Previously, I’ve described how to capture a network traffic log from Microsoft Edge, Google Chrome, and applications based on Chromium or Electron.

In this post, I aim to catalog some guidance for looking at these logs to help find the root cause of captured problems and otherwise make sense of the data collected.

Last Update: April 24, 2020I expect to update this post over time as I continue to gain experience in analyzing network logs.

Choose A Viewer – Fiddler or Catapult

After you’ve collected the net-export-log.json file using the about:net-export page in the browser, you’ll need to decide how to analyze it.

The NetLog file format consists of a JSON-encoded stream of event objects that are logged as interesting things happen in the network layer. At the start of the file there are dictionaries mapping integer IDs to symbolic constants, followed by event objects that make use of those IDs. As a consequence, it’s very rare that a human will be able to read anything interesting from a NetLog.json file using just a plaintext editor or even a JSON parser.

The most common (by far) approach to reading NetLogs is to use the Catapult NetLog Viewer, a HTML/JavaScript application which loads the JSON file and parses it into a much more readable set of events.

An alternative approach is to use the NetLog Importer for Telerik Fiddler.

Importing NetLogs to Fiddler

For Windows users who are familiar with Fiddler, the NetLog Importer extension for Fiddler is easy-to-use and it enables you to quickly visualize HTTP/HTTPS requests and responses. The steps are easy:

  1. Install the NetLog Importer,
  2. Open Fiddler, ideally in Viewer mode fiddler.exe -viewer
  3. Click File > Import > NetLog JSON
  4. Select the JSON file to import

In seconds, all of the HTTP/HTTPS traffic found in the capture will be presented for your review. If the log was compressed before it was sent to you, the importer will automatically extract the first JSON file from a chosen .ZIP or .GZ file, saving you a step.

In addition to the requests and responses parsed from the log, there are a number of pseudo-Sessions with a fake host of NETLOG that represent metadata extracted from the log:

These pseudo-sessions include:

  • RAW_JSON contains the raw constants and event data. You probably will never want to examine this view.
  • CAPTURE_INFO contains basic data about the date/time of the capture, what browser and OS version were used, and the command line arguments to the browser.
  • ENABLED_EXTENSIONS contains the list of extensions that are enabled in this browser instance. This entry will be missing if the log was captured using the –log-net-log command line argument.
  • URL_REQUESTS contains a dictionary mapping every event related to URL_REQUEST back to the URL Requests to which it belongs. This provides a different view of the events that were used in the parsing of the Web Sessions added to the traffic list.
  • SECURE_SOCKETS contains a list of all of the HTTPS sockets that were established for network requests, including the certificates sent by the server and the parameters requested of any client certificates. The Server certificates can be viewed by saving the contents of a –BEGIN CERTIFICATE– entry to a file named something.cer. Alternatively, select the line, hit CTRL+C, click Edit > Paste As Sessions, select the Certificates Inspector and press its Content Certificates button.

You can then use Fiddler’s UI to examine each of the Web Sessions.


The NetLog format currently does not store request body bytes, so those will always be missing (e.g. on POST requests).

Unless the Include Raw Bytes option was selected by the user collecting the capture, all of the response bytes will be missing as well. Fiddler will show a “dropped” notice when the body bytes are missing:

If the user did not select the Include Cookies and Credentials option, any Cookie or Authorization headers will be stripped down to help protect private data:

Scenario: Finding URLs

You can use Fiddler’s full text search feature to look for URLs of interest if the traffic capture includes raw bytes. Otherwise, you can search the Request URLs and headers alone.

On any session, you can use Fiddler’s “P” keystroke (or the Select > Parent Request context menu command) to attempt to walk back to the request’s creator (e.g. referring HTML page).

You can look for the traffic_annotation value that reflects why a resource was requested by looking for the X-Netlog-Traffic_Annotation Session Flag.

Scenario: Cookie Issues

If Fiddler sees that cookies were not set or sent due to features like SameSiteByDefault cookies, it will make a note of that in the Session using a psuedo $NETLOG-CookieNotSent or $NETLOG-CookieNotSet header on the request or response:

Closing Notes

If you’re interested in learning more about this extension, see announcement blog post and the open-source code.

While the Fiddler Importer is very convenient for analyzing many types of problems, for others, you need to go deeper and look at the raw events in the log using the Catapult Viewer.

Viewing NetLogs with the Catapult Viewer

Opening NetLogs with the Catapult NetLog Viewer is even simpler:

  1. Navigate to the web viewer
  2. Select the JSON file to view

If you find yourself opening NetLogs routinely, you might consider using a shortcut to launch the Viewer in an “App Mode” browser instance: msedge.exe --app=https://netlog-viewer.appspot.com/#import

The App Mode instance is a standalone window which doesn’t contain tabs or other UI:

Note that the Catapult Viewer is a standalone HTML application. If you like, you can save it as a .HTML file on your local computer and use it even when completely disconnected from the Internet. The only advantage to loading it from appspot.com is that the version hosted there is updated from time to time.

Along the left side of the window are tabs that offer different views of the data– most of the action takes place on the Events tab.


