Interesting report, thanks. The main problem here seems to be that we allow ProgIDs to be launched as external protocol handlers on Windows, and that is pretty scary on its own.

Adding some people who might have thoughts on how to prevent that.

confirm it would also affect popular application like java with default installation config. (which use jarfile: in regedit,  and do not check extension so bypassed the download check , also bypass the sandbox) 

The reason for allowing ProgIDs is that the difference between a ProgID and a custom protocol is small, from Windows's point of view. Both are registered under HKEY_CLASSES_ROOT\<string>\, but custom protocols have an extra "URL Protocol" key with an empty string.

So when we are launching an external protocol, we ought to make sure if the protocol is a valid one by checking a "URL Protocol" key exists under the registered class.

I tested this on Windows by creating a .url file pointing to an iTunes link in Windows Explorer:

[InternetShortcut]
URL=itmss://itunes.apple.com/deeplink?app=music&p=subscribe&ign-refClientId=3z1mamAFz6MRz4XczD3ozBJwRKWNQ
IDList=
HotKey=0
[{000214A0-0000-0000-C000-000000000046}]
Prop3=19,0

When iTunes is installed, this shortcut works properly. But when I remove HKEY_CLASSES_ROOT\itmss\"URL Protocol" key, Explorer complains with "Unable to open this Internet Shortcut. The protocol itmss does not have a registered program."

So this would be my suggested fix. But as a quick bandaid, we can also add python.file and jarfile to the denided schemes: https://cs.chromium.org/chromium/src/chrome/browser/external_protocol/external_protocol_handler.cc?rcl=5e3bf2a1f618d292d253ee5b0236fcd2cb909c50&l=36

Tentative patch: https://chromium-review.googlesource.com/c/chromium/src/+/1256208

This also affects Windows 10, by the way.

nasko, wfh: As Windows experts, would you be able to take a look at comment #5 and see if the fix makes sense to you?  

the fix in #5 seems reasonable, but I don't know if it might break behavior people are relying on...?

Probably will, but there is a workaround (adding the relevant registry key) so I'm not overly concerned about breakage.

+grt for bug visibility

sorry , the online demo URL changed to http://1vpctucm.3cm.me/chrome_rce.html
if you want to check the patch is effective or not, visit it please.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/d19a75fc26fd0ab1ce79ef3d1c1c9b3cc1fbd098

commit d19a75fc26fd0ab1ce79ef3d1c1c9b3cc1fbd098
Author: Mustafa Emre Acer <meacer@chromium.org>
Date: Mon Oct 08 18:15:14 2018

Validate external protocols before launching on Windows

Bug:  889459 
Change-Id: Id33ca6444bff1e6dd71b6000823cf6fec09746ef
Reviewed-on: https://chromium-review.googlesource.com/c/1256208
Reviewed-by: Greg Thompson <grt@chromium.org>
Commit-Queue: Mustafa Emre Acer <meacer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#597611}
[modify] https://crrev.com/d19a75fc26fd0ab1ce79ef3d1c1c9b3cc1fbd098/chrome/browser/shell_integration_win.cc

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Hi ma7h1as.l@ - thanks for the report! The VRP Panel decided that this bug should be tracked as medium severity, and awarded $1,000 - cheers!

This bug has been closed for more than 14 weeks. Removing security view restrictions.

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot