NISCC Vulnerability Advisory 236929: Vulnerability Issues in TCP

[David Ahmad <da () securityfocus com>]

http://www.uniras.gov.uk/vuls/2004/236929/index.htm

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NISCC Vulnerability Advisory 236929

Vulnerability Issues in TCP


Version Information 

Advisory Reference 236929 
Release Date 20 April 2004 
Last Revision 20 April 2004 
Version Number 1.0 
 

What is Affected?

The vulnerability described in this advisory affects implementations of the Transmission Control Protocol (TCP) that 
comply with the Internet Engineering Task Force.s (IETF.s) Requests For Comments (RFCs) for TCP, including RFC 793, the 
original specification, and RFC 1323, TCP Extensions for High Performance.

TCP is a core network protocol used in the majority of networked computer systems today. Many vendors include support 
for this protocol in their products and may be impacted to varying degrees. Furthermore any network service or 
application that relies on a TCP connection will also be impacted, the severity depending primarily on the duration of 
the TCP session. 


Severity

The impact of this vulnerability varies by vendor and application, but in some deployment scenarios it is rated 
critical. Please see the vendor section below for further information. Alternatively contact your vendor for product 
specific information.

If exploited, the vulnerability could allow an attacker to create a Denial of Service condition against existing TCP 
connections, resulting in premature session termination. The resulting session termination will affect the application 
layer, the nature and severity of the effects being dependent on the application layer protocol. The primary dependency 
is on the duration of the TCP connection, with a further dependency on knowledge of the network (IP) addresses of the 
end points of the TCP connection.

The Border Gateway Protocol (BGP) is judged to be potentially most affected by this vulnerability.

BGP relies on a persistent TCP session between BGP peers. Resetting the connection can result in medium term 
unavailability due to the need to rebuild routing tables and route flapping.  Route flapping may result in route 
dampening (suppression) if the route flaps occur frequently within a short time interval.  The overall impact on BGP is 
likely to be moderate based on the likelihood of successful attack. If the TCP MD5 Signature Option and anti-spoofing 
measures are used then the impact will be low as these measures will successfully mitigate the vulnerability.

There is a potential impact on other application protocols such as DNS (Domain Name System) and SSL (Secure Sockets 
Layer) in the case of zone transfers and ecommerce transactions respectively, but the duration of the sessions is 
relatively short and the sessions can be restarted without medium term unavailability problems. In the case of SSL it 
may be difficult to guess the source IP address.

Data injection may be possible. However, this has not been demonstrated and appears to be problematic. 


Summary

The issue described in this advisory is the practicability of resetting an established TCP connection by sending 
suitable TCP packets with the RST (Reset) or SYN (Synchronise) flags set.

The packets need to have source and destination IP addresses that match the established connection as well as the same 
source and destination TCP ports.

The fact that TCP sessions can be reset by sending suitable RST and SYN packets is a design feature of TCP according to 
RFC 793, but a reset attack is only possible at all because the source IP address and TCP port can be forged or 
.spoofed..

Although denial of service using crafted TCP packets is a well known weakness of TCP, until recently it was believed 
that a successful denial of service attack was not achievable in practice. The reason for this is that the receiving 
TCP implementation checks the sequence number of the RST or SYN packet, which is a 32 bit number, giving a probability 
of 1/232 of guessing the sequence number correctly (assuming a random distribution).

The discoverer of the practicability of the RST attack was Paul A. Watson, who describes his research in his paper 
.Slipping In The Window: TCP Reset Attacks., presented at the CanSecWest 2004 conference. He noticed that the 
probability of guessing an acceptable sequence number is much higher than 1/232 because the receiving TCP 
implementation will accept any sequence number in a certain range (or .window.) of the expected sequence number. The 
window makes TCP reset attacks practicable.

Any application protocol which relies on long term TCP connections and for which the source and destination IP 
addresses and TCP ports are known or can be easily guessed will be vulnerable to at least denial of service attacks. 


Details

TCP is the transport layer protocol designed to provide connection-oriented reliable delivery of IP packets. To do this 
TCP uses a mixture of flags, to indicate state, and sequence numbers, to identify the order in which the packets are to 
be reassembled.

TCP also provides a number, called an acknowledgement number, that is used to indicate the sequence number of the next 
packet expected. The packets are reassembled by the receiving TCP implementation only if their sequence numbers fall 
within a range of the acknowledgement number (called a "window"). The acknowledgement number is not used in a RST 
packet because a reset does not expect a packet in return. (To be completely accurate, although the last statement is 
true for a RST packet without the ACK flag set, used to indicate that a TCP port is closed, a RST/ACK is used to 
terminate an active connection in the event of error. In a RST/ACK packet an acknowledgement number is included in the 
packet, although it is not checked by the receiving TCP implementation.)

RFC 793, p36, states the following:

"In all states except SYN-SENT, all reset (RST) segments are validated by checking their SEQ-fields [sequence numbers]. 
A reset is valid if its sequence number is in the window. In the SYN-SENT state (a RST received in response to an 
initial SYN), the RST is acceptable if the ACK field acknowledges the SYN."

Resets must be processed immediately. RFC 793, p25, says "[.] [E]ven when the receive window is zero, a TCP must 
process the RST and URG fields of all incoming segments."

It is also possible to perform the same attack with SYN (synchronise) packets. An established connection will abort by 
sending a RST if it receives a duplicate SYN packet with initial sequence number within the TCP window. RFC 793, p31 
states:

.The principle reason for the three-way handshake is to prevent old duplicate connection initiations from causing 
confusion. To deal with this, a special control message, reset, has been devised. [.] If the TCP is in one of the 
synchronized states (ESTABLISHED, FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK, TIME-WAIT), it aborts the 
connection and informs its user..

TCP window sizes are negotiated in the initial 3-way handshake used to set up a TCP connection, with higher values 
serving to improve throughput in some circumstances. Vendor-chosen defaults also influence the selection. In any case, 
the larger the window size, the greater is the probability that a randomly chosen TCP sequence number will lie within 
the window range. This is the basis for the attack.

A TCP connection is defined by a 4-tuple comprising source and destination IP addresses, and source and destination 
ports. An attacker seeking to disrupt an existing TCP connection must supply the 4-tuple correctly. As the source port 
varies, additional work is generally called for on the part of the attacker. However, research (referenced below) has 
shown that the process of source port selection on many platforms includes predictable elements, so that the attack 
remains practicable. By weighting 'likely' source port values carefully, an attacker can disrupt TCP implementations 
that employ a range of window sizes.

Application layer protocols that are critically affected are those that:

.  Depend on long lived TCP connections 
.  Have known or easy-to-guess IP address end points 
.  Have easy to an easy-to-guess source TCP port 

As noted above BGP does use long lived TCP connections, and the IP addresses and source port (and destination port) are 
sometimes available through the use of BGP looking glasses (multi-source, multi-destination trace route tools) or DNS 
resource records. Using .trace route. commands can provide information on peering point IP addresses. Thus BGP is 
likely to be critically affected by the TCP vulnerability.

These denial of service attacks can be carried out by single machine, or by multiple co-operating systems (to form a 
distributed denial of service attack).

It is also possible to inject packets, which will be processed if they are in the window. The difficulty with data 
injection attacks is that the receiving TCP implementation will reassemble the packets received according to sequence 
number, dropping any duplicate packets.


Vendor specific information will be released as it becomes available and if vendor permission has been received. 
Subscribers are advised to check the following URL regularly for updates:

http://www.uniras.gov.uk/vuls/2004/236929/index.htm

[Please note that updates to this advisory will not be notified by email.]

This vulnerability has been assigned the CVE name CAN-2004-0230.

The Open Source Vulnerability Database ID number for this vulnerability is 4030.


Mitigation

The following mitigation steps are still being evaluated and may be incomplete. Customers should work with vendors for 
the workaround most appropriate for the product in question.