If the problem only exists on one browser instance, check the Command Line parameters and Active Field trials sections on the Import tab to see if there’s an experimental flag that may have caused the breakage. Similarly, check the Modules tab to see if there are any browser extensions that might explain the problem.

Each URL Request has a traffic_annotation value which is a hash you can look up in annotations.xml. That annotation will help you find what part of Chromium generated the network request:

Most requests generated by web content will be generated by the blink_resource_loader, navigations will have navigation_url_loader and requests from features running in the browser process are likely to have other sources.

Scenario: DNS Issues

Look at the DNS tab on the left, and HOST_RESOLVER_IMPL_JOB entries in the Events tab.

One interesting fact: The DNS Error page performs an asynchronous probe to see whether the configured DNS provider is working generally. The Error page also has automatic retry logic; you’ll see a duplicate URL_REQUEST sent shortly after the failed one, with the VALIDATE_CACHE load flag added to it. In this way, you might see a DNS_PROBE_FINISHED_NXDOMAIN error magically disappear if the user’s router’s DNS flakes.

Scenario: Cookie Issues

Look for COOKIE_INCLUSION_STATUS events for details about each candidate cookie that was considered for sending or setting on a given URL Request. In particular, watch for cookies that were excluded due to SameSite or similar problems.

Scenario: HTTPS Handshaking Issues

Look for SSL_CONNECT_JOB and CERT_VERIFIER_JOB entries. Look at the raw TLS messages on the SOCKET entries.

Scenario: HTTPS Certificate Issues

Note: While NetLogs are great for capturing certs, you can also get the site’s certificate from the browser’s certificate error page.

The NetLog includes the certificates used for each HTTPS connection in the base64-encoded SSL_CERTIFICATES_RECEIVED events.

You can just copy paste each certificate (including ---BEGIN to END---) out to a text file, name it log.cer, and use the OS certificate viewer to view it. Or you can use Fiddler’s Inspector, as noted above.

If you’ve got Chromium’s repo, you can instead use the script at \src\net\tools\print_certificates.py to decode the certificates. There’s also a cert_verify_tool in the Chromium source you might build and try. For Mac, using verify-cert to check the cert and dump-trust-settings to check the state of the Root Trust Store might be useful.

In some cases, running the certificate through an analyzer like https://crt.sh/lintcert can flag relevant problems.

Scenario: Authentication Issues

Look for HTTP_AUTH_CONTROLLER events, and responses with the status codes 401, 403, and 407.

For instance, you might find that the authentication fails with ERR_INVALID_AUTH_CREDENTIALS unless you enable the browser’s DisableAuthNegotiateCnameLookup policy (Kerberos has long been very tricky).

Scenario: Debugging Proxy Configuration Issues

See Debugging the behavior of Proxy Configuration Scripts.

Got a great NetLog debugging tip I should include here? Please leave a comment and teach me!


I’ve written about Browser Proxy Configuration a few times over the years, and I’m delighted that Chromium has accurate & up-to-date documentation for its proxy support.

One thing I’d like to call out is that Microsoft Edge’s new Chromium foundation introduces a convenient new debugging feature for debugging the behavior of Proxy AutoConfiguration (PAC) scripts.

To use it, simply add alert() calls to your PAC script, like so:

alert("!!!!!!!!! PAC script start parse !!!!!!!!");
function FindProxyForURL(url, host) {
alert("Got request for (" + url+ " with host: " + host + ")");
return "PROXY";
alert("!!!!!!!!! PAC script done parse !!!!!!!!");

Then, collect a NetLog trace from the browser:

msedge.exe --log-net-log=C:\temp\logFull.json --net-log-capture-mode=IncludeSocketBytes

…and reproduce the problem.

Save the NetLog JSON file and reload it into the NetLog viewer. Search in the Events tab for PAC_JAVASCRIPT_ALERT events:

Even without adding new alert() calls, you can also look for HTTP_STREAM_JOB_CONTROLLER_PROXY_SERVER_RESOLVED events to see what proxy the proxy resolution process determined should be used.

One current limitation of the current logging is that if the V8 Proxy Resolver process…

… crashes (e.g. because Citrix injected a DLL into it), there’s no mention of that crash in the NetLog; it will just show DIRECT. Until the logging is enhanced, users can hit SHIFT+ESC to launch the browser’s task manager and check to see whether the utility process is alive.

Try using the System Resolver

In some cases (e.g. when using DirectAccess), you might want to try using Windows’ proxy resolution code rather than the code within Chromium.

The --winhttp-proxy-resolver command line argument will direct Chrome/Edge to call out to Windows’ WinHTTP Proxy Service for PAC processing.

Differences in WPAD/PAC Processing

  • The WinHTTP Proxy Service caches proxy authentication credentials and reuses them across browser launches; Chromium does not.
  • The WinHTTP Proxy Service caches WPAD determination across process launches. Chromium does not and will need to redetect the proxy each time the browser reopens.
  • Internet Explorer/WinINET/Edge Legacy call the PAC script’s FindProxyForURLEx function (introduced to unlock IPv6 support), if present, and FindProxyForURL if not.
  • Chrome/Edge/Firefox only call the FindProxyForURL function and do not call the Ex version.
  • Internet Explorer/WinINET/Edge Legacy expose a getClientVersion API that is not defined in other PAC environments.
  • Chrome/Edge may return different results than IE/WinINET/EdgeLegacy from the myIpAddress function when connected to a VPN.

