OpenBSD's IPv6 mbufs remote kernel buffer overflow
Core Security Technologies - CoreLabs Advisory
Date Published: 2007-03-13
Last Update: 2007-03-13
Advisory ID: CORE-2007-0219
Bugtraq ID: 22901
CVE Name: CVE-2007-1365
Title: OpenBSD's IPv6 mbufs remote kernel buffer overflow
Class: Buffer Overflow
Remotely Exploitable: Yes
Locally Exploitable: No
- 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 chain.
- 2007-03-05: Core develops proof of concept code that demonstrates remote code execution in the kernel context by exploiting the mbuf overflow.
- 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 website.
- 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.
Release Mode: FORCED RELEASE
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 configuration.
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.
OpenBSD 4.1 prior to Feb. 26th, 2006.
OpenBSD 4.0 Current
OpenBSD 4.0 Stable
All other releases that implement the IPv6 protocol stack may be vulnerable.
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, or;
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):
We can see that the mbuf contains another structure of type m_ext (/sys/mbuf.h):
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.
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