Note: I expect to update this post over time. Last update: 5/8/2019.

Compatibility Deltas

As our new Edge Insider builds roll out to the public, we’re starting to triage reports of compatibility issues where Edge76 (the new Chromium-based Edge,  aka Anaheim) behaves differently than the old Edge (Edge18, aka Spartan) and/or Google Chrome.

In general, Edge76 will behave very similarly to Chrome, with the caveat that, to date, only Dev and Canary channels have been released. When looking at Chrome behavior, be sure to compare against the corresponding Chrome Dev and Canary channels.

However, we expect there will be some behavioral deltas between Edge76 and its Chrome-peer versions, so I’ll note those here too.

Note: I’ve previously blogged about interop issues between Edge18 and Chrome.


  • For security reasons, Edge76 and Chrome block navigation to file:// URLs from non-file URLs. If a browser user clicks on a file: link on a webpage, nothing happens (except an error message in the Developer Tools console, noting “Not allowed to load local resource: file://host/whatever”). In contrast, Edge18 (like Internet Explorer before it) allowed HTTP/HTTPS-served pages in your Intranet Zone to navigate to URLs that use the file:// URL protocol; only pages in the Internet Zone were blocked from such navigations. No override for this block is available.


  • Unlike IE/Edge18, Edge76/Chrome do not support DirectInvoke, a scheme whereby a download is converted into the launch of an application with a URL argument. DirectInvoke is most commonly used when launching Office documents and when running ClickOnce applications. For now, users can workaround the lack of ClickOnce support by installing an extension.
  • Edge76/Chrome do not support the proprietary msSaveBlob or msSaveOrOpenBlob APIs supported in Edge18. In most cases, you should instead use an A element with a download attribute.
  • Edge18 did not support navigation to or downloading from data URLs via the download attribute; Edge76/Chrome allow the download of data URLs up to 2mb in length. In most cases, you should prefer blob urls.

HTTPS – TLS Protocol

  • Edge76 and Chrome enable TLS/1.3 by default; Edge18 does not support TLS/1.3 prior to Windows 10 19H1, and even on that platform it is disabled by default (and known to be buggy).
  • Edge76 and Chrome support a different list of TLS ciphers than Edge18.
  • Edge76 and Chrome send GREASE tokens in HTTPS handshakes; Edge18 does not.
  • Edge76 and Chrome prohibit connections for HTTP/2 traffic from using banned (weak) ciphers, showing ERR_SPDY_INADEQUATE_TRANSPORT_SECURITY if the server attempts to use such ciphers. Edge18 did not enforce this requirement. This has primarily impacted intranet websites served by IIS on Windows Server 2012 where the server was either misconfigured or does not have the latest updates installed. Patching the server and/or adjusting its TLS configuration will resolve the problem.

HTTPS – Certificates

  • Edge76 and Chrome require that a site’s certificate contain its domain name in the SubjectAltName (SAN) field. Edge 18 permits the certificate to omit the SAN and if the domain name is in the Subject Common Name (CN) field. (All public CAs use the SAN; certificates that chain to a local/enterprise trusted root may need to be updated).
  • Edge76 and Chrome require certificates that chain to trusted root CAs to be logged in Certificate Transparency (CT). This generally isn’t a problem because public roots are supposed to log in CT as a part of their baseline requirements. However, certain organizations (including Microsoft and CAs) have hybrid roots which are both publicly trusted and issue privately within the organization. As a result, loading pages may error out with NET::ERR_CERTIFICATE_TRANSPARENCY_REQUIRED. To mitigate this, such organizations must either start logging internal certificates in CT, or set one of three policies under HKLM\SOFTWARE\Policies\Microsoft\Edge\. Edge18 does not support CT.
  • Edge76 and Chrome use a custom Win32 client certificate picker UI, while Edge18 uses the system’s default certificate picker.


  • Edge76 and Chrome support the Leave Secure Cookies Alone spec, which blocks HTTP pages from setting cookies with the Secure attribute and restricts the ways in which HTTP pages may interfere with cookies sent to HTTPS pages. Legacy Edge does not have these restrictions.
  • Edge76 and Chrome support Cookie prefixes (restrictions on cookies whose names begin with the prefixes __Secure- and __Host-). Legacy Edge does not enforce these restrictions.
  • Edge76, Chrome, and Firefox ignore Set-Cookie headers with values over 4096 characters in length (including cookie-controlling directives like SameSite). In contrast, IE and Edge18 permit cookies with name-value pairs up to 5118 characters in length.

Authentication and Login

  • In Edge76, Edge18, and Firefox, running the browser in InPrivate mode disables automatic Integrated Windows Authentication. Chrome and Internet Explorer do not disable automatic authentication in private mode. You can disable automatic authentication in Chrome by launching it with a command line argument: chrome.exe --auth-server-whitelist="_"
  • Edge18/Edge76 integrates a built-in single-sign-on (SSO) provider, such that configured account credentials are automatically injected into request headers for configured domains; this feature is disabled in InPrivate mode. Chrome does not have this behavior for Microsoft accounts.
  • Edge18 supports Azure Active Directory’s Conditional Access feature. For Chrome, an extension is required. Edge76 has not yet integrated support for this feature.



Gravestone reading RIP Fallbacks



Just over 5 years ago, I wrote a blog post titled “Misbehaving HTTPS Servers Impair TLS 1.1 and TLS 1.2.”

In that post, I noted that enabling versions 1.1 and 1.2 of the TLS protocol in IE would cause some sites to load more slowly, or fail to load at all. Sites that failed to load were sending TCP/IP RST packets when receiving a ClientHello message that indicated support for TLS 1.1 or 1.2; sites that loaded more slowly relied on the fact that the browser would retry with an earlier protocol version if the server had sent a TCP/IP FIN instead.

TLS version fallbacks were an ugly but practical hack– they allowed browsers to enable stronger protocol versions before some popular servers were compatible. But version fallback incurs real costs:

  • security – a MITM attacker can trigger fallback to the weakest supported protocol
  • performance – retrying handshakes takes time
  • complexity – falling back only in the right circumstances, creating new connections as needed
  • compatibility – not all clients are willing or able to fallback (e.g. Fiddler would never fallback)

Fortunately, server compatibility with TLS 1.1 and 1.2 has improved a great deal over the last five years, and browsers have begun to remove their fallbacks; first fallback to SSL 3 was disabled and now Firefox 37+ and Chrome 50+ have removed fallback entirely.

In the rare event that you encounter a site that needs fallback, you’ll see a message like this, in Chrome:

Google Chrome 52 error

or in Firefox:

Firefox 45 error

Currently, both Internet Explorer and Edge fallback; first a TLS 1.2 handshake is attempted:

TLS 1.2 ClientHello bytes

and after it fails (the server sends a TCP/IP FIN), a TLS 1.0 attempt is made:

TLS 1.0 ClientHello bytes

This attempt succeeds and the site loads in IE. If you analyze the affected site using SSLLabs, you can see that it has a lot of problems, the key one for this case is in the middle:

Grade F on SSLLabs

This is repeated later in the analysis:

TLS Version intolerant for versions 1.2 and 1.3

The analyzer finds that the server refuses not only TLS 1.2 but also the upcoming TLS 1.3.

Unfortunately, as an end-user, there’s not much you can safely do here, short of contacting the site owners and asking them to update their server software to support modern standards. Fortunately, this problem is rare– the Chrome team found that only 0.0017% of TLS connections triggered fallbacks, and this tiny number is probably artificially high (since a spurious connection problem will trigger fallback).