Notes for Other Browsers

  • Prior to Windows 8, IE showed PAC alert() notices in a modal dialog box. It no longer does so and alert() is a no-op.
  • Firefox shows alert() messages in the Browser Console (hit Ctrl+Shift+J); note that Firefox’s Browser Console is not the Web Console where web pages’ console.log statements are shown.


Brave, Mozilla Firefox, Google Chrome and Microsoft Edge presented on our current privacy work at the Enigma 2020 conference in late January. The talks were mostly high-level, but there were a few feature-level slides for each browser.

My ~10 minute presentation on Microsoft Edge was first, followed by Firefox, Chrome, and Brave.

At 40 minutes in a 35min Q&A session starts, first with questions from the panel moderator, followed by questions from the audience.

This post is intended to collect a random set of questions I’ve been asked multiple times about the new Chromium-based Edge. I’ll add to it over time. I wouldn’t call this a FAQ because these questions, while repeated, are not frequently asked.

Last Update: May 7, 2020

Can I get a list of all of the command line arguments for Edge?

Unfortunately, we are not today publishing the list of command line arguments, although in principle we could use the same tool Chromium does to parse our source and generate a listing.

In general, our command-line arguments are the same as those in Chrome (Warning: This list may be outdated), with the exception of marketing names (e.g. Chrome uses --incognito while msedge.exe uses --inprivate) and restricted words (sometimes Edge replaces blacklist with denylist and whitelist with allowlist).

Can I block my employees from accessing the edge://flags page?

You can add “edge://flags” to the URLBlocklist if desired. Generally, we don’t recommend using this policy to block edge://* pages as doing so could have unexpected consequences.

Note that, even if you block access to edge://flags, a user is still able to modify the JSON data storage file backing that page: %LocalAppData%\Microsoft\Edge\User Data\Local State using Notepad or any other text editor.

Similarly, a user might specify command line arguments when launching msedge.exe to change a wide variety of settings.

Can I disable certain ciphers, like 3DES, in the new Edge?

The new Edge does not use SChannel, so none the prior SChannel cipher configuration policies or settings have any effect on the new Edge.

Group Policy may be used to configure the new Edge’s SSLVersionMin (which does impact available cipher suites, but doesn’t disable all of the ciphers considered “Weak” by SSLLabs.com’s test).

Chromium explicitly made a design/philosophical choice (see this and this) not to support disabling individual cipher suites via policy. Ciphersuites in the new Edge may be disabled using a command-line flag:

msedge.exe --cipher-suite-denylist=0x000a https://ssllabs.com

A few other notes:

  • The cipher suite in use is selected by the server from the list offered by the client. So if an organization is worried about ciphers used within their organization, they can simply direct their servers to only negotiate cipher suites acceptable to them.
  • The Chrome team has begun experimenting with disabling some weaker/older ciphersuites; see https://crbug.com/658905
  • If an Enterprise has configured IE Mode, the IE Mode tab’s HTTPS implementation is still controlled by Internet Explorer / Windows / SChannel policy, not the new Edge Chromium policies.
  • If TLS/1.3 is enabled, you cannot use the cipher-suite-denylist to disable ciphers 0x1301, 0x1302, and 0x1303. TLS1.3 spec: “A TLS-compliant application MUST implement the TLS_AES_128_GCM_SHA256 [GCM] cipher suite and SHOULD implement the TLS_AES_256_GCM_SHA384 [GCM] and TLS_CHACHA20_POLY1305_SHA256 [RFC8439] cipher suites (see Appendix B.4).”

Can I use TLS/1.3 in the new Edge?

TLS/1.3 is supported natively within the new Chromium-based Edge on all platforms.

Chromium-based Edge does not rely upon OS support for TLS. Windows’ IE 11 and Legacy Edge do not yet support TLS/1.3, but are expected to support TLS/1.3 in a future Windows 10 release.

For the time being, enabling both TLS/1.3 and TLS/1.2 is a best practice for servers.

Can I turn off TLS/1.3 in the new Edge?

You can set the SSLVersionMax command line argument, but the associated Group Policy was removed in Chrome 75.

msedge.exe --ssl-version-max=tls1.2 https://ssllabs.com

Can Extensions be installed automatically?

Enterprises can make extension install automatically and prevent disabling them using the ExtensionInstallForcelist Policy. Admins can also install extensions (but allow users to disable them) using the ExtensionSettings policy with the installation_mode set to normal_installed.

Here are the details to install extensions directly via the Windows Registry. Please note that if you want to install extensions from the Chrome WebStore, then you must provide the Chrome store id and update url: https://clients2.google.com/service/update2/crx.

Can specific file types be set to auto-open? Can I change my mind?

After downloading a file, you can click the “…” menu next to the download item and choose “Always open files of this type” from the context menu:

This option is not available for all file types (e.g. file types deemed dangerous cannot be auto-opened).

One challenge with this UI is that after you set this option, the download bar will not be shown for this file type any longer, leaving you no way to untick the “Always open files of this type” menu item.