In the absence of vendor patching of the TCP implementation, the following are general mitigating steps:

.  Implement IP Security (IPSEC) which will encrypt traffic at the network layer, so TCP information will not be 
visible 
.  Reduce the TCP window size (although this could increase traffic loss and subsequent retransmission) 
.  Do not publish TCP source port information 


It should be noted that IPSEC provides confidentiality and authentication services at the network layer, and can 
provide a measure of trust in the authenticity of the end points as well as encryption of traffic between the end 
points.  However, in the context of the current attack IPSEC will reject RST and SYN packets that are not part of a 
secure IP packet stream.

To change the TCP window size, in some Unix variants you can set a value of the default TCP windows size by using the 
.sysctl. program (.ndd -set. in the case of Sun Solaris). In the case of Microsoft Windows NT/2000/XP/2003, the default 
window size can be changed by modifying the value of the 
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters key. As noted above, great care should be 
exercised when altering the default TCP window size as network performance could be adversely affected.

In the case of BGP, the following may counter the problem:

.  Implement ingress and egress filtering to check that the traffic entering or leaving the network has a source IP 
address that is expected on the router/firewall interface that receives the traffic 
.  Implement the TCP MD5 Signature Option to checksum the TCP packet carrying the BGP application data (see RFC 2385), 
being careful to set and maintain strong (i.e. difficult to guess) passwords to which the MD5 checksum is applied.  
Also see RFC 3562 which discusses the security requirements of this keying material. 
.  Limit the amount of information available through looking glasses and DNS resource records, being careful not to 
expose TCP port information unnecessarily 


The IETF ingress filtering standard is defined in RFC 2827. A discussion of egress filtering can be found at 
http://www.sans.org/y2k/egress.htm.

The use of the TCP MD5 Signature Option will prevent the exploitation of this vulnerability. Router customers should 
implement this on all BGP peering points if it is supported by the router, upgrading the router firmware if necessary.


Solution

Please refer to the Vendor Information section of this advisory for implementation specific remediation.

Some vendors will have reduced the likelihood of successful denial of service by amending the TCP implementation to 
issue a further acknowledgment packet challenge for RST and SYN packets that do not have exactly the expected sequence 
number.

The Internet Engineering Task Force (IETF) has published an Internet Draft to co-incide with the release of this 
advisory.  The text of this draft is available from the IETF web site:
http://www.ietf.org/internet-drafts/draft-ietf-tcpm-tcpsecure-00.txt

NISCC has produced best practice guidelines for BGP available at
http://www.niscc.gov.uk/BGP Filtering Guide.pdf

Secure configuration templates for BGP implementations on Cisco IOS and Juniper JunOS can be found at:

.  Cisco  http://www.cymru.com/Documents/secure-bgp-template.html  
.  Juniper  http://www.qorbit.net/documents/junos-bgp-template.pdf  


Guidance on tuning of the IP stack for a number of different UNIX operating systems is available at 
http://www.cymru.com/Documents/ip-stack-tuning.html 


Vendor Information 

The following vendors have provided information about how their products are affected by these vulnerabilities.

Please note that JPCERT/CC have released a Japanese language advisory for this vulnerability which contains additional 
information regarding Japanese vendors. This advisory is available at http://www.jpcert.or.jp/at/2004/at040003.txt.





Certicom Internet Initiative Japan, Inc   
Check Point InterNiche   
Cisco Juniper Networks   
Cray Inc NEC   
Hitachi Polycom   
Innovaphone Yamaha   




Certicom 
  Certicom's SSL software developer toolkits (SDK), requires a transport mechanism, however it is not restricted to 
TCP. The default implementation that is shipped with the product utilizes the supported operating system's TCP/IP 
stack. Certicom recognizes that the indicated vulnerability is against the protocol stack itself and not directly the 
application on top. As our products (SSL Plus, SSL Plus for Java, Security Builder SSL-C, and Security Builder SSL-J), 
are primarily used in a web server environment, a denial of service attack is important to us and our customers. As 
there is no patch or workaround that Certicom can implement within our products, we feel that we are not directly 
vulnerable to this advisory. Certicom's website is www.certicom.com. 
Check Point 
  The latest release for VPN-1/FireWall-1 (R55 HFA-03) contains a protection against this vulnerability.  The 
protection applies to both the firewall device and to hosts behind the firewall.

