(fwd) FreeBSD Security Advisory FreeBSD-SA-01:39.tcp-isn (fwd)

From: Tim Newsham (newsham@lava.net)
Date: 05/03/01


From: newsham@lava.net (Tim Newsham)
To: security@freebsd.org
Date: Wed, 2 May 2001 15:13:07 -1000 (HST)


hmm.. I think you may have gotten the attack description
and conditions wrong.

Attacks are performed against live, already authenticated
connections. As such, rsh and rlogin are no more
susceptible to attack than other unencrypted sessions.
All sessions, reguardless of use of encryption, are
susceptible to being shut down prematurely.

Filtering out priveledged ports will have no impact
on this vulnerability.

Tim N.

> =============================================================================
> FreeBSD-SA-01:39 Security Advisory
> FreeBSD, Inc.
>
> Topic: TCP initial sequence number generation contains
> statistical vulnerability
>
> Category: core
> Module: kernel
> Announced: 2001-05-02
> Credits: Tim Newsham <tim.newsham@guardent.com>
> Niels Provos <provos@OpenBSD.org> for the revised algorithm
> Affects: All released versions of FreeBSD 3.x, 4.x prior to 4.3.
> FreeBSD 3.5-STABLE prior to the correction date.
> FreeBSD 4.2-STABLE prior to the correction date.
> Corrected: 2001-05-02 (FreeBSD 3.5-STABLE)
> 2001-04-18 (FreeBSD 4.3-RC)
> 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.
>
> The algorithm used to generate TCP initial sequence numbers was
> subject to statistical analysis, which allows an attacker to guess a
> range of values likely to be in use by a given server at a moment in
> time, based on observation of the value at a previous time (for
> example, by initiating a TCP connection to an open port on the
> server).
>
> Note that this vulnerability is different to the vulnerability
> described in Security Advisory 00:52 (which dealt with failure of the
> PRNG used in the ISN generation algorithm; this advisory relates to a
> higher-level weakness in the algorithm itself).
>
> In order for this to be successfully exploited, the attacker must also
> satisfy the following conditions:
>
> a) be able to initiate a TCP connection to an open port on the server.
>
> 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 3.x and 4.x prior to the correction date
> including 3.5.1-RELEASE and 4.2-RELEASE are vulnerable to this
> problem. The problem was corrected prior to the release of FreeBSD
> 4.3-RELEASE by using the TCP ISN generation algorithm obtained from
> OpenBSD, which uses a more sophisticated randomization method that is
> believed not to be vulnerable to the problem described here.
>
> A more satisfactory, long-term solution would be to implement the
> algorithm described in RFC 1948; plans are underway to implement this
> algorithm for FreeBSD, and it is likely that it will be included in
> future releases of FreeBSD.
>
> 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
>
> Possible workarounds for the vulnerability include one or more 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. As of
> FreeBSD 4.2-RELEASE these services were not enabled by default.
>
> 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 (ingress
> filtering) 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 10.0.0.0/24
> network are allowed to obtain passwordless rlogin access to a server,
> then external users should be prevented from sending packets with
> 10.0.0.0/24 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 4.3-RELEASE or
> 3.5.1-STABLE after the respective correction dates.
>
> 2) To patch your present system: download the relevant patch from the
> below location, and execute the following commands as root:
>
> [FreeBSD 4.1/4.2 base system]
>
> This patch has been verified to apply to FreeBSD 4.1 and 4.2 only. It
> may or may not apply to older releases. Users of FreeBSD 4.1 must
> apply the patch from advisory 00:52 before applying this patch.
>
> # fetch ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/patches/SA-01:39/tcp-isn-4.2.patch
> # fetch ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/patches/SA-01:39/tcp-isn-4.2.patch.asc
>
> Verify the detached PGP signature using your PGP utility.
>
> # cd /usr/src/sys/netinet
> # patch -p < /path/to/patch
>
> [ Recompile your kernel as described in
> http://www.freebsd.org/handbook/kernelconfig.html and reboot the
> system ]
>
> [FreeBSD 3.5.1 base system]
>
> The following patch applies to FreeBSD 3.5.1-RELEASE which has already
> had the patch from advisory 00:52 applied.
>
> # fetch ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/patches/SA-01:39/tcp-isn-3.5.1-stable.patch
> # fetch ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/patches/SA-01:39/tcp-isn-3.5.1-stable.patch.asc
>
> The following patch applies to unpatched FreeBSD 3.5.1-RELEASE only.
> It may or may not apply to older, unsupported releases.
>
> # fetch ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/patches/SA-01:39/tcp-isn-3.5.1-rel.patch
> # fetch ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/patches/SA-01:39/tcp-isn-3.5.1-rel.patch.asc
>
> Verify the detached PGP signature using your PGP utility.
>
> # cd /usr/src/sys/netinet
> # patch -p < /path/to/patch
>
> [ Recompile your kernel as described in
> http://www.freebsd.org/handbook/kernelconfig.html and reboot the
> system ]
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>
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