__________________________________________________________ The U.S. Department of Energy Computer Incident Advisory Capability ___ __ __ _ ___ / | /_\ / \___ __|__ / \ \___ __________________________________________________________ INFORMATION BULLETIN Samba smbd Buffer Overrun Vulnerability [Samba 2.2.8 Upgrade] March 17, 2003 19:00 GMT Number N-055 [REVISED June 2, 2003] ______________________________________________________________________________ PROBLEM: A buffer overrun condition exists in the SMB/CIFS packet fragment re-assembly code in Samba's smbd which would allow an attacker to cause smbd to overwrite arbitrary areas of memory in its own process address space. This could allow a skilled attacker to inject binary specific exploit code into smbd. PLATFORM: Samba 2.0.x to 2.2.7a inclusive. DAMAGE: An external attacker could remotely and anonymously gain Super User (root) privileges. SOLUTION: If you are running Samba 2.2.7 or above apply the appropriate patch. If you are running an earlier version of Samba, upgrade to version 2.2.8 or prohibit external access to TCP ports 139 and 445. ______________________________________________________________________________ VULNERABILITY The risk is HIGH. The SMB/CIFS protocol implemented by Samba is ASSESSMENT: vulnerable to many attacks, even without specific security holes. The details of this vulnerability have been distributed via the Internet. ______________________________________________________________________________ LINKS: CIAC BULLETIN: http://www.ciac.org/ciac/bulletins/n-055.shtml ORIGINAL BULLETIN: http://www.samba.org/samba/whatsnew/samba-2.2.8.html ADDITIONAL Sun Alert #53580 - patches for Solaris 9 INFORMATION: http://sunsolve.sun.com/pub-cgi/retrieve.pl?doc=fsalert% 2F53580&zone_32=category%3Asecurity ______________________________________________________________________________ Revision History 6/2/03 - added link to Sun Alert #53580 for information on Solaris 9 patches. [***** Start Samba 2.2.8 Upgrade *****] The Samba Team announces Samba 2.2.8 **************************************** * IMPORTANT: Security bugfix for Samba * **************************************** This release provides an important security fix outlined in the release notes that follow. This is the latest stable release of Samba and the version that all production Samba servers should be running for all current bug-fixes. The source code can be downloaded from : http://download.samba.org/samba/ftp/ in the file samba-2.2.8.tar.gz or samba-2.2.8.tar.bz2. Both archives have been signed using the Samba Distribution Key. Binary packages will be released shortly for major platforms and can be found at http://download.samba.org/samba/ftp/Binary_Packages/ As always, all bugs are our responsibility. --Sincerely The Samba Team Summary ------- The SuSE security audit team, in particular Sebastian Krahmer, has found a flaw in the Samba main smbd code which could allow an external attacker to remotely and anonymously gain Super User (root) privileges on a server running a Samba server. This flaw exists in previous versions of Samba from 2.0.x to 2.2.7a inclusive. This is a serious problem and all sites should either upgrade to Samba 2.2.8 immediately or prohibit access to TCP ports 139 and 445. Advice created by Andrew Tridgell, the leader of the Samba Team, on how to protect an unpatched Samba server is given at the end of this section. The SMB/CIFS protocol implemented by Samba is vulnerable to many attacks, even without specific security holes. The TCP ports 139 and the new port 445 (used by Win2k and the Samba 3.0 alpha code in particular) should never be exposed to untrusted networks. Description ----------- A buffer overrun condition exists in the SMB/CIFS packet fragment re-assembly code in smbd which would allow an attacker to cause smbd to overwrite arbitrary areas of memory in its own process address space. This could allow a skilled attacker to inject binary specific exploit code into smbd. This version of Samba adds explicit overrun and overflow checks on fragment re-assembly of SMB/CIFS packets to ensure that only valid re-assembly is performed by smbd. In addition, the same checks have been added to the re-assembly functions in the client code, making it safe for use in other services. Credit ------ This security flaw was discovered and reported to the Samba Team by Sebastian Krahmer of the SuSE Security Audit Team. The fix was prepared by Jeremy Allison and reviewed by engineers from the Samba Team, SuSE, HP, SGI, Apple, and the Linux vendor engineers on the Linux Vendor security mailing list. The Samba Team would like to thank SuSE and Sebastian Krahmer for their excellent auditing work and for drawing attention to this flaw. Patch Availability ----------------- As this is a security issue, patches for this flaw specific to earlier versions of Samba will be posted on the samba-technical@samba.org mailing list as requested. ************************************ Protecting an unpatched Samba server ************************************ Samba Team, March 2003 This is a note on how to provide your Samba server some protection against the recently discovered remote security hole if you are unable to upgrade to the fixed version immediately. Even if you do upgrade you might like to think about the suggestions in this note to provide you with additional levels of protection. Using host based protection --------------------------- In many installations of Samba the greatest threat comes for outside your immediate network. By default Samba will accept connections from any host, which means that if you run an insecure version of Samba on a host that is directly connected to the Internet you can be especially vulnerable. One of the simplest fixes in this case is to use the 'hosts allow' and 'hosts deny' options in the Samba smb.conf