The secret to changing your mind is to visit edge://settings/downloads and click the Clear all button next to the File types which are opened automatically after downloading list. There is no way to clear just one file type from the list short of editing the profile’s PREFERENCES json directly.

Presently, no Group Policy is available to force file types (except PDF) to open automatically, but this is a common enterprise request. The Master Preferences file can be configured with this option, but those defaults are only used when creating new browser profiles, and users may change them.

Can I go directly to single-word (Intranet) sites without doing a search first?

Can I avoid doing a search with a notification bar saying “Did you mean to go to <http://payroll&gt;?” In Internet Explorer, there was a “Go to an intranet site for a single word entry in the Address bar” checkbox in the Advanced Settings.

Use the GoToIntranetSiteForSingleWordEntryInAddressBar policy to change the default behavior.

Starting in Edge 77 (and Chrome 77), the prompt shown when launching an AppProtocol from the browser was changed to remove the “Always allow” checkbox. That change was made, in large part, because this prompt is the only thing standing between every arbitrary site on the Internet (loaded inside your browser’s sandbox) and a full-trust application on your computer (running outside of the browser’s sandbox). See the prior blog post for details on why AppProtocols are so scary.

After Edge 77, when you try to launch a Microsoft Teams meeting, for instance, you’ll see a UI like this:

Unfortunately, there’s a downside to this security improvement.

The same prompt that protects users from malicious content on https://BadGuy.example also shows every single time the legitimate Microsoft Teams website tries to open its related application. Users complain that the security prompt feels redundant, and IT departments have howled that they’ll have to retrain users and field helpdesk calls.

Starting in Edge 82.0.425.0 Canary, a new flag is available:

Visit edge://flags/#edge-exclude-schemes-per-origin, set the flag to Enabled, and restart the browser. After doing so, you’ll see that the prompt now includes a new checkbox: “Always allow <hostname> to open links of this type in the associated app”:

By storing exemptions on a per-site, per-scheme basis, attack surface is significantly reduced, because only sites you’ve specifically allowed in the past are permitted to bypass the prompt.

This change will also be available in browsers based on Chromium 84.

Some notes on this change:

  • Exemptions are stored on a per-scheme, per-origin basis (e.g. “Allow teams: from https://teams.microsoft.com“, so if multiple origins use the same scheme, you’ll need to exempt each one.
  • Stored exemptions are origin specific: “https://site.example&#8221; and “https://www.site.example” and “http://site.example” are all different origins.
  • Stored exemptions are only available for secure origins (basically, HTTPS, HTTP-to-Localhost, and FILE).
  • This checkbox is visible by default in Edge 84, but can be disabled using the existing Group Policy.
  • At present, there is no Group Policy for an admin to push exemption pairs to the client. We are investigating this request.
  • To clear stored exemptions, you may continue to use the “Cookies and other site data” checkbox in the Clear Browsing Data dialog box. Note that you can set the time range to anything you like– all Origin+Scheme exemptions will be cleared.

You can experiment with this feature using the AppProtocol test page.


It’s an interesting time. Microsoft now maintains three different web browsers:

  • Internet Explorer 11
  • Microsoft Edge Legacy (Spartan, v18 and below)
  • Chromium-based Microsoft Edge (v79+)

If you’re using Internet Explorer 11, you should stop; sometimes, this is easier said than done.

If you’re using Legacy Microsoft Edge, you should upgrade to the new Microsoft Edge which is better in almost every way. When you install the Stable version of the new Microsoft Edge (either by downloading it or eventually by using WindowsUpdate), it will replace your existing Legacy Edge with the new version.

What if I still need to test in Legacy Edge?

If you’re a web developer and need to keep testing your sites and services in the legacy Microsoft Edge, you’ll need to set a registry key to prevent the Edge installer from removing the entry points to the old Edge.

Simply import this registry script before the new Edge is installed. When the AllowSxS key is set to 1, the new Edge installer will keep the old entry point, renaming it to “Microsoft Edge Legacy”:

Thereafter, you can use both versions of Edge on the same PC.

If you didn’t have this registry key set and your legacy Edge entry points have disappeared when you installed the new Edge, you can use the Add or Remove Programs applet in the system control panel to uninstall the new Edge, then set the registry key, then reinstall the new Edge.

Note: If you’re a Web Developer, you should also be testing in the Edge Beta or Edge Dev builds because these will allow you to see the changes coming to Edge before your users do. These builds install side-by-side (replacing no browser) and can be installed from https://MicrosoftEdgeInsider.com.

What if my company has sites that only work in Internet Explorer?

In order to help speed migration to the new Microsoft Edge, it offers an Internet Explorer Mode feature when running on Windows. IE Mode allows IT administrators to configure PCs running Windows 7, 8.1, and 10 such that specified sites will load inside a browser tab that uses the Internet Explorer 11 rendering engine.

