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Category:   Device (Firewall)  >   Juniper ScreenOS Vendors:   Juniper, NetScreen
(Juniper Issues Fix for ScreenOS) Multiple Vendor TCP Stack Implementations Let Remote Users Deny Service
SecurityTracker Alert ID:  1009887
SecurityTracker URL:
CVE Reference:   CVE-2004-0230   (Links to External Site)
Date:  Apr 20 2004
Impact:   Denial of service via network
Fix Available:  Yes  Vendor Confirmed:  Yes  
Version(s): prior to 5.0r6
Description:   A vulnerability was reported in several TCP stack implementations. A remote user may be able to cause denial of service conditions using a TCP reset attack. Multiple vendors are affected, including Juniper's Screen OS (formerly NetScreen).

The UK National Infrastructure Security Co-Ordination Centre (NISCC) reported that some implementations of the Transmission Control Protocol (TCP) are particularly vulnerable to TCP reset attacks. A remote user can cause TCP sessions to terminate prematurely, causing denial of service conditions.

The specific impact on applications that use TCP depends on the mechanisms built into the application to address premature TCP session termination.

According to the report, NISCC considers the Border Gateway Protocol (BGP) to be one of the most affected applications, as it relies on a persistent TCP session between BGP peer entities. Premature termination of an underlying TCP session may require routing tables to be rebuilt and may cause "route flapping". In the case of BGP, using the TCP MD5 Signature Option and anti-spoofing measures can mitigate the vulnerability.

Other applications, such as Domain Name System (DNS) and (Secure Sockets Layer) SSL based applications may also be affected, but to a lesser degree, the report said.

A remote user can reportedly send a TCP packet with the RST (reset) flag set (or the SYN flag) with the appropriate spoofed source and destination IP addresses and TCP ports to cause the TCP session to be terminated. Ordinarily, the remote user may have the probability of 1 in 2^32 of guessing the correct sequence number, the report said. However, in actuality, a remote user may be able to guess an appropriate sequence number with much greater probability because many implementations will accept any sequence number within a certain window of the expected sequence number. The Associate Press reports that the proper number can be guessed within as few as four attempts, requiring only seconds to achieve.

The report credits Paul A. Watson for discovering a practical method for conducting TCP reset attacks (presented in "Slipping In The Window: TCP Reset Attacks" at the CanSecWest 2004 conference).

The report indicates that the following vendors are affected [this is not an inclusive list]:

- Cray Inc. is vulnerable on their UNICOS, UNICOS/mk and UNICOS/mp systems

- Check Point is affected, but has issued a protection mechanism in the latest release for VPN-1/FireWall-1 (R55 HFA-03) that can protect both the firewall device and hosts located behind the firewall.

- Internet Initiative Japan, Inc (IIJ) is affected.

- InterNiche NicheStack and NicheLite are affected.

- Juniper Networks products are affected.

- Cisco products are affected, including IOS and non-IOS based devices.

Other vendors are assessing the impact of this flaw.

The NISCC Vulnerability Advisory 236929 is available at:

Impact:   A remote user can cause denial of service on the target TCP session. The specific impact depends on the specific vendor implementation.
Solution:   Juniper has issued a fixed version of NetScreen ScreenOS (5.0r6), available at:

Juniper indicates that NetScreen firewall customers should do one or more of the following [quote]:
1) Configure Anti-Spoof protection as appropriate.
2) Use secure protocols such as ssh, HTTPS, BGP4 w/ MD5 Authentication and IPSec which are more resistant to attack.
3) Limit management to dedicated and/or internal interfaces
4) Upgrade to ScreenOS 5.0r6 which enhances the stateful firewall functionality to protect devices on the network.

Vendor URL: (Links to External Site)
Cause:   State error

Message History:   This archive entry is a follow-up to the message listed below.
Apr 20 2004 Multiple Vendor TCP Stack Implementations Let Remote Users Deny Service

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