CVE-2021-43860 (Other, published 2022-01-12T22:15Z)
Description: value: Flatpak is a Linux application sandboxing and distribution framework. Prior to versions 1.12.3 and 1.10.6, 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. So maliciously crafted apps can give themselves hidden permissions. Users who have Flatpaks installed from untrusted sources are at risk in case the Flatpak has a maliciously crafted metadata file, either initially or in an update. This issue is patched in versions 1.12.3 and 1.10.6. As a workaround, users can manually check the permissions of installed apps by checking the metadata file or the xa.metadata key on the commit metadata.
Severity: MEDIUM (CVSS v3 base score: 8.6)
Weakness: CWE-276 \u2014 Incorrect Default Permissions
During installation, installed file permissions are set to allow anyone to modify those files.
Repository: https://github.com/flatpak/flatpak
Files changed:
tests/Makefile-test-matrix.am.inc
tests/Makefile.am.inc
tests/test-metadata-validation.sh
Commit message: Add test for metadata validation

This tests for invalid metadata, missing xa.metadata and mismatched
values in xa.metadata and the real metadata, including the embedded
null leading to the hidden permissions of CVE-2021-43860.


Vulnerable code:

Fixed code:
	tests/test-metadata-validation.sh \
	tests/test-metadata-validation.sh \
#!/bin/bash
#
# Copyright (C) 2021 Matthew Leeds <mwleeds@protonmail.com>
#
# SPDX-License-Identifier: LGPL-2.0-or-later

set -euo pipefail

. $(dirname $0)/libtest.sh

echo "1..7"

setup_repo

COUNTER=1

create_app () {
    local OPTIONS="$1"
    local DIR=`mktemp -d`

    mkdir ${DIR}/files
    echo $COUNTER > ${DIR}/files/counter
    let COUNTER=COUNTER+1

    local INVALID=""
    if [[ $OPTIONS =~ "invalid" ]]; then
        INVALID=invalidkeyfileline
    fi
    cat > ${DIR}/metadata <<EOF
[Application]
name=org.test.Malicious
runtime=org.test.Platform/${ARCH}/master
$INVALID

[Context]
EOF
    if [[ $OPTIONS =~ "mismatch" ]]; then
        echo -e "filesystems=host;" >> ${DIR}/metadata
    fi
    if [[ $OPTIONS =~ "hidden" ]]; then
        echo -ne "\0" >> ${DIR}/metadata
        echo -e "\nfilesystems=home;" >> ${DIR}/metadata
    fi
    local XA_METADATA=--add-metadata-string=xa.metadata="$(head -n6 ${DIR}/metadata)"$'\n'
    if [[ $OPTIONS =~ "no-xametadata" ]]; then
        XA_METADATA="--add-metadata-string=xa.nometadata=1"
    fi
    ostree commit --repo=repos/test --branch=app/org.test.Malicious/${ARCH}/master ${FL_GPGARGS} "$XA_METADATA" ${DIR}/
    if [[ $OPTIONS =~ "no-cache-in-summary" ]]; then
        ostree --repo=repos/test ${FL_GPGARGS} summary -u
        # force use of legacy summary format
        rm -rf repos/test/summary.idx repos/test/summaries
    else
        update_repo
    fi
    rm -rf ${DIR}
}

cleanup_repo () {
    ostree refs --repo=repos/test --delete app/org.test.Malicious/${ARCH}/master
    update_repo
}

create_app "hidden"

if ${FLATPAK} ${U} install -y test-repo org.test.Malicious 2>install-error-log; then
    assert_not_reached "Should not be able to install app with hidden permissions"
fi

assert_file_has_content install-error-log "not matching expected metadata"

assert_not_has_dir $FL_DIR/app/org.test.Malicious/current/active

cleanup_repo

ok "app with hidden permissions can't be installed (CVE-2021-43860)"

create_app no-xametadata

# The install will fail because the metadata in the summary doesn't match the metadata on the commit
# The missing xa.metadata in the commit got turned into "" in the xa.cache
if ${FLATPAK} ${U} install -y test-repo org.test.Malicious 2>install-error-log; then
    assert_not_reached "Should not be able to install app with missing xa.metadata"
fi

assert_file_has_content install-error-log "not matching expected metadata"

assert_not_has_dir $FL_DIR/app/org.test.Malicious/current/active

cleanup_repo

ok "app with no xa.metadata can't be installed"

create_app "no-xametadata no-cache-in-summary"

# The install will fail because there's no metadata in the summary or on the commit
if ${FLATPAK} ${U} install -y test-repo org.test.Malicious 2>install-error-log; then
    assert_not_reached "Should not be able to install app with missing metadata"
fi
assert_file_has_content install-error-log "No xa.metadata in local commit"

assert_not_has_dir $FL_DIR/app/org.test.Malicious/current/active

cleanup_repo

ok "app with no xa.metadata and no metadata in summary can't be installed"

create_app "invalid"

if ${FLATPAK} ${U} install -y test-repo org.test.Malicious 2>install-error-log; then
    assert_not_reached "Should not be able to install app with invalid metadata"
fi
assert_file_has_content install-error-log "Metadata for .* is invalid"

assert_not_has_dir $FL_DIR/app/org.test.Malicious/current/active

cleanup_repo

ok "app with invalid metadata (in summary) can't be installed"

create_app "invalid no-cache-in-summary"

if ${FLATPAK} ${U} install -y test-repo org.test.Malicious 2>install-error-log; then
    assert_not_reached "Should not be able to install app with invalid metadata"
fi
assert_file_has_content install-error-log "Metadata for .* is invalid"

assert_not_has_dir $FL_DIR/app/org.test.Malicious/current/active

cleanup_repo

ok "app with invalid metadata (in commit) can't be installed"

create_app "mismatch no-cache-in-summary"

if ${FLATPAK} ${U} install -y test-repo org.test.Malicious 2>install-error-log; then
    assert_not_reached "Should not be able to install app with non-matching metadata"
fi
assert_file_has_content install-error-log "Commit metadata for .* not matching expected metadata"

assert_not_has_dir $FL_DIR/app/org.test.Malicious/current/active

cleanup_repo

ok "app with mismatched metadata (in commit) can't be installed"

create_app "mismatch"

if ${FLATPAK} ${U} install -y test-repo org.test.Malicious 2>install-error-log; then
    assert_not_reached "Should not be able to install app with non-matching metadata"
fi
assert_file_has_content install-error-log "Commit metadata for .* not matching expected metadata"

assert_not_has_dir $FL_DIR/app/org.test.Malicious/current/active

cleanup_repo

ok "app with mismatched metadata (in summary) can't be installed"