  • IE Mode is not designed for or available to consumers.
  • Because IE Mode relies upon the IE11 binaries on the current machine, it is not available in Edge for MacOS, iOS, or Android.
  • IE Mode tabs run inside the legacy security sandbox (weaker than the regular Edge sandbox) and ActiveX controls like Silverlight are available to web pages.
  • IE Mode does not share a cache, cookies, or web storage with Microsoft Edge, so scenarios that depend upon using these storage mechanisms in a cross-site+cross-engine context will not work correctly. IT administrators should carefully set their policies such that user flows occur within a single engine.
  • Most Edge browser extensions will not work on IE Mode tabs–extensions which only look at the tab’s URL should work, but extensions which try to view or modify the page content will not function correctly.

In an ideal world, users will migrate to the latest version of Microsoft Edge as quickly as possible, and enjoy a faster, more compatible, more reliable browser. Nevertheless, Microsoft will continue to patch both Legacy Edge and Internet Explorer 11 according to their existing support lifecycle.



In late 2004, I was the Program Manager for Microsoft’s clipart website, delivering a million pieces of clipart to Microsoft Office customers every day. It was great fun. But there was a problem– our “Clip of the Day” feature, meant to spotlight a new and topical piece of clipart every day, wasn’t changing as expected.

After much investigation (could the browser itself really be wrong?!?), I wrote to the IE team to complain about what looked like bugs in its caching implementation. In a terse reply, I was informed that the handful of people then left on the browser team were only working on critical security fixes, and my caching problems weren’t nearly important enough to even look at.

That night, unable to sleep, I tossed and turned and fumed at the seeming arrogance of the job link in the respondent’s email signature… “Want to change the world? Join the new IE team today!

Gradually, though, I calmed down and reasoned it through… While the product wasn’t exactly beloved, everyone I knew with a computer used Internet Explorer. Arrogant or not, it was probably accurate that there was nothing I could do with my career at that time that would have as big an impact as joining the IE team. And, I smugly realized that if I joined the team, I’d get access to the IE source code, and could go root out those caching bugs myself.

I reached out to the IE lead for an informational interview the following day, and passed an interview loop shortly thereafter.

After joining the team, I printed out the source code for the network stack and sat down with a red pen. There were no fewer than six different bugs causing my “Clip of the Day gets stuck” issue. When my devs fixed the last of them, I mentioned this and my story to my GPM (boss’ boss).

Does this mean you’re a retention risk?” Tony asked.

Maybe after we fix the rest of these…” I retorted, pointing at the pile of paper with almost a hundred red circles.

No one in the world loved IE as much as I did, warts and all. Investigating, documenting, and fixing problems in Internet Explorer was a nearly all-consuming passion throughout my twenties. Internet Explorer pioneered a broad range of (mostly overlooked) innovations, and in rediscovering them, I felt like one of the characters on Lost — a castaway in a codebase whose brilliant designers were long gone. IE9 was a fantastic, best-of-its-time browser, and I’ll forever be proud of it. But as IE9 wound down and the Windows 8 adventure began, it was already clear that its lead would not last against the Chrome juggernaut.

I shipped IE7, IE8, IE9, and IE10, leaving Microsoft in late 2012, shortly after IE10 was finished, to build Fiddler for Telerik.

In 2015, I changed my default browser to Chrome. In 2016, I joined the Chrome Security team. I left Google in the summer of 2018 and rejoined the Microsoft Edge team, and that summer and fall I spent 50% of my time rediscovering bugs that I’d first found in IE and blogged about a decade before.

Fortunately, Edge’s faster development pace meant that we actually got to fix some of the bugs this time, but Chrome’s advantages in nearly every dimension left Edge very much in an underdog status. Fortunately, the other half of my time was spent working on our (then) secret project to replatform the next version of our Edge browser atop the open-source Chromium project.

We’ve now shipped our best browser ever — the Chromium-based Microsoft Edge. I hope you’ll try it out.

It’s with love that I beg you… please let Internet Explorer retire to the great bitbucket in the sky. It’s time. It’s been time for a long time.

Burndown List

Last night, as I read the details of yet another 0-day security bug in Internet Explorer, I posted the following throwaway tweet, which netted a surprising number of interactions:

I expected the usual slew of “Yeah, IE is terrible,” and “IE was always terrible,” and “Somebody tell my {boss,school,parents}” responses, but I didn’t really expect serious replies. I got some, however, and they’re interesting.

Shared Credentials

Internet Explorer shares a common networking stack (WinINET) and Cookie Jar (for Intranet/Trusted sites) with many native code applications on Windows, including Windows Explorer. Tim identifies a scenario where Windows Explorer relies on an auth cookie being found in the WinINET cookie jar, put there by Internet Explorer. We’ve seen similar scenarios in some Microsoft Office flows.

Depending on a cookie set by Internet Explorer might’ve been somewhat reasonable in 2003, but Vista/IE7’s introduction of Protected Mode (and cookie jar partitioning) in 2006 made this a fragile architecture. The fact that anything depends upon it in 2020 is appalling.

Thoughts: I need to bang on some doors. This is depressing.

Certificate Issuance

Developers who apply digital signatures to their apps and server operators who expose their sites over HTTPS do so using a digital certificate. In ideal cases, getting a certificate is automatic and doesn’t involve a browser at all, but some Certificate Authorities require browser-based flows. Those flows often demand that the user use either Internet Explorer or Firefox because the former supports ActiveX Controls for certificate issuance, while Firefox, until recently, supported the Keygen element.

