Core Security Technologies Advisory 2009.0820

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Core Security Technologies - CoreLabs Advisory  
http://www.coresecurity.com/corelabs/  
  
Dnsmasq Heap Overflow and Null-pointer Dereference on TFTP Server  
  
  
1. *Advisory Information*  
  
Title: Dnsmasq Heap Overflow and Null-pointer Dereference on TFTP Server  
Advisory ID: CORE-2009-0820  
Advisory URL: http://www.coresecurity.com/content/dnsmasq-vulnerabilities  
Date published: 2009-08-31  
Date of last update: 2009-08-31  
Vendors contacted: Simon Kelley  
Release mode: Coordinated release  
  
  
2. *Vulnerability Information*  
  
Class: Buffer overflow  
Remotely Exploitable: Yes  
Locally Exploitable: No  
Bugtraq ID: 36120, 36121  
CVE Name: CVE-2009-2957, CVE-2009-2958  
  
  
3. *Vulnerability Description*  
  
Dnsmasq is a lightweight DNS forwarder and DHCP server. A  
vulnerability has been found that may allow an attacker to execute  
arbitrary code on servers or home routers running dnsmasq[1] with the  
TFTP service[2][3] enabled ('--enable-tfp'). This service is not  
enabled by default on most distributions; in particular it is not  
enabled by default on OpenWRT or DD-WRT. Chances of successful  
exploitation increase when a long directory prefix is used for TFTP.  
Code will be executed with the privileges of the user running dnsmasq,  
which is normally a non-privileged one.  
  
Additionally there is a potential DoS attack to the TFTP service by  
exploiting a null-pointer dereference vulnerability.  
  
  
4. *Vulnerable packages*  
  
. dnsmasq 2.40.  
. dnsmasq 2.41.  
. dnsmasq 2.42.  
. dnsmasq 2.43.  
. dnsmasq 2.44.  
. dnsmasq 2.45.  
. dnsmasq 2.46.  
. dnsmasq 2.47.  
. dnsmasq 2.48.  
. dnsmasq 2.49.  
. Older versions are probably affected too, but they were not checked.  
  
  
5. *Non-vulnerable packages*  
  
. dnsmasq 2.50  
  
  
6. *Vendor Information, Solutions and Workarounds*  
  
If the TFTP service is enabled and patching is not available  
immediately, a valid workaround is to filter TFTP for untrusted hosts  
in the network (such as the Internet). This is the default  
configuration when enabling TFTP on most home routers.  
  
Patches are already available from the software author. Most  
distributions should release updates for binary packages soon.  
  
  
7. *Credits*  
  
The heap-overflow vulnerability (CVE-2009-2957) was discovered during  
Bugweek 2009 by Pablo Jorge and Alberto Solino from the team "Los  
Herederos de Don Pablo" of Core Security Technologies.  
  
The null-pointer dereference (CVE-2009-2958) was reported to the  
author of dnsmasq independently by an uncredited code auditor. It was  
merged with this advisory for user's convenience.  
  
  
8. *Technical Description*  
  
  
8.1. *Heap Overflow vulnerability (CVE-2009-2957, BID 36121)*  
  
First let's focus on the overflow vulnerability. The 'tftp_request'  
calls 'strncat' on 'daemon->namebuff', which has a predefined size of  
'MAXDNAME' bytes (defaulting to 1025).  
  
/-----------  
  
else if (filename[0] == '/')  
daemon->namebuff[0] = 0;  
strncat(daemon->namebuff, filename, MAXDNAME);  
  
- -----------/  
  
  
  
This may cause a heap overflow because 'daemon->namebuff' may already  
contain data, namely the configured 'daemon->tftp_prefix' passed to  
the daemon via a configuration file.  
  
/-----------  
  
if (daemon->tftp_prefix)  
{  
if (daemon->tftp_prefix[0] == '/')  
daemon->namebuff[0] = 0;  
strncat(daemon->namebuff, daemon->tftp_prefix, MAXDNAME)  
  
- -----------/  
  
  
  
The default prefix is '/var/tftpd', but if a longer prefix is used,  
arbitrary code execution may be possible.  
  
Sending the string resulting from the execution of the following  
python snippet to a vulnerable server, with a long enough directory  
prefix configured, should crash the daemon.  
  
/-----------  
  
import sys  
sys.stdout.write( '\x00\x01' + "A"*1535 + '\x00' + "netascii" + '\x00' )  
  
- -----------/  
  
  
  
  
8.2. *Null-pointer Dereference vulnerability (CVE-2009-2958, BID 36120)*  
  
Now onto the null-pointer dereference. The user can crash the service  
by handcrafting a packet, because of a problem on the guard of the  
first if inside this code loop:  
  
/-----------  
  
while ((opt = next(&p, end)))  
{  
if (strcasecmp(opt, "blksize") == 0 &&  
(opt = next(&p, end)) &&  
!(daemon->options & OPT_TFTP_NOBLOCK))  
{  
transfer->blocksize = atoi(opt);  
if (transfer->blocksize < 1)  
transfer->blocksize = 1;  
if (transfer->blocksize > (unsigned)daemon->packet_buff_sz - 4)  
transfer->blocksize = (unsigned)daemon->packet_buff_sz - 4;  
transfer->opt_blocksize = 1;  
transfer->block = 0;  
}  
  
if (strcasecmp(opt, "tsize") == 0 && next(&p, end) &&  
!transfer->netascii)  
{  
transfer->opt_transize = 1;  
transfer->block = 0;  
}  
}  
  
- -----------/  
  
The problem exists because the guard of the first if includes the  
result of 'opt = next(&p, end)' as part of the check. If this returns  
'NULL', the guard will fail and in the next if 'strcasecmp(opt,  
"tsize")' will derrefence the null-pointer.  
  