configuration file to only allow access to your server from a specific range of hosts. An example might be: hosts allow = 127.0.0.1 192.168.2.0/24 192.168.3.0/24 hosts deny = 0.0.0.0/0 The above will only allow SMB connections from 'localhost' (your own computer) and from the two private networks 192.168.2 and 192.168.3. All other connections will be refused connections as soon as the client sends its first packet. The refusal will be marked as a 'not listening on called name' error. Using interface protection -------------------------- By default Samba will accept connections on any network interface that it finds on your system. That means if you have a ISDN line or a PPP connection to the Internet then Samba will accept connections on those links. This may not be what you want. You can change this behavior using options like the following: interfaces = eth* lo bind interfaces only = yes that tells Samba to only listen for connections on interfaces with a name starting with 'eth' such as eth0, eth1, plus on the loopback interface called 'lo'. The name you will need to use depends on what OS you are using. In the above I used the common name for ethernet adapters on Linux. If you use the above and someone tries to make a SMB connection to your host over a PPP interface called 'ppp0', they will get a TCP connection refused reply. In that case no Samba code is run at all as the operating system has been told not to pass connections from that interface to any process. Using a firewall ---------------- Many people use a firewall to deny access to services that they don't want exposed outside their network. This can be a very good idea, although I would recommend using it in conjunction with the above methods so that you are protected even if your firewall is not active for some reason. If you are setting up a firewall then you need to know what TCP and UDP ports to allow and block. Samba uses the following: UDP/137 - used by nmbd UDP/138 - used by nmbd TCP/139 - used by smbd TCP/445 - used by smbd The last one is important as many older firewall setups may not be aware of it, given that this port was only added to the protocol in recent years. Using a IPC$ share deny ----------------------- If the above methods are not suitable, then you could also place a more specific deny on the IPC$ share that is used in the recently discovered security hole. This allows you to offer access to other shares while denying access to IPC$ from potentially untrustworthy hosts. To do that you could use: [ipc$] hosts allow = 192.168.115.0/24 127.0.0.1 hosts deny = 0.0.0.0/0 this would tell Samba that IPC$ connections are not allowed from anywhere but the two listed places (localhost and a local subnet). Connections to other shares would still be allowed. As the IPC$ share is the only share that is always accessible anonymously this provides some level of protection against attackers that do not know a username/password for your host. If you use this method then clients will be given a 'access denied' reply when they try to access the IPC$ share. That means that those clients will not be able to browse shares, and may also be unable to access some other resources. I don't recommend this method unless you cannot use one of the other methods listed above for some reason. Upgrading Samba --------------- Of course the best solution is to upgrade Samba to a version where the bug has been fixed. If you wish to also use one of the additional measures above then that would certainly be a good idea. Please check regularly on http://www.samba.org/ for updates and important announcements. **************************************** **************************************** [***** End Samba 2.2.8 Upgrade *****] _______________________________________________________________________________ CIAC wishes to acknowledge the contributions of Samba for the information contained in this bulletin. _______________________________________________________________________________ CIAC, the Computer Incident Advisory Capability, is the computer security incident response team for the U.S. Department of Energy (DOE) and the emergency backup response team for the National Institutes of Health (NIH). CIAC is located at the Lawrence Livermore National Laboratory in Livermore, California. CIAC is also a founding member of FIRST, the Forum of Incident Response and Security Teams, a global organization established to foster cooperation and coordination among computer security teams worldwide. CIAC services are available to DOE, DOE contractors, and the NIH. CIAC can be contacted at: Voice: +1 925-422-8193 (7x24) FAX: +1 925-423-8002 STU-III: +1 925-423-2604 E-mail: ciac@ciac.org Previous CIAC notices, anti-virus software, and other information are available from the CIAC Computer Security Archive. World Wide Web: http://www.ciac.org/ Anonymous FTP: ftp.ciac.org PLEASE NOTE: Many users outside of the DOE, ESnet, and NIH computing communities receive CIAC bulletins. If you are not part of these communities, please contact your agency's response team to report incidents. Your agency's team will coordinate with CIAC. The Forum of Incident Response and Security Teams (FIRST) is a world-wide organization. A list of FIRST member organizations and their constituencies can be obtained via WWW at http://www.first.org/. This document was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor the University of California nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial products, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation or favoring by the United States Government or the University of California. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or the University of California, and shall not be used for advertising or product endorsement purposes. 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