WebCrypto, now supported in all modern browsers, serves as a modern replacement for these deprecated approaches, and some certificate issuers are starting to build issuance flows atop it.

Thoughts: We all need to send some angry emails. Companies in the Trust space should not be built atop insecure technologies.

Banking, especially in Asia

A fascinating set of circumstances led to Internet Explorer’s dominance in Asian markets. First, early browsers had poor support for Unicode and East Asian character sets, forcing website developers to build their own text rendering atop native code plugins (ActiveX). South Korea mandated use of a locally-developed cipher (SEED) for banking transactions[1], and this cipher was not implemented by browser developers… ActiveX again to the rescue. Finally, since all users were using IE, and were accustomed to installing ActiveX controls, malware started running rampant, so banks and other financial institutions started bundling “security solutions” (aka rootkits) into their ActiveX controls. Every user’s browser was a battlefield with warring native code trying to get the upper hand. A series of beleaguered Microsoft engineers (including Ed Praitis, who helped inspire me to make my first significant code commits to the browser) spent long weeks trying to keep all of this mess working as we rearchitected the browser, built Protected Mode and later Enhanced Protected Mode, and otherwise modernized a codebase nearing its second decade.

Thoughts: IE marketshare in Asia may be higher than other places, but it can’t be nearly as high as it once was. Haven’t these sites all pivoted to mobile apps yet?

Reader Survey: Do you have any especially interesting scenarios where you’re forced to use Internet Explorer? Sound off in the comments below!


Q: I get that IE is terrible, but I’m an enterprise admin and I own 400 websites running lousy websites written by a vendor in a hurry back in 2004. These sites will not be updated, and my employees need to keep using them. What can I do?

A: The new Chromium-based Edge has an IE Mode; you can configure your users so that Edge will use an Internet Explorer tab when loading those sites, directly within Edge itself.

Q: Uh, isn’t IE Mode a security risk?

A: Any use of an ancient web engine poses some risk, but IE Mode dramatically reduces the risk, by ensuring that only sites selected by the IT Administrator load in IE mode. Everything else seamlessly transitions back to the modern, performant and secure Chromium Edge engine.

Q: What about Web Browser Controls (WebOCs) inside my native code applications?

A: In many cases, WebOCs inside a native application are used to render trusted content delivered from the application itself, or from a server controlled by the application’s vendor. In such cases, and presuming that all content is loaded over HTTPS, the security risk of the use of a WebOC is significantly lower. Rendering untrusted HTML in a WebOC is strongly discouraged, as WebOCs are even less secure than Internet Explorer itself. For compatibility reasons, numerous security features are disabled-by-default in WebOCs, and the WebOC does not run content in any type of process sandbox.

Looking forward, the new Chromium-based WebView2 control should be preferred over WebOCs for scenarios that require the rendering of HTML content within an application.

Q: Does this post mean anything has changed with regard to Internet Explorer’s support lifecycle, etc?

A: No. Internet Explorer will remain a supported product until its support lifecycle runs out. I’m simply begging you to not use it except to download a better browser.


[1] The SEED cipher wasn’t just a case of the South Korean government suffering from not-invented-here, but instead a response to the fact that the US Government at the time forbid export of strong crypto.

Problems in accessing websites can often be found and fixed if the network traffic between the browser and the website is captured as the problem occurs. This short post explains how to capture such logs.

Capturing Network Traffic Logs

If someone asked you to read this post, chances are good that you were asked to capture a web traffic log to track down a bug in a website or your web browser.

Fortunately, in Google Chrome or the new Microsoft Edge (version 76+), capturing traffic is simple:

  1. Optional but helpful: Close all browser tabs but one.
  2. Navigate the tab to chrome://net-export
  3. In the UI that appears, press the Start Logging to Disk button.
  4. Choose a filename to save the traffic to. Tip: Pick a location you can easily find later, like your Desktop.
  5. Reproduce the networking problem in a new tab. If you close or navigate the //net-export tab, the logging will stop automatically.
  6. After reproducing the problem, press the Stop Logging button.
  7. Share the Net-Export-Log.json file with whomever will be looking at it. Optional: If the resulting file is very large, you can compress it to a ZIP file.
Network Capture UI

Privacy-Impacting Options

In some cases, especially when you dealing with a problem in logging into a website, you may need to set either the Include cookies and credentials or Include raw bytes options before you click the Start Logging button.

Note that there are important security & privacy implications to selecting these options– if you do so, your capture file will almost certainly contain private data that would allow a bad actor to steal your accounts or perform other malicious actions. Share the capture only with a person you trust and do not post it on the Internet in a public forum.

Tutorial Video

If you’re more of a visual learner, here’s a short video demonstrating the traffic capture process.

In a followup post, I explore how developers can analyze captured traffic.


Appendix A: Capture on Startup

In rare cases, you may need to capture network data early (e.g. to capture proxy script downloads and the like. To do that, close Edge, then run

msedge.exe --log-net-log=C:\some_path\some_file_name.json --net-log-capture-mode=IncludeSocketBytes

Note: This approach also works for Electron JS applications like Microsoft Teams:

%LOCALAPPDATA%\Microsoft\Teams\current\Teams.exe --log-net-log=C:\temp\TeamsNetLog.json

I suspect that this is only going to capture the network traffic from the Chromium layer of Electron apps (e.g. web requests from the nodeJS side will not be captured) but it still may be very useful.

