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Behavior of Download File Types in Chrome

components/safe_browsing/content/resources/README.md

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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/+/main/components/safe_browsing/content/resources/README.md

Procedure for adding/modifying file type(s)

  • Edit download_file_types.asciipb and enums.xml.
  • Get it reviewed, submit.
  • The Component Updater system will notice those files and import them, but will not push them yet.
  • Wait 1-3 day for this to run on Canary to verify it doesn't crash Chrome.
  • Push it to all users via component update:

Procedure for rollback

While Omaha allows rollback through the release manager, the Chrome client will reject updates with lower version numbers. (This is important for running new versions on Canary/Dev channel). Rolling back a bad version is best achieved by:

  • Reverting the changes on the Chromium source tree.
  • Submitting a new CL incrementing the version number.
  • Push the newest version, as above.

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 used for known safe types. The verdict won't be used.

    • NO_PING: Don’t send any pings. This file is allowlisted. 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 choosing 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_TYPE_ANY" (i.e. platform is not set), that will be used. Otherwise,

    3. The default_file_type's settings will be filled in.

    Warning: When specifying a new platform_settings for a file type, be sure to specify values for all necessary settings. The platform_settings will override all of the default_file_type's settings, and this may result in a change in behavior for platform_settings left unspecified. For example, see crbug.com/946558.

  • 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.

      • Note that file types at this level can affect how our partner team calculates the warning volume. Please reach out before adding a new file type under this danger level.

    • 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 users preference enables Safe Browsing or 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.

    If the SafeBrowsingForTrustedSourcesEnabled policy is set and the download originates from a Trusted source, no warnings will be shown even for types with a danger_level of DANGEROUS or ALLOW_ON_USER_GESTURE.

  • 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.max_file_size_to_analyze: (optional). Size in bytes of the largest file that the analyzer is willing to inspect; for instance, a zip file larger than the threshold will not be unpacked to allow scanning of the files within.

  • 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 FULL_PING for all platforms, so that unknown file types generate pings.