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# Behavior of Download File Types in Chrome
This describes how to adjust file-type download behavior in
Chrome including interactions with Safe Browsing. The metadata described
here, and stored in `download_file_types.asciipb`, will be both baked into
Chrome released and pushable to Chrome between releases (via
`FileTypePolicies` class). http://crbug.com/596555
Rendered version of this file: https://chromium.googlesource.com/chromium/src/+/master/chrome/browser/resources/safe_browsing/README.md
## Procedure for adding/modifying file type(s)
* **Edit** `download_file_types.asciipb` and update `enums.xml`
* Get it reviewed, **submit.**
* **Push** it to all users via component update:
* Wait 1-3 day for this to run on Canary to verify it doesn't crash Chrome.
* In a synced checkout, run the following to generate protos for all
platforms and push them to GCS. Replace the arg with your build directory:
* % `chrome/browser/resources/safe_browsing/push_file_type_proto.py -d
out-gn/Debug`
* It will ask you to double check its actions before proceeding. It will
fail if you're not a member of
`chrome-file-type-policies-pushers@google.com`, since that's required for
access to the GCS bucket.
* The Component Updater system will notice those files and push them to
users withing ~6 hours. If not, contact `waffles@.`
## Guidelines for a DownloadFileType entry:
See `download_file_types.proto` for all fields.
* `extension`: (required) Value must be unique within the config. It should be
lowercase ASCII and not contain a dot. If there _is_ a duplicate,
first one wins. Only the `default_file_type` should leave this unset.
* `uma_value`: (required) must be unique and match one in the
`SBClientDownloadExtensions` enum in `enums.xml`.
* `is_archive`: `True` if this filetype is a container for other files.
Leave it unset for `false`.
* `ping_setting`: (required). This controls what sort of ping is sent
to Safe Browsing and if a verdict is checked before the user can
access the file.
* `SAMPLED_PING`: Don't send a full Safe Browsing ping, but
send a no-PII "light-ping" for a random sample of SBER users.
This should be the default for unknown types. The verdict won't
be used.
* `NO_PING`: Don’t send any pings. This file is whitelisted. All
NOT_DANGEROUS files should normally use this.
* `FULL_PING`: Send full pings and use the verdict. All dangerous
file should use this.
* `platform_settings`: (repeated) Zero or more settings to differentiate
behavior by platform. Keep them sorted by platform. At build time,
this list will be filtered to contain exactly one setting by chosing
as follows before writing out the binary proto.
1. If there's an entry matching the built platform,
that will be preferred. Otherwise,
2. If there's a "PLATFORM_ANY" (i.e. `platform` is not set),
that will be used. Otherwise,
3. The `default_file_type`'s settings will be filled in.
* `platform_settings.danger_level`: (required) Controls how files should be
handled by the UI in the absence of a better signal from the Safe Browsing
ping. This applies to all file types where `ping_setting` is either
`SAMPLED_PING` or `NO_PING`, and downloads where the Safe Browsing ping
either fails, is disabled, or returns an `UNKNOWN` verdict. Exceptions are
noted below.
The warning controlled here is a generic "This file may harm your computer."
If the Safe Browsing verdict is `UNCOMMON`, `POTENTIALLY_UNWANTED`,
`DANGEROUS_HOST`, or `DANGEROUS`, Chrome will show that more severe warning
regardless of this setting.
This policy also affects also how subresources are handled for *"Save As
..."* downloads of complete web pages. If any subresource ends up with a
file type that is considered `DANGEROUS` or `ALLOW_ON_USER_GESTURE`, then
the filename will be changed to end in `.download`. This is done to prevent
the file from being opened accidentally.
* `NOT_DANGEROUS`: Safe to download and open, even if the download
was accidental. No additional warnings are necessary.
* `DANGEROUS`: Always warn the user that this file may harm their
computer. We let them continue or discard the file. If Safe
Browsing returns a `SAFE` verdict, we still warn the user.
* `ALLOW_ON_USER_GESTURE`: Potentially dangerous, but is likely harmless if
the user is familiar with host and if the download was intentional. Chrome
doesn't warn the user if both of the following conditions are true:
* There is a user gesture associated with the network request that
initiated the download.
* There is a recorded visit to the referring origin that's older than
the most recent midnight. This is taken to imply that the user has a
history of visiting the site.
In addition, Chrome skips the warning if the download was explicit (i.e.
the user selected "Save link as ..." from the context menu), or if the
navigation that resulted in the download was initiated using the Omnibox.
* `platform_settings.auto_open_hint`: (required).
* `ALLOW_AUTO_OPEN`: File type can be opened automatically if the user
selected that option from the download tray on a previous download
of this type.
* `DISALLOW_AUTO_OPEN`: Never let the file automatically open.
Files that should be disallowed from auto-opening include those that
execute arbitrary or harmful code with user privileges, or change
configuration of the system to cause harmful behavior immediately
or at some time in the future. We *do* allow auto-open for files
that upon opening sufficiently warn the user about the fact that it
was downloaded from the internet and can do damage. **Note**:
Some file types (e.g.: .local and .manifest) aren't dangerous
to open. However, their presence on the file system may cause
potentially dangerous changes in behavior for other programs. We
allow automatically opening these file types, but always warn when
they are downloaded.
* TODO(nparker): Support this: `platform_settings.unpacker`:
optional. Specifies which archive unpacker internal to Chrome
should be used. If potentially dangerous file types are found,
Chrome will send a full-ping for the entire file. Otherwise, it'll
follow the ping settings. Can be one of UNPACKER_ZIP or UNPACKER_DMG.
## Guidelines for the top level DownloadFileTypeConfig entry:
* `version_id`: Must be increased (+1) every time the file is checked in.
Will be logged to UMA.
* `sampled_ping_probability`: For what fraction of extended-reporting
users' downloads with unknown extensions (or
ping_setting=SAMPLED_PING) should we send light-pings? [0.0 .. 1.0]
* `file_types`: The big list of all known file types. Keep them
sorted by extension.
* `default_file_type`: Settings used if a downloaded file is not in
the above list. `extension` is ignored, but other settings are used.
The ping_setting should be SAMPLED_PING for all platforms.