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CodeQL workshop for JavaScript: Finding unsafe calls to the jQuery $ function

  • Analyzed language: JavaScript
  • Difficulty level: 200

Overview

Problem statement

jQuery is an extremely popular, but old, open source JavaScript library designed to simplify things like HTML document traversal and manipulation, event handling, animation, and Ajax. The jQuery library supports modular plugins to extend its capabilities. Bootstrap is another popular JavaScript library, which has used jQuery's plugin mechanism extensively. However, the jQuery plugins inside Bootstrap used to be implemented in an unsafe way that could make the users of Bootstrap vulnerable to cross-site scripting (XSS) attacks. This is when an attacker uses a web application to send malicious code, generally in the form of a browser side script, to a different end user.

Four such vulnerabilities in Bootstrap jQuery plugins were fixed in this pull request, and each was assigned a CVE.

The core mistake in these plugins was the use of the omnipotent jQuery $ function to process the options that were passed to the plugin. For example, consider the following snippet from a simple jQuery plugin:

let text = $(options.textSrcSelector).text();

This plugin decides which HTML element to read text from by evaluating options.textSrcSelector as a CSS-selector, or that is the intention at least. The problem in this example is that $(options.textSrcSelector) will execute JavaScript code instead if the value of options.textSrcSelector is a string like "<img src=x onerror=alert(1)>". The values in options cannot always be trusted.

In security terminology, jQuery plugin options are a source of user input, and the argument of $ is an XSS sink.

The pull request linked above shows one approach to making such plugins safer: use a more specialized, safer function like $(document).find instead of $.

let text = $(document).find(options.textSrcSelector).text();

In this challenge, we will use CodeQL to analyze the source code of Bootstrap, taken from before these vulnerabilities were patched, and identify the vulnerabilities.

Setup instructions for Visual Studio Code

To take part in the workshop you will need to follow these steps to get the CodeQL development environment set up:

  1. Install the Visual Studio Code IDE.
  2. Download and install the CodeQL extension for Visual Studio Code. Full setup instructions are here.
  3. Set up the starter workspace.
    • Important: Don't forget to git clone --recursive or git submodule update --init --remote, so that you obtain the standard query libraries.
  4. Open the starter workspace: File > Open Workspace > Browse to vscode-codeql-starter/vscode-codeql-starter.code-workspace.
  5. Download the esbena_bootstrap-pre-27047_javascript CodeQL database.
  6. Unzip the database.
  7. Import the unzipped database into Visual Studio Code:
    • Click the CodeQL icon in the left sidebar.
    • Place your mouse over Databases, and click the + sign that appears on the right.
    • Choose the unzipped database directory on your filesystem.
  8. Create a new file, name it UnsafeDollarCall.ql, save it under codeql-custom-queries-javascript.

Documentation links

If you get stuck, try searching our documentation and blog posts for help and ideas. Below are a few links to help you get started:

Workshop

The workshop is split into several steps. You can write one query per step, or work with a single query that you refine at each step.

Each step has a Hint that describe useful classes and predicates in the CodeQL standard libraries for JavaScript and keywords in CodeQL. You can explore these in your IDE using the autocomplete suggestions (Ctrl+Space) and jump-to-definition command (F12).

Each step has a Solution that indicates one possible answer. Note that all queries will need to begin with import javascript, but for simplicity this may be omitted below.

Finding calls to the jQuery $ function

  1. Find all function call expressions, such as alert("hello world") and speaker.sayHello("world").

    Hint

    A function call is called a CallExpr in the CodeQL JavaScript library.

    Solution 
    from CallExpr dollarCall
select dollarCall

  2. Identify the expression that is used as the first argument for each call, , such as alert(<first-argument>) and speaker.sayHello(<first-argument>).

    Hint
    • Add another variable to your from clause. This can be named dollarArg and have type Expr.
    • Add a where clause.
    • CallExpr has a predicate getArgument(int) to find the argument at a 0-based index.
    Solution 
    from CallExpr dollarCall, Expr dollarArg
where dollarArg = dollarCall.getArgument(0)
select dollarArg

  3. Filter your results to only those calls to a function named $, such as $("hello world") and speaker.$("world").

    Hint
    • CallExpr has a predicate getCalleeName() to find the name of the function being called.
    • Use the and keyword to add conditions to your query.
    • Use the = operator to assert that two values are equal.
    Solution 
    from CallExpr dollarCall, Expr dollarArg
where
  dollarArg = dollarCall.getArgument(0) and
  dollarCall.getCalleeName() = "$"
select dollarArg

  4. So far we have looked for the function name $. Are there other ways of calling the jQuery $ function? Perhaps the CodeQL library can handle these for us?

