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Mac OS X vpnd local format string
May 29 2007 11:26AM
NGSSoftware Insight Security Research (nisr ngssoftware com)
Name: Mac OS X vpnd local format string
Release Date: 29 May 2007
Discover: Chris Anley <chris (at) ngssoftware (dot) com [email concealed]>
Vendor Reference: 26417237
Systems Affected: OS X Server 10.4.9 and prior
Discovered: 15 March 2007
Reported: 19 March 2007
Fixed: 24 May 2007
Published: 29 May 2007
The 'vpnd' command shipped with OS X runs setuid root, and is vulnerable
to a format string attack.
The vpnd command, when run with the '-i' parameter, is vulnerable to a
format string attack. The command is setuid root, and is world-executable.
This allows any local user to execute arbitrary code as root, though the
vulnerable code is only accessible by default on server versions of OS
X. It is possible for a client version of OS X to be configured in a
vulnerable manner, though this requires extensive configuration changes
and is unlikely to happen by accident.
Apple:~ shellcoders$ sw_vers
ProductName: Mac OS X Server
Apple:~ shellcoders$ vpnd -n -i _ABCD_%268\$x
2007-03-15 17:07:07 GMT Server '_ABCD_%268$x' starting...
2007-03-15 17:07:07 GMT Server ID '_ABCD_41424344' invalid
2007-03-15 17:07:07 GMT Error processing prefs file
#0 0x90011cb8 in __vfprintf ()
#1 0x9002a90c in vsnprintf ()
#2 0x9002a41c in vsyslog ()
#3 0x00003150 in vpnlog ()
#4 0x00004b80 in process_prefs ()
#5 0x000028d4 in main ()
The source code for vpnd is available from the Apple Darwin source code
download site. The relevant code is in the ppp package. The code is
distributed under the Apple Public Source License, available at
The bug occurs in the process_prefs() function in vpnoptions.c.
The user-specified server name is passed into the snprintf() function as
data, and the resulting string is then passed to the vpnlog() function,
as the format_str parameter. Although the server name is limited to 64
characters (with '%.64s') it is still straightforward to exploit the
bug, and NGS have written a reliable exploit.
This issue was fixed by Apple in Security Update 2007-005, released on
the 24th May 2007. NGS would like to thank the Apple Security Team for
their professional and prompt response to this issue.
NGSSoftware Insight Security Research
+44(0)208 401 0070
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