[tahoe-lafs-trac-stream] [Tahoe-LAFS] #2506: Magic Folder: enforce that downloaded files are only written below the magic folder directory

[Tahoe-LAFS]

#2506: Magic Folder: enforce that downloaded files are only written below the
magic folder directory
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 Reporter:  daira                                |          Owner:  daira
     Type:  defect                               |         Status:  new
 Priority:  major                                |      Milestone:
Component:  code-frontend-magic-folder           |  undecided
 Keywords:  otf-magic-folder-objective4 test-    |        Version:  n/a
  needed security unicode path                   |  Launchpad Bug:
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 https://github.com/tahoe-lafs/tahoe-
 lafs/blob/665c36e45cab3487f279ccae44f79f606613c8f2/src/allmydata/frontends/magic_folder.py#L539

 Since `name` comes from another client's DMD and is an arbitrary string,
 it can be an absolute path, or contain "`..`" path elements, or start with
 "`~`", which would cause the file to be written somewhere that might not
 be below this client's magic folder directory.

 We should use `FilePath.preauthChild` to prevent this, and to provide a
 clearer type distinction between relative and absolute paths.

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Ticket URL: <https://tahoe-lafs.org/trac/tahoe-lafs/ticket/2506>
Tahoe-LAFS <https://Tahoe-LAFS.org>
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