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. 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 a new type

  • Edit download_file_types.asciipb. Update histograms.xml
  • Get it reviewed, submit.
  • Push via component update (PROCEDURE TBD)

Guidelines for a DownloadFileType entry:

See download_file_types.proto for all fields.

  • extension: 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: Value must be unique and match one in the SBClientDownloadExtensions enum in histograms.xml.

  • is_archive: True if this filetype is a container for other files. Leave it unset for false.

  • platform_settings: (repeated) Must have one entry with an unset platform field, and optionally additional entries with overrides for one or more platforms. An unset platform field acts as a default for any platforms that don't have an override. There should not be two settings with the same platform, but if there are, first one wins. Keep them sorted by platform.

  • platform_settings.danger_level: (required)

    • NOT_DANGEROUS: Safe to download and open, even if the download was accidental.
    • 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: Warn the user normally but skip the warning if there was a user gesture or the user visited this site before midnight last night (i.e. is a repeat visit). If Safe Browsing returns a SAFE verdict for this file, it won't show a warning.

  • 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.

  • platform_settings.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.

  • 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_type: 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.