Appendix B: References

As your browser navigates from page to page, servers are informed of the URL from where you’ve come from using the Referer HTTP header1; the document.referrer DOM property reveals the same information to JavaScript.

Similarly, as the browser downloads the resources (images, styles, JavaScript) within webpages, the Referer header on the request allows the resource’s server to determine which page is requesting the resource.

The Referrer is omitted in some cases, including:

  • When the user navigates via some mechanism other than a link in the page (e.g. choosing a bookmark or using the address box)
  • When navigating from HTTPS pages to HTTP pages
  • When navigating from a resource served by a protocol other than HTTP(S)
  • When the page opts-out (details in a moment)


The Referrer mechanism can be very useful, because it helps a site owner understand from where their traffic is originating. For instance, WordPress automatically generates this dashboard which shows me where my blog gets its visitors:


I can see not only which Search Engines send me the most users, but also which specific posts on Reddit are driving traffic my way.

Privacy Implications

Unfortunately, this default behavior has a significant impact on privacy, because it can potentially leak private and important information.

Imagine, for example, that you’re reviewing a document your mergers and acquisitions department has authored, with the URL https://contoso.com/​Q4/PotentialAcquisitionTargetsUpTo5M.docx. Within that document, there might have a link to https://fabrikam.com​/financialdisclosures.htm. If you were to click that link, the navigation request to Fabrikam’s server will contain the full URL of the document that led you there, potentially revealing information that your firm would’ve preferred to keep quiet.

Similarly, your search queries might contain something you don’t mind Bing knowing (“Am I required to disclose a disease before signing up for HumongousInsurance.com?”) but that you didn’t want to immediately reveal to the site where you’re looking for answers.

If your web-based email reader puts your email address in the URL, or includes the subject of the current email, links you click in that email might be leaking information you wish to keep private.

The list goes on and on. This class of threat was noted almost thirty years ago:

Referrer Policy

Websites have always had ways to avoid leaking information to navigation targets, usually involving nonstandard navigation mechanisms (e.g. meta refresh) or by wrapping all links so that they go through an innocuous page (e.g. https://example.net/offsitelink.aspx).

However, these mechanisms were non-standard, cumbersome, and would not control the referrer information sent when downloading resources embedded in pages. To address these limitations, Referrer Policy was developed and implemented by most browsers2.


Referrer Policy allows a website to control what information is sent in Referer headers and exposed to the document.referrer property. As noted in the spec, the policy can be specified in several ways:

  • Via the Referrer-Policy HTTP response header.
  • Via a meta element with a name of referrer.
  • Via a referrerpolicy content attribute on an aareaimgiframe, or link element.
  • Via the noreferrer link relation on an aarea, or link element.
  • Implicitly, via inheritance.

The policy can be any of the following:

  • no-referrer – Do not send a Referer.
  • unsafe-url – Send the full URL (lacking only auth info and fragment), even on navigations from HTTPS to HTTP.
  • no-referrer-when-downgrade – Don’t send the Referer when navigating from HTTPS to HTTP. [The longstanding default behavior of browsers.]
  • strict-origin-when-cross-origin – For a same-origin navigation, send the URL. For a cross-origin navigation, send only the Origin of the referring page. Send nothing when navigating from HTTPS to HTTP. [Spoiler alert: The new default.]
  • origin-when-cross-origin For a same-origin navigation, send the URL. For a cross-origin navigation, send only the Origin of the referring page. Send the Referer even when navigating from HTTPS to HTTP.
  • same-origin – Send the Referer only for same-origin navigations.
  • origin – Send only the Origin of the referring page.
  • strict-origin – Send only the Origin of the referring page; send nothing when navigating from HTTPS to HTTP.
  • empty string – Inherit, or use the default

As you can see, there are quite a few policies. That’s partly due to the strict- variations which prevent leaking even the origin information on HTTPS->HTTP navigations.

Improving Defaults

With this background out of the way, the Chromium team has announced that they plan to change the default Referrer Policy from no-referrer-when-downgrade to strict-origin-when-cross-origin. This means that cross-origin navigations will no longer reveal path or query string information, significantly reducing the possibility of unexpected leaks.

As with other big privacy changes, this change is slated to ship in v80, the code has been in for five years and you can enable it in Chrome 78+ and Edge 78+:

  1. Visit chrome://flags/#reduced-referrer-granularity
  2. Set the feature to Enabled
  3. Restart your browser

I’ve published a few toy test cases for playing with Referrer Policy here.

As noted in their Intent To Implement, the Chrome team are not the first to make changes here. As of Firefox 70 (Oct 2019), the default referrer policy is set to strict-origin-when-cross-origin, but only for requests to known-tracking domains, OR while in Private mode. In Safari ITP, all cross-site HTTP referrers and all cross-site document.referrers are downgraded to origin. Brave forges the Referer (sending the Origin of the target, not the source) when loading 3rd party resources.