    The CodeQL standard library for JavaScript has a built-in predicate jquery() to describe references to $. Expand the hint for details, and modify your query to use it.

    Hint
    • Calling the predicate jquery() returns all values that refer to the $ function.
    • To find all calls to this function, use the predicate getACall().
    • Notice that when you call jquery(), getACall(), and getAnArgument() in succession, you get return values of type DataFlow::Node, not Expr. These are data flow nodes. They describe a part of the source program that may have a value, and let us do more complex reasoning about this value. We'll learn more about these in the next section.
    • You can change your dollarArg variable to have type DataFlow::Node, or convert the data flow node back into an Expr using the predicate asExpr().
    Solution 
    from Expr dollarArg
where
  dollarArg = jquery().getACall().getArgument(0).asExpr()
select dollarArg

    OR

    from DataFlow::Node dollarArg
where
  dollarArg = jquery().getACall().getArgument(0)
select dollarArg

Finding jQuery plugin options

jQuery plugins are usually defined by assigning a value to a property of the $.fn object:

$.fn.copyText = function() { ... } // this function is a jQuery plugin

In this step, we will find such plugins, and their options.

Consider creating a new query for these next few steps, or commenting out your earlier solutions and using the same file. We will use the earlier solutions again in the next section.

  1. You have already seen how to find references to the jQuery $ function. Now find all places in the code that read the property $.fn.

    Hint - Declare a new variable of type `DataFlow::Node` to hold the results. - Notice that `jQuery()` returns a value of type `DataFlow::SourceNode`. Source nodes are places in the program that introduce a new value, from which the flow of data may be tracked. - `DataFlow::SourceNode` has a predicate named `getAPropertyRead(string)`, which finds all reads of a particular property on the same object. The string argument is the name of the property.
    Solution 
    from DataFlow::Node n
where n = jquery().getAPropertyRead("fn")
select n

  2. Find the functions that are assigned to a property of $.fn. These are jQuery plugins.

    Remember the previous example:

    $.fn.copyText = function() { ... } // this function is a jQuery plugin

    There might be some variation in how this code is written. For example, we might see intermediate assignments to local variables:

    let fn = $.fn
let f = function() { ... } // this function is a jQuery plugin
fn.copyText = f

    The use of intermediate variables and nested expressions are typical source code examples that require use of local data flow analysis to detect.

    Data flow analysis helps us answer questions like: does this expression ever hold a value that originates from a particular other place in the program?

    We have already encountered data flow nodes, described by the DataFlow::Node CodeQL class. They are places in the program that have a value. They are returned by useful predicates like jquery() in the library.

    These nodes are separate and distinct from the AST (Abstract Syntax Tree, which represents the basic structure of the program) nodes, to allow for flexibility in how data flow is modeled.

    We can visualize the data flow analysis problem as one of finding paths through a directed graph, where the nodes of the graph are data flow nodes, and the edges represent the flow of data between those elements. If a path exists, then the data flows between those two nodes.

    The CodeQL JavaScript data flow library is very expressive. It has several classes that describe different places in the program that can have a value. We have seen SourceNodes; there are many other forms such as ValueNodes, FunctionNodes, ParameterNodes, and CallNodes. You can find our more in the documentation.

    When we are looking for the flow of information to or from these nodes within a single function or scope, this is called local data flow analysis. The CodeQL library has several predicates available on different types of data flow node that reason about local data flow.

    You have already seen one such predicate: SourceNode.getAPropertyRead(). To complete this step of the workshop, look at the hint for another useful predicate.

    Hint
    • DataFlow::SourceNode has a predicate named getAPropertySource(), which finds a source node whose value is stored in a property of this node.
    • In the previous step, we used getAPropertyRead(string) to identify the source node $.fn. Now try to find a value stored in a property of this source node $.fn.
    Solution 
    from DataFlow::Node plugin
where plugin = jquery().getAPropertyRead("fn").getAPropertySource()
select plugin

  3. Find the last parameter of the jQuery plugin functions that you identified in the previous step. These parameters are the plugin options.

