Samba nmbd Invalid Length Denial of Service Vulnerability

iDEFENSE Security Advisory 09.13.04a

www.idefense.com/application/poi/display?id=138&type=vulnerabilities

September 13, 2004

I. BACKGROUND

Samba is a software suite that provides file and print services to

SMB/CIFS clients, such as Microsoft Windows platforms. For more

information see:

http://www.samba.org/

II. DESCRIPTION

Remote exploitation of an input validation error in Samba allows an

attacker to crash the Samba nmbd server. The nmbd is a server, typically

listening on UDP port 138, understands and can reply to NetBIOS over IP

name service requests, and participates in the browsing protocols that

comprise the Windows "Network Neighborhood" view. Due to an input

validation error, a malformed UDP packet can cause the nmbd server to

crash while attempting to access memory outside of what is available.

The vulnerability specifically exists in the process_logon_packet()

function when it handles a SAM_UAS_CHANGE request. Part of this packet

contains a count of the number of structures that follow. No check is

made against the length of the packet to determine whether it is

possible to have as many structures in it as it claims. If a large value

is supplied, but a small number of structures are supplied, nmbd will

reference memory outside of the packet it has been supplied. This may

cause the nmbd process to crash.

The following is a trace of exploitation, showing the server no longer

responding to an nmblookup. The nmblookup tool is used to query NetBIOS

names and map them to IP addresses.

sh-2.05b$ nmblookup -A 10.1.0.240

Looking up status of 10.1.0.240

FEDORA1 <00> - B <ACTIVE>

FEDORA1 <03> - B <ACTIVE>

FEDORA1 <20> - B <ACTIVE>

..__MSBROWSE__. <01> - <GROUP> B <ACTIVE>

MYGROUP <00> - <GROUP> B <ACTIVE>

MYGROUP <1b> - B <ACTIVE>

MYGROUP <1c> - B <ACTIVE>

MYGROUP <1e> - <GROUP> B <ACTIVE>

sh-2.05b$ ./n 10.1.0.240 138 fedora1

Samba 3.x nmbd remote DoS exploit (0day)

Attacking 10.1.0.240:138 ..

Done, nmbd should be killed now.

sh-2.05b$ nmblookup -A 10.1.0.240

Looking up status of 10.1.0.240

sh-2.05b$

III. ANALYSIS

This vulnerability is only exploitable if the daemon has been configured

to process domain logons. This vulnerability does not allow arbitrary

code execution. When the nmbd process dies, it no longer returns

information about the server, and the host is no longer accessible by

referencing its name.

IV. DETECTION

iDEFENSE has confirmed Samba 3.0.2 is vulnerable. Analysis of the

source suggests that version 3.0.4 is also vulnerable. Samba 2.x does

not include the affected code and, therefore, is not affected by this

vulnerability. The line 'domain logons = yes' must also occur in

smb.conf for this issue to be exploitable. Note that removal of this

line from the configuration file, although it will prevent exploitation,

may also affect the Samba server's functionality.

The vendor has confirmed that Samba 3.0.x prior to and including v3.0.6

are vulnerable.

V. WORKAROUND

iDEFENSE is currently unaware of any effective workarounds for this

issue. Removal of the line "domain logons = yes" from the smb.conf

file

for the server will prevent exploitation but may also affect the Samba

server's functionality.

VI. VENDOR RESPONSE

The patch file for Samba 3.0.5 addressing [the] bug

(samba-3.0.5-DoS.patch) can be downloaded from:

http://download.samba.org/samba/ftp/patches/security/

VII. CVE INFORMATION

The Common Vulnerabilities and Exposures (CVE) project has assigned the

names CAN-2004-0808 to these issues. This is a candidate for inclusion

in the CVE list (http://cve.mitre.org),

which standardizes names for

security problems.

VIII. DISCLOSURE TIMELINE

09/02/2004 Initial vendor notification

09/02/2004 iDEFENSE clients notified

09/02/2004 Vendor response

09/13/2004 Coordinated public disclosure

IX. CREDIT

The discoverer of this vulnerability wishes to remain anonymous.

Get paid for vulnerability research

http://www.idefense.com/poi/teams/vcp.jsp

X. LEGAL NOTICES

Copyright (c) 2004 iDEFENSE, Inc.

Permission is granted for the redistribution of this alert

electronically. It may not be edited in any way without the express

written consent of iDEFENSE. If you wish to reprint the whole or any

part of this alert in any other medium other than electronically, please

email customerservice (at) idefense (dot) com [email concealed] for permission.

Disclaimer: The information in the advisory is believed to be accurate

at the time of publishing based on currently available information. Use

of the information constitutes acceptance for use in an AS IS condition.

There are no warranties with regard to this information. Neither the

author nor the publisher accepts any liability for any direct, indirect,

or consequential loss or damage arising from use of, or reliance on,

this information.

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