  
9. *Report Timeline*  
  
. 2009-08-20:  
Core Security Technologies notifies Simon Kelley of the vulnerability,  
including technical details of the vulnerability in an advisory draft.  
  
. 2009-08-21:  
Simon Kelley acknowledges the vulnerability and confirms to be working  
on a patch. He also informs that he is aware that most home router  
distributions have tftp turned off by default, and firewalled, and  
suggests this should be mentioned on the advisory. Simon also mentions  
that a NULL-pointer dereference bug has also been discovered on that  
code, and suggests merging both bugs in the same advisory. Monday  
31/08 is accepted as a possible release date for this advisory, and  
help is offered in contacting package maintainers of dnsmasq for most  
operating systems.  
  
. 2009-08-21:  
Core changes the advisory draft to accommodate Simon's suggestions.  
About the NULL-pointer dereference, Core mentions the terms it thinks  
appropriate for the bug to be merged into this advisory, and details  
how this would affect the following procedures, such as asking for a  
CVE/Bugtraq ID.  
  
. 2009-08-23:  
Simon Kelley contacts Core back, saying that the terms for the  
null-pointer derrefence bug to be included in the advisory are ok. He  
also mentions that the finder of this bug prefers to remain uncredited  
in this advisory. Details are sent by him about the new bug so that  
the advisory draft can be updated to include it.  
  
. 2009-08-23:  
Core asks for proper CVE and Bugtraq ID numbers, specifying it  
believes each vulnerability reported in this advisory should be  
assigned its own.  
  
. 2009-08-23:  
Vincent Danen, from Red Hat's Security Response Team contacts Core in  
order to discuss both vulnerabilities by a secure communications  
channel, and offers its help in obtaining proper CVE numbers,  
specifying they also believe a separate number should be assigned to  
each vulnerability.  
  
. 2009-08-23:  
Core replies to Vincent Danen by sending its gpg key. Core also  
mentions separate CVE numbers have already been asked.  
  
. 2009-08-23:  
Core replies to Simon Kelley, including a new advisory draft with both  
bugs merged.  
  
. 2009-08-23:  
Core receives proper CVE and Bugtraq ID numbers for both bugs, and  
sends them to Red Hat and Simon Kelley.  
  
. 2009-08-31:  
The advisory CORE-2009-0820 is published.  
  
  
  
10. *References*  
  
[1] http://www.thekelleys.org.uk/dnsmasq/doc.html  
[2] http://www.isi.edu/in-notes/ien/ien133.txt  
[3] http://en.wikipedia.org/wiki/Trivial_File_Transfer_Protocol  
  
  
11. *About CoreLabs*  
  
CoreLabs, the research center of Core Security Technologies, is  
charged with anticipating the future needs and requirements for  
information security technologies. We conduct our research in several  
important areas of computer security including system vulnerabilities,  
cyber attack planning and simulation, source code auditing, and  
cryptography. Our results include problem formalization,  
identification of vulnerabilities, novel solutions and prototypes for  
new technologies. CoreLabs regularly publishes security advisories,  
technical papers, project information and shared software tools for  
public use at: http://www.coresecurity.com/corelabs.  
  
  
12. *About Core Security Technologies*  
  
Core Security Technologies develops strategic solutions that help  
security-conscious organizations worldwide develop and maintain a  
proactive process for securing their networks. The company's flagship  
product, CORE IMPACT, is the most comprehensive product for performing  
enterprise security assurance testing. CORE IMPACT evaluates network,  
endpoint and end-user vulnerabilities and identifies what resources  
are exposed. It enables organizations to determine if current security  
investments are detecting and preventing attacks. Core Security  
Technologies augments its leading technology solution with world-class  
security consulting services, including penetration testing and  
software security auditing. Based in Boston, MA and Buenos Aires,  
Argentina, Core Security Technologies can be reached at 617-399-6980  
or on the Web at http://www.coresecurity.com.  
  
  
13. *Disclaimer*  
  
The contents of this advisory are copyright (c) 2009 Core Security  
Technologies and (c) 2009 CoreLabs, and may be distributed freely  
provided that no fee is charged for this distribution and proper  
credit is given.  
  
  
14. *PGP/GPG Keys*  
  
This advisory has been signed with the GPG key of Core Security  
Technologies advisories team, which is available for download at  
http://www.coresecurity.com/files/attachments/core_security_advisories.asc.  
  
  
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