    Hint
    • Modify your from clause so that the variable that describes that jQuery plugin is of type DataFlow::FunctionNode. As the name suggests, this is a data flow node that refers to a function definition.
    • DataFlow::FunctionNode has a predicate named getLastParameter().
    • If you want to add a new variable to describe the parameter, it can be of type DataFlow::ParameterNode.
    Solution 
    from DataFlow::FunctionNode plugin, DataFlow::ParameterNode optionsParam
where
  plugin = jquery().getAPropertyRead("fn").getAPropertySource() and
  optionsParam = plugin.getLastParameter()
select plugin, optionsParam

Putting it all together

We have now identified (a) places in the program which receive jQuery plugin options (which may be untrusted data) and (b) places in the program which are passed to the jQuery $ function and may be interpreted as HTML. We now want to tie these two together to ask: does the untrusted data from a jQuery plugin option ever flow to the potentially unsafe $ call?

This is also a data flow problem. However, it is larger in scope that the problems we have tackled so far, because the plugin options and the $ call may be in different functions. We call this a global data flow problem.

In this section we will create a path problem query capable of looking for global data flow, by populating this template:

/**
 * @name Cross-site scripting vulnerable plugin
 * @kind path-problem
 * @id js/xss-unsafe-plugin
 */
import javascript
import DataFlow::PathGraph

class Config extends TaintTracking::Configuration {
  Config() { this = "Config" }
  override predicate isSource(DataFlow::Node source) {
    exists(/** TODO fill me in from Section 2 **/ |
      source = /** TODO fill me in from Section 2 **/
    )
  }
  override predicate isSink(DataFlow::Node sink) {
    sink = /** TODO fill me in from Section 1 **/
  }
}

from Config config, DataFlow::PathNode source, DataFlow::PathNode sink
where config.hasFlowPath(source, sink)
select sink, source, sink, "Potential XSS vulnerability in plugin."

  1. Complete the isSource predicate using the query you wrote for Section 2.

    Hint
    • You can translate from a query clause to a predicate by:
      • Converting the variable declarations in the from part to the variable declarations of an exists
      • Placing the where clause conditions (if any) in the body of the exists
      • Adding a condition which equates the select to one of the parameters of the predicate.
    • Remember that the source of untrusted data is the jQuery plugin options parameter.
    Solution 
      override predicate isSource(DataFlow::Node source) {
    exists(DataFlow::FunctionNode plugin |
      plugin = jquery().getAPropertyRead("fn").getAPropertySource() and
      source = plugin.getLastParameter()
    )
  }

  2. Complete the isSink predicate by using the query you wrote for Section 1.

    Hint
    • Complete the same process as above.
    • We already found a DataFlow::Node in Section 1 as the result of calling jquery() and predicates on it.
    • Remember that the first argument of a call to $ is a sink for XSS vulnerabilities.
    Solution 
      override predicate isSink(DataFlow::Node sink) {
    sink = jquery().getACall().getArgument(0)
  }

  3. You can now run the completed query. This should find 5 results on the unpatched Bootstrap codebase.

    Completed query 
    /**
* @name Cross-site scripting vulnerable plugin
* @kind path-problem
* @id js/xss-unsafe-plugin
*/

import javascript
import DataFlow::PathGraph

class Configuration extends TaintTracking::Configuration {
  Configuration() { this = "XssUnsafeJQueryPlugin" }

  override predicate isSource(DataFlow::Node source) {
    exists(DataFlow::FunctionNode plugin |
      plugin = jquery().getAPropertyRead("fn").getAPropertySource() and
      source = plugin.getLastParameter()
    )
  }

  override predicate isSink(DataFlow::Node sink) {
    sink = jquery().getACall().getArgument(0)
  }
}

from Configuration cfg, DataFlow::PathNode source, DataFlow::PathNode sink
where cfg.hasFlowPath(source, sink)
select sink, source, sink, "Potential XSS vulnerability in plugin."

We have created a query from scratch to find this problem. A production version of this query can be found as part of the default set of CodeQL security queries: UnsafeJQueryPlugin.ql. You can see the results on a vulnerable copy of Bootstrap that has been analyzed on LGTM.com, our free open source analysis platform.

What's next?

Acknowledgements

This is a reduced version of a Capture-the-Flag challenge devised by @esbena, available at https://securitylab.github.com/ctf/jquery. Try out the full version! Thanks to our moderators for valuable feedback on the workshop.