Cisco PIX SSH/telnet dDOS vulnerability CSCdy51810 Nov 05 2002 09:20PM
Nils Reichen (nreichen lanexpert ch) (1 replies)

Security Advisory 05.11.02:

Title : Cisco PIX SSH/telnet DOS vulnerability CSCdy51810

Reporter : Nils Reichen LANexpert SA

Affected software : PIX OS 6.2.2 (and probably old version)

Risk : High

Date : November 5, 2002

URL: Full description should be posted in few days on


[1] Summary

A vulnerability in the TCP/IP stack allow a remote

attacker run a

denial of service attack against the PIX firewall.

This vulnerability is due to a wrong handling of the

subnet address

by the PIX OS stack. If the SSH or telnet daemon is

used, the PIX

will answer to connection request sent to the subnet


The use of the subnet address as destination bypass the


other filter.

DDOS attack exploiting this vulnerability may produce

memory fragmentation.

[2] Affected software

Version 6.2.2 has been confirmed as vulnerable.

Older versions have not been confirmed, but due to the

stack level of this vulnerability, they could be

supposed vulnerable.

[3] Patch

Interim build is available for Cisco


through the Cisco Technical Assistance Center


[4] Testing environments

PIX 515

OS version 6.2.2

Only few inside hosts allowed to access the PIX using

SSH / telnet

Naptha tool v1.1 from BindView's RAZOR Security Team

[5] Required Knowledge


PIX firewall

[6] Technical Details

The PIX firewall respond to TCP SYN packet sent to the

subnet address (first IP of the subnet) for SSH and

telnet service. This behavior is seen if at least one

inside host is allowed to access the PIX using SSH

and/or telnet. In this case,

the TCP three-way handshaking will be completed for any

external host targeting

the subnet address.

Test conduced with the Naptha tool v1.1 (from

BindView's RAZOR team) show the free memory counter

displayed with the "show memory" decreasing. This test

was performed by opening

TCP connections using subnet address as destination and

never close it.

The "show memory" command shows the largest contiguous

free memory.

During the test using one attacker host, the free

memory shown decreased at 8kbytes/sec.

A DDOS attack may lead to fragment all the free memory.

TCPdump trace:

07:32:38.409393 > MY.NET.97.128.22:

S 1381191936:1381191936(0) win 32120 <mss

1460,sackOK,timestamp 542804848[|tcp]> (DF) (ttl 48, id

33829, len 60)

07:32:38.409556 MY.NET.97.128.22 >

S [tcp sum ok] 872965664:872965664(0) ack 1381191937

win 4096 <mss 1460> (ttl 255, id 6173, len 44)

07:32:38.452593 > MY.NET.97.128.22:

. [tcp sum ok] 1:1(0) ack 1 win 32120 (DF) (ttl 48, id

33831, len 40)

07:32:38.453312 MY.NET.97.254.22 >

P 872965665:872965684(19) ack 1381191937 win 4096 (ttl

255, id 6174, len 59)

07:32:38.497426 > MY.NET.97.254.22:

R [tcp sum ok] 1381191937:1381191937(0) win 0 (ttl 239,

id 33833, len 40)

07:32:48.482733 > MY.NET.97.128.22:

F [tcp sum ok] 1:1(0) ack 1 win 32120 (DF) (ttl 48, id

34171, len 40)

[7] Exploit Code

Not included in this advisory on purpose

[8] Workaround

Filter inbound SSH and telnet traffic targeted to the

PIX external subnet address

and interface address on the upstream router.

[9] Timeline

Aug 28, 2002 Issue discovered

Aug 30, 2002 Vendor notified, Cisco Systems PSIRT team

notified by the TAC

Sep 03, 2002 Vendor confirmed the issue, bug referenced:


Sep 04, 2002 IDS Europe mailing list notified

Sep 13, 2002 New build with fix available

Nov 05, 2002 Public disclosure on Bugtraq mailing list

[10] Correlation / Vendor status

No Common Vulnerabilities and Exposures (CVE)

identification assigned now

Vendor referenced ID for this issue: CSCdy51810

New OS build with patch released:

Vendor don't have plans to issue security advisory,

vendor PSIRT

(Product Security Incident Response Team) is

considering the problem as "unexpected behavior".

[11] Credit

Discovery and exploitation Research:

Nils Reichen GCIA CCIE#6763

nreichen (at) lanexpert (dot) ch [email concealed]

[12] Disclaimer

The information within this paper may change without


Use of this information constitutes acceptance for use

in an AS IS

condition. There are NO warranties with regard to this


In no event shall the author or an entity where he

belongs be liable

for any damages whatsoever arising out of or

in connection with the use or spread of this information.

Any use of this information is at the user's own risk.

[13] Feedback

Please send suggestions, updates, and comments to:

Nils Reichen LANexpert SA


Official : nreichen (at) lanexpert (dot) ch [email concealed]

[ reply ]
Re: Cisco PIX SSH/telnet dDOS vulnerability CSCdy51810 Nov 11 2002 05:52PM
Sharad Ahlawat (sahlawat cisco com)


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