Dnsmasq < 2.50 - Heap Overflow / Null Pointer Dereference

-----BEGIN PGP SIGNED MESSAGE-----

Hash: SHA1



        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.

-----BEGIN PGP SIGNATURE-----

Version: GnuPG v1.4.7 (MingW32)

Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org



iD8DBQFKp9rOyNibggitWa0RAjkbAJ0SLIFwI1CMF7IOHSDv+Fg0DwFNQwCfWsZm

wa3syAdyXlixVdQhdk5vcK0=

=tfqM

-----END PGP SIGNATURE-----



# milw0rm.com [2009-09-09]