** Also affects: flatpak (Ubuntu Focal)
Importance: Undecided
Status: New
** Also affects: flatpak (Ubuntu Impish)
Importance: Undecided
Status: New
** Also affects: flatpak (Ubuntu Bionic)
Importance: Undecided
Status: New
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https://bugs.launchpad.net/bugs/1957716
Title:
Update for CVE-2021-43860 and second github advisory
Status in flatpak package in Ubuntu:
In Progress
Status in flatpak source package in Bionic:
New
Status in flatpak source package in Focal:
New
Status in flatpak source package in Impish:
New
Bug description:
[Links]
https://github.com/flatpak/flatpak/security/advisories/GHSA-qpjc-vq3c-572j
https://github.com/flatpak/flatpak/security/advisories/GHSA-8ch7-5j3h-g4fx
https://security-tracker.debian.org/tracker/CVE-2021-43860
[Impact]
Versions in Ubuntu right now:
Jammy: 1.12.2-2
Impish: 1.10.2-3ubuntu0.1
Focal: 1.6.5-0ubuntu0.4
Bionic: 1.0.9-0ubuntu0.4
Affected versions:
all
Patched versions:
1.12.3, 1.10.6
[Test Case]
Unknown
[Regression Potential]
Flatpak has a test suite, which is run on build across all relevant
architectures and passes.
There is also a manual test plan
https://wiki.ubuntu.com/Process/Merges/TestPlan/flatpak .
Flatpak has autopkgtests enabled
http://autopkgtest.ubuntu.com/packages/f/flatpak .
Regression potential is low, and upstream is very responsive to any
issues raised.
[Patches]
There are two separate github advisories but only one of them has a CVE.
The advisory with the CVE has 5 patches, the other has 2 patches.
[Other Information]
For the first advisory with the CVE:
Ryan Gonzalez discovered that Flatpak doesn't properly validate that
the permissions displayed to the user for an app at install time match
the actual permissions granted to the app at runtime, in the case that
there's a null byte in the metadata file of an app. Therefore apps can
grant themselves permissions without the consent of the user.
Flatpak shows permissions to the user during install by reading them
from the "xa.metadata" key in the commit metadata. This cannot contain
a null terminator, because it is an untrusted GVariant. Flatpak
compares these permissions to the actual metadata, from the "metadata"
file to ensure it wasn't lied to.
However, the actual metadata contents are loaded in several places
where they are read as simple C-style strings. That means that, if the
metadata file includes a null terminator, only the content of the file
from before the terminator gets compared to xa.metadata. Thus, any
permissions that appear in the metadata file after a null terminator
are applied at runtime but not shown to the user. Maliciously crafted
apps can use this to give themselves hidden permissions.
In addition, a similar weakness was discovered, where if the
permissions in the summary metadata are invalid, they would not be
displayed to the user, but the the actual permissions would be
granted, even though it didn't match the invalid version.
For the second advisory:
flatpak-builder applies finish-args last in the build. At this point
the build directory will have the full access that is specified in the
manifest, so running flatpak build against it will gain that
permissions. Normally this will not be done, so this is not problem.
However, if --mirror-screenshots-url is specified, then flatpak-
builder will launch flatpak build --nofilesystem=host appstream-utils
mirror-screenshots after finalization, which can lead to issues even
with the --nofilesystem=host protection.
These changes result in a behaviour change as debian have noted in
their changelog:
* Behaviour changes, as a result of how GHSA-8ch7-5j3h-g4fx was fixed:
- --nofilesystem=host is now special-cased to negate all --filesystem
permissions. Previously, it would cancel out --filesystem=host but
not --filesystem=/some/dir.
- --nofilesystem=home is now special-cased to negate several
home-directory-related filesystem permssions such as
--filesystem=xdg-config/foo, not just --filesystem=host.
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