Public Vulnerability Reports

Apple Mac OS X AppleTalk mbuf Kernel Heap Overflow Vulnerability

11.14.07

BACKGROUND

AppleTalk, a set of networking protocols developed by Apple, was originally implemented on early Mac operating systems. Although it is a legacy protocol, it is still supported on the latest version of Mac OS X. AppleTalk is compiled into the default kernel, but must be turned on in order to be used. More information can be found at the following URL.

http://docs.info.apple.com/article.html?artnum=50039

DESCRIPTION

Local exploitation of a heap based buffer overflow in Apple Inc.'s OS X may allow an attacker to execute arbitrary code in kernel context.

The vulnerability exists within a function responsible for allocating an mbuf. mbufs are a BSD concept, long used by BSD kernels to allocate buffers for storing network related data.

When allocating an mbuf buffer, the kernel performs a comparison using two signed integers, one of which is controlled by the user, to determine how many bytes to allocate. If a user passes a negative value, a minimally sized buffer will be allocated due to the signed comparison. The calling function will usually interpret the user controlled value as an unsigned value, and this results in the allocated buffer being overflowed.

ANALYSIS

Successful exploitation of this vulnerability will result in the execution of arbitrary code in kernel context. Unsuccessful attempts will likely crash the system. Exploitation has proven to be non-trivial.

In order to exploit this vulnerability, a system would have to have AppleTalk turned on. It would likely be used on a network consisting of older Mac hosts since previous versions of Mac relied on it to implement Apple File Sharing.

DETECTION

iDefense has confirmed the existence of this vulnerability in Mac OS X 10.4.10, Workstation and Server editions. Previous versions may also be affected.

To determine if AppleTalk is running, the following command can be executed on the command line.

 $ appletalk -s 

WORKAROUND

Disabling AppleTalk will prevent exploitation of this vulnerability. Executing the following command will disable AppleTalk if it is enabled.

 # appletalk -d 

VENDOR RESPONSE

Apple addressed this vulnerability within their Mac OS X 2007-008 security update. More information is available at the following URL.

http://docs.info.apple.com/article.html?artnum=307041

CVE INFORMATION

The Common Vulnerabilities and Exposures (CVE) project has assigned the name CVE-2007-4268 to this issue. This is a candidate for inclusion in the CVE list (http://cve.mitre.org/), which standardizes names for security problems.

DISCLOSURE TIMELINE

08/08/2007 Initial vendor notification
08/09/2007 Initial vendor response
11/14/2007 Coordinated public disclosure

CREDIT

This vulnerability was discovered by Sean Larsson of VeriSign iDefense Labs.

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LEGAL NOTICES

Copyright © 2007 Verisign, Inc.

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Disclaimer: The information in the advisory is believed to be accurate at the time of publishing based on currently available information. Use of the information constitutes acceptance for use in an AS IS condition. There are no warranties with regard to this information. Neither the author nor the publisher accepts any liability for any direct, indirect, or consequential loss or damage arising from use of, or reliance on, this information.