Understand the Limits

Note that this new default is “opt-out”– a page can still choose to send unrestricted referral URLs if it chooses. As an author, I selfishly hope that sites like Reddit and Hacker News might do so.

Also note that this new default does not in any way limit JavaScript’s access to the current page‘s URL. If your page at https://contoso.com/SuperSecretDoc.aspx includes a tracking script:


… the HTTPS request for track.js will send Referer: https://contoso.com/, but when the script runs, it will have access to the full URL of its execution context (https://contoso.com/SuperSecretDoc.aspx) via the window.location.href property.

Test Your Sites

If you’re a web developer, you should test your sites in this new configuration and update them if anything is unexpectedly broken. If you want the browser to behave as it used to, you can use any of the policy-specification mechanisms to request no-referrer-when-downgrade behavior for either an entire page or individual links.

Or, you might pick an even stricter policy (e.g. same-origin) if you want to prevent even the origin information from leaking out on a cross-site basis. You might consider using this on your Intranet, for instance, to help prevent the hostnames of your Intranet servers from being sent out to public Internet sites.

Stay private out there!


1 The misspelling of the HTTP header name is a historical error which was never corrected.

2 Notably, Safari, IE11, and versions of Edge 18 and below only supported an older draft of the Referrer policy spec, with tokens never (matching no-referrer), always (matching unsafe-url), origin (unchanged) and default (matching no-referrer-when-downgrade). Edge 18 supported origin-when-cross-origin, but only for resource subdownloads.

For a small number of users of Chromium-based browsers (including Chrome and the new Microsoft Edge) on Windows 10, after updating to 78.0.3875.0, every new tab crashes immediately when the browser starts.

Impacted users can open as many new tabs as they like, but each will instantly crash:



As of Chrome 81.0.3992, the page will show the string Error Code: STATUS_INVALID_IMAGE_HASH.

What’s going wrong?

This problem relates to a security/reliability improvement made to Chromium’s sandboxing. Chromium runs each of the tabs (and extensions) within locked down (“sandboxed”) processes:


In Chrome 78, a change was made to prevent 3rd-party code from injecting itself into these sandboxed processes. 3rd-party code is a top source of browser reliability and performance problems, and it has been a longstanding goal for browser vendors to get this code out of the web platform engine.

This new feature relies on setting a Windows 10 Process Mitigation policy that instructs the OS loader to refuse to load binaries that aren’t signed by Microsoft. Edge 13 enabled this mitigation in 2015, and the Chromium change brings parity to the new Edge 78+. Notably, Chrome’s own DLLs aren’t signed by Microsoft so they are specially exempted by the Chromium sandboxing code.

Unfortunately, the impact of this change is that the renderer is killed (resulting in the “Aw snap” page) if any disallowed DLL attempts to load, for instance, if your antivirus software attempts to inject its DLLs into the renderer processes. For example, Symantec Endpoint Protection versions before 14.2 are known to trigger this problem.

If you encounter this problem, you should follow the following steps:

Update any security software you have to the latest version.

Other than malware, security software is the other likely cause of code being unexpectedly injected into the renderers.

Temporarily disable the new protection

You can temporarily launch the browser without this sandbox feature to verify that it’s the source of the crashes.

  1. Close all browser instances (verify that there are no hidden chrome.exe or msedge.exe processes using Task Manager)
  2. Use Windows+R to launch the browser with the command line override:
  msedge.exe --disable-features=RendererCodeIntegrity


  chrome.exe --disable-features=RendererCodeIntegrity

Ensure that the tab processes work properly when code integrity checks are disabled.

If so, you’ve proven that code integrity checks are causing the crashes.

Hunt down the culprit

Navigate your browser to the URL chrome://conflicts#R to show the list of modules loaded by the client. Look for any files that are not Signed By Microsoft or Google.

If you see any, they are suspects. (There will likely be a few listed as Shell Extensions; e.g. 7-Zip.dll, that do not cause this problem)– check for an R in the Process types column to find modules loading in the Renderers.

You should install any available updates for any of your suspects to see if doing so fixes the problem.

Check the Event Log

The Windows Event Log will contain information about modules denied loading. Open Event Viewer. Expand Applications and Services Logs > Microsoft > Windows > CodeIntegrity > Operational and look for events with ID 3033. The detail information will indicate the name and location of the DLL that caused the crash:CodeIntegrity

Optional: Use Enterprise Policy to disable the new protection

If needed, IT Adminstrators can disable the new protection using the RendererCodeIntegrity policy for Chrome and Edge. You should outreach to the software vendors responsible for the problematic applications and request that they update them.

Other possible causes

Note that it’s possible that you could have a PC that encounters symptoms like this (all subprocesses crash) but not a result of the new code integrity check. In such cases, the Error Code on the crash page will be something other than STATUS_INVALID_IMAGE_HASH.

  • For instance, Chromium once had an obscure bug in its sandboxing code that caused all sandboxes to crash depending on the random memory mapping of Address Space Layout Randomization.
  • Similarly, Chrome and Edge still have an active bug where all renderers crash on startup if the PC has AppLocker enabled and the browser is launched elevated (as Administrator).