Please refer to the Check Point web site for further information at:
http://www.checkpoint.com/techsupport/alerts/tcp_dos.html. 
Cisco 
  Place holder. 
Cray Inc 
  Cray Inc. is vulnerable on their UNICOS, UNICOS/mk and UNICOS/mp systems.  Spr's have been opened to track this 
issue.  Please contact your local Cray Service Representative for more information. 
Hitachi 
  Hitachi is investigating the potential impact to Hitachi's products. 
Innovaphone 
  Not vulnerable. 
Internet Initiative Japan, Inc (IIJ) 
  IIJ will release a new firmware to fix this vulnerability.  Details are available on their web site at 
http://www.seil.jp/en/ann/announce_en_20040421_01.txt. 
InterNiche 
  === NicheStack v2.0 TCP/IP ===

InterNiche Technologies has updated its NicheStack v2.0 TCP/IP product to handle the scenarios described in NISCC 
Vulnerability Notice #236929.  The patch is available to all InterNiche customers in accordance with the terms of their 
current support agreements.

More information can be found on www.iNiche.com or through support () iNiche com 


=== NicheLite v2.0 TCP/IP ===

InterNiche Technologies has updated its NicheLite v2.0 TCP/IP product to handle the scenarios described in NISCC 
Vulnerability Notice #236929.  The patch is available to all InterNiche customers in accordance with the terms of their 
current support agreements. 

More information can be found on www.iNiche.com or through support () iNiche com  
Juniper Networks 
  Juniper Networks products are susceptible to this vulnerability. Software is available that implements several 
mechanisms to mitigate the associated risks. Customers should contact Juniper Networks Technical Assistance Center for 
availability and download instructions.

Additional information is posted on our web site at https://www.juniper.net/support.  
NEC 
  NEC is aware of this vulnerability and is trying to determine potential impacts on our products. 
Polycom 
  Polycom has investigated the potential impact to our products for NISCC Advisory 236929.

Specific product information will be provided at http://www.polycom.com/securitycenter. 
Yamaha 
  Pending. 


Acknowledgements

NISCC wishes to thank the following:

.  Steve Bellovin, Rob Thomas and Paul Watson for their contributions to this advisory. 
.  Cisco Systems Inc. and Juniper Networks Inc. for their help with the content of this advisory and for their support 
during the disclosure process. 
.  JPCERT/CC for their assistance in co-ordinating this disclosure in Japan. 


Contact Information

The NISCC Vulnerability Management Team can be contacted as follows:


Email vulteam () niscc gov uk 
(Please quote the advisory reference in the subject line.) 
Telephone +44 (0)20 7821 1330 Extension 4511 
(Monday to Friday 08:30 - 17:00) 
Fax +44 (0)20 7821 1686 
Post Vulnerability Management Team
NISCC
PO Box 832
London
SW1P 1BG 

We encourage those who wish to communicate via email to make use of our PGP key. This is available from 
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What is NISCC?

For further information regarding the UK National Infrastructure Security Co-Ordination Centre, please visit the NISCC 
web site at: 
http://www.niscc.gov.uk/aboutniscc/index.htm

Reference to any specific commercial product, process or service by trade name, trademark manufacturer or otherwise, 
does not constitute or imply its endorsement, recommendation, or favouring by NISCC. The views and opinions of authors 
expressed within this notice shall not be used for advertising or product endorsement purposes.

Neither shall NISCC accept responsibility for any errors or omissions contained within this advisory. In particular, 
they shall not be liable for any loss or damage whatsoever, arising from or in connection with the usage of information 
contained within this notice.

? 2004 Crown Copyright

Revision History

April 20, 2004:  Initial release (1.0) 
    

<End of NISCC Vulnerability Advisory>


-- 
David Mirza Ahmad
Symantec 

PGP: 0x26005712
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