BLeak: Automatically Debugging Memory Leaks in Web Applications
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Latest commit caf161e Feb 21, 2018

BLeak v1.0.1

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BLeak automatically finds, ranks, and diagnoses memory leaks in the client-side of web applications.

BLeak uses a short developer-provided script to drive the application in a loop through specific visual states (e.g., the inbox view and email view of a mail client) as an oracle to find memory leaks. In our experience, BLeak's precision is often 100% (e.g., no false positives), and fixing the leaks it finds reduces heap growth by 94% on average on a corpus of real production web apps.

For more information please see the preprint of our academic paper, which will appear at PLDI 2018.


The following must be installed for BLeak to work:

  • mitmproxy
  • Python 3.6 or greater
    • Our mitmproxy plugin uses new Python async features


npm install -g bleak-detector

After installing, you should be able to run bleak from the commandl ine.


  1. Build BLeak (see above).
  2. Write a configuration file for your web application (see below).
  3. Run bleak run --config path/to/config.js --out path/to/where/you/want/output
    • The output directory should be unique for this specific run of BLeak, otherwise it will overwrite files in the directory. It will be created if needed.
  4. Wait. BLeak typically runs in <10 minutes, but its speed depends on the number of states in your loop and the speed of your web application.
  5. Run the BLeak Results Viewer by running bleak viewer and navigating to http://localhost:8889/ in a web browser. Upload path/to/where/you/want/output/bleak_results.json to the web application to view the results!
    • Alternatively, BLeak prints out a report in bleak_report.log in the same directory, but the results viewer presents additional information not captured in that log file.

Configuration File

BLeak uses a configuration file to find memory leaks in the client-side of a web application. Only a few fields are required.

exports = {
  // URL to the web application.
  url: "http://path/to/my/site",
  // Runs your program in a loop. Each item in the array is a `state`. Each `state` has a "check"
  // function, and a "next" function to transition to the next state in the loop. These run
  // in the global scope of your web app.
  // BLeak assumes that the app is in the first state when it navigates to the URL. If you specify
  // optional setup states, then it assumes that the final setup state transitions the web app to
  // the first state in the loop.
  // The last state in the loop must transition back to the first.
  loop: [
    // First state
      // Return 'true' if the web application is ready for `next` to be run.
      check: function() {
        // Example: `group-listing` must be on the webpage
        return !!document.getElementById('group-listing');
      // Transitions to the next state.
      next: function() {
        // Example: Navigate to the first thread
    // Second (and last) state
      check: function() {
        // Example: Make sure the body of the thread has loaded.
        return !!document.getElementById('thread-body');
      // Since this is the last state in the loop, it must transition back to the first state.
      next: function() {
        // Example: Click back to group listing

  // (Optional) Number of loop iterations to perform during leak detection (default: 8)
  iterations: 8,

  // (Optional) An array of states describing how to login to the application. Executed *once*
  // to set up the session. See 'config.loop' for a description of a state.
  login: [
      check: function() {
        // Return 'true' if the element 'password-field' exists.
        return !!document.getElementById('password-field');
      next: function() {
        // Log in to the application.
        const pswd = document.getElementById('password-field');
        const uname = document.getElementById('username-field');
        const submitBtn = document.getElementById('submit');
        uname.value = 'spongebob';
        pswd.value = 'squarepants';;
  // (Optional) An array of states describing how to get from config.url to the first state in
  // the loop. Executed each time the tool explicitly re-navigates to config.url. See
  // config.loop for a description of states.
  setup: [

  // (Optional) How long (in milliseconds) to wait for a state transition to finish before declaring an error.
  // Defaults to 10 minutes
  timeout: 10 * 60 * 1000,
  // (Optional) How long (in milliseconds) to wait between a check() returning 'true' and transitioning to the next step or taking a heap snapshot.
  // Default: 1000
  postCheckSleep: 1000,
  // (Optional) How long (in milliseconds) to wait between transitioning to the next step and running check() for the first time.
  // Default: 0
  postNextSleep: 0,
  // (Optional) How long (in milliseconds) to wait between submitting login credentials and reloading the page for a run.
  // Default: 5000
  postLoginSleep: 5000,
  // (Optional) An array of numerical IDs identifying leaks with fixes in your code. Used to
  // evaluate memory savings with different leak configurations and the effectiveness of bug fixes.
  // In the code, condition the fix on $$$SHOULDFIX$$$(ID), or add logic to `exports.rewrite` (see below),
  // and BLeak will run the web app with the fixes applied.
  fixedLeaks: [0, 1, 2],
  // (Optional) Proxy re-write rule that runs in a Node.js environment, *not* in the browser.
  // Lets you rewrite the web app's JavaScript/HTML/CSS to test bug fixes. Especially useful for evaluating
  // fixes on web apps you do not control.
  // Return a Node.js Buffer containing the replacement resource contents, or the original contents if not
  // modifying.
  rewrite: function(url /* URL of the resource */,
                    type /* MIME type of resource */,
                    data /* Contents of resource, as a Node.js Buffer */,
                    fixes /* Array of numerical IDs corresponding to bug fixes that are active during the session (see fixedLeaks) */) {
    function hasFix(n) {
      return fixes.indexOf(n) !== -1;
    // Example: Filter out non-JavaScript resources.
    if (type.indexOf("javascript") !== -1) {
      if (url.indexOf("19/common.js") !== -1) {
        let src = data.toString();
        // Example: Replace a specific string in `19/common.js` to fix bug 0.
        if (hasFix(0)) {
          src = src.replace(`window.addEventListener("scroll",a,!1)`, 'window.onscroll=a');
        return Buffer.from(src, 'utf8');
    return data;


Interested in fixing bugs or building on BLeak? Excellent! Read below on how to build BLeak from source and run our unit tests.


  • Yarn package manager
    • NPM may work, but we do not test against it


# Install NPM dependencies (only need to run once)
yarn install
# Build BLeak
yarn run build


yarn test

Debugging Tips

The bleak executable (runnable via ./bleak once built) has a number of useful debug commands. For example, use proxy-session to debug issues with BLeak's proxy / diagnoses phase.