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Date: Wed, 4 Jan 2023 23:47:12 +0100
From: Gabriel Corona <>
Subject: Code execution through MIME-type association of Mono interpreter and
 security expectations of MIME type associations

On Debian and derivatives, the mono-runtime-common package associates
the application/x-ms-dos-executable MIME type with the Mono CLR
interpreter [1]. This makes it very easy for an attacker to trigger
arbitrary code execution through programs such as Chromium [2], Firefox
[3] and Thunderbird [4] when the Mono packages are installed.

This has been fixed in package [5] which is available
in Debian testing, Debian Sid and Ubuntu Lunar (23.04). This has
currently not been fixed in any stable distribution.

On Firefox and Thunderbird, a user interface is used to let the user
confirm which program to use to open the file. In this case, we can
trick the user into thinking he is about to open the file with a
innocuous program by serving the file with a special MIME type such as
inode/directory or x-scheme-handler/trash [3,4]. These MIME types are
typically associated with a file manager. When called this way, several
file managