#VU51443 Improper Neutralization of Special Elements in Output Used by a Downstream Component in Flatpak - CVE-2021-21381

Vulnerability identifier: #VU51443

Vulnerability risk: Medium

CVSSv4.0: 1.2 [CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:A/VC:L/VI:L/VA:N/SC:N/SI:N/SA:N/E:U/U:Green]

CVE-ID: CVE-2021-21381

CWE-ID: CWE-74

Exploitation vector: Network

Exploit availability: No

Vulnerable software:
Flatpak
Server applications / Frameworks for developing and running applications

Vendor: Flatpak

Description

The vulnerability allows a remote attacker to bypass implemented security restrictions.

The vulnerability exists due to improper input validation within the "file forwarding" feature. By putting the special tokens `@@` and/or `@@u` in the Exec field of a Flatpak app's .desktop file, a malicious app publisher can trick flatpak into behaving as though the user had chosen to open a target file with their Flatpak app, which automatically makes that file available to the Flatpak app.

Mitigation
Install updates from vendor's website.

Vulnerable software versions

Flatpak: 0.9.4 - 0.99.3, 1.0.0 - 1.0.9, 1.1.0 - 1.10.1, 1.2.0 - 1.2.5, 1.3.0 - 1.3.4, 1.4.0 - 1.4.4, 1.5.0 - 1.5.2, 1.6.0 - 1.6.5, 1.7.1 - 1.7.3, 1.8.0 - 1.8.5, 1.9.1 - 1.9.3

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External links
https://github.com/flatpak/flatpak/commit/8279c5818425b6812523e3805bbe242fb6a5d961
https://github.com/flatpak/flatpak/commit/a7401e638bf0c03102039e216ab1081922f140ae
https://github.com/flatpak/flatpak/commit/eb7946bb6248923d8c90fe9b84425fef97ae580d
https://github.com/flatpak/flatpak/pull/4156
https://github.com/flatpak/flatpak/releases/tag/1.10.2
https://github.com/flatpak/flatpak/security/advisories/GHSA-xgh4-387p-hqpp
https://www.debian.org/security/2021/dsa-4868

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Q & A

Can this vulnerability be exploited remotely?

Yes. This vulnerability can be exploited by a remote non-authenticated attacker via the Internet.

Is there known malware, which exploits this vulnerability?

No. We are not aware of malware exploiting this vulnerability.