cryptographic key. d) be able to guess or infer the return TCP data from the server to the spoofed client (if any), to which he will not have access, All versions of FreeBSD prior to the correction date including 4.1.1 and 3.5.1 are vulnerable to this problem. The FreeBSD Security Officer would like to thank the Hacker Emergency Response Team for working with us to bring this matter to our attention, and to coordinate the release of this advisory. III. Impact Systems running insecure protocols which blindly trust a TCP connection which appears to come from a given IP address without requiring other authentication of the originator are vulnerable to spoofing by a remote attacker, potentially yielding privileges or access on the local system. Examples of such protcols and services are: the rlogin/rsh/rexec family when used to grant passwordless access (e.g. via .rhosts or hosts.equiv files); web server address-based access controls on scripts which do not require user authentication and which control privileged resources; tcp-wrappers host access controls around services which do not authenticate the connection further; lpr address-based access controls, and others. Note that the rlogin family of protocols when configured to use Kerberos or UNIX passwords are not vulnerable to this attack since they authenticate connections (using Kerberos tickets in the former case, and account passwords in the latter). Source address based authentication in the rlogin family of protocols is not used by default, and must be specifically enabled through use of a per-user .rhosts file, or a global /etc/hosts.equiv file. Attackers can also forge TCP connections to arbitrary TCP protocols (including protocols not vulnerable to the spoofing attack described above) and simulate the effects of failed remote access attempts from a target machine (e.g. repeated attempts to guess a password), potentially misleading the administrators of the server into thinking they are under attack from the spoofed client. IV. Workaround Note that in order to exploit the vulnerability an attacker must make several real connection attempts in close succession to a port on the target machine (e.g. a web server). Since in order for the attack to be successful the machine must be quiescent (i.e. not accepting any other connections), this rapid connection activity followed by a connection to an insecure service may provide a signature which can be used to detect and trace the attacker. Possible workarounds for the vulnerability include one or both of the following: 1) Disable all insecure protocols and services including rlogin, rsh and rexec (if configured to use address-based authentication), or reconfigure them to not authenticate connections based solely on originating address. In general, the rlogin family should not be used anyway - the ssh family of commands (ssh, scp, slogin) provide a secure alternative which is included in FreeBSD 4.0 and above. To disable the rlogin family of protocols, make sure the /etc/inetd.conf file does not contain any of the following entries uncommented (i.e. if present in the inetd.conf file they should be commented out as shown below:) #shell stream tcp nowait root /usr/libexec/rshd rshd #login stream tcp nowait root /usr/libexec/rlogind rlogind #exec stream tcp nowait root /usr/libexec/rexecd rexecd Be sure to restart inetd by sending it a HUP signal after making any changes: # kill -HUP `cat /var/run/inetd.pid` Audit the use of other services including those noted in section III above and either disable the service, or if possible require it to use a stronger form of authentication. See workaround 3) below. 2) Impose IP-level packet filters on network perimeters or on local affected machines to prevent access from any outside party to a vulnerable internal service using a "privileged" source address. For example, if machines on the internal network are allowed to obtain passwordless rlogin access to a server, then external users should be prevented from sending packets with source addresses from the outside network into the internal network. This is standard good security policy. Note however that if an external address must be granted access to local resources then this type of filtering cannot be applied. It also does not defend against spoofing attacks from within the network perimeter. Consider disabling this service until the affected machines can be patched. 3) Enable the use of IPSEC to authenticate (and/or encrypt) vulnerable TCP connections at the IP layer. A system which requires authenticaion of all incoming connections to a port using IPSEC cannot be spoofed using the attack described in this advisory, nor can TCP sessions be hijacked by an attacker with access to the packet stream. FreeBSD 4.0 and later include IPSEC functionality in the kernel, and 4.1 and later include an IKE daemon, racoon, in the ports collection. Configuration of IPSEC is beyond the scope of this document, however see the following web resources: http://www.freebsd.org/handbook/ipsec.html http://www.netbsd.org/Documentation/network/ipsec/ http://www.kame.net/ V. Solution Note that address-based authentication is generally weak, and should be avoided even in environments running with the sequence numbering improvements. Instead, cryptographically-protected protocols and services should be used wherever possible. 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. 2a) FreeBSD 3.x systems Download the patch and detached PGP signature from the following locations, and verify the signature using your PGP utility. ftp://ftp.freebsd.org/pub/FreeBSD/CERT/patches/SA-00:52/tcp-iss-3.x.patch ftp://ftp://ftp.freebsd.org/pub/FreeBSD/CERT/patches/SA-00:52/tcp-iss-3.x.patch.asc # cd /usr/src/sys/ # patch -p < /path/to/patch [ Recompile your kernel as described in http://www.freebsd.org/handbook/kernelconfig.html and reboot the system ] 2b) FreeBSD 4.x systems Apply the patch below and recompile your kernel. 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. ftp://ftp.freebsd.org/pub/FreeBSD/CERT/patches/SA-00:52/tcp-iss.patch ftp://ftp.freebsd.org/pub/FreeBSD/CERT/patches/SA-00:52/tcp-iss.patch.asc # cd /usr/src/sys/netinet # patch -p < /path/to/patch_or_advisory [ Recompile your kernel as described in http://www.freebsd.org/handbook/kernelconfig.html and reboot the system ] Patch for vulnerable 4.x systems: Index: tcp_seq.h =================================================================== RCS file: /usr2/ncvs/src/sys/netinet/tcp_seq.h,v retrieving revision 1.11 retrieving revision 1.12 diff -u -r1.11 -r1.12 --- tcp_seq.h 1999/12/29 04:41:02 1.11 +++ tcp_seq.h 2000/09/29 01:37:19 1.12 @@ -91,7 +91,7 @@ * number in the range [0-0x3ffff] that is hard to predict. */ #ifndef tcp_random18 -#define tcp_random18() ((random() >> 14) & 0x3ffff) +#define tcp_random18() (arc4random() & 0x3ffff) #endif #define TCP_ISSINCR (122*1024 + tcp_random18()) Index: tcp_subr.c =================================================================== RCS file: /usr2/ncvs/src/sys/netinet/tcp_subr.c,v retrieving revision 1.80 retrieving revision 1.81 diff -u -r1.80 -r1.81 --- tcp_subr.c 2000/09/25 23:40:22 1.80 +++ tcp_subr.c 2000/09/29 01:37:19 1.81 @@ -178,7 +178,7 @@ { int hashsize; - tcp_iss = random(); /* wrong, but better than a constant */ + tcp_iss = arc4random(); /* wrong, but better than a constant */ tcp_ccgen = 1; tcp_cleartaocache(); [****** End FreeBSD Advisory ******]

CIAC wishes to acknowledge the contributions of FreeBSD, Inc. for the information contained in this bulletin.
CIAC services are available to DOE, DOE Contractors, and the NIH. CIAC can be contacted at:
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    E-mail:          ciac@llnl.gov
    World Wide Web:  http://www.ciac.org/
                     (same machine -- either one will work)
    Anonymous FTP:   ftp.ciac.org
                     (same machine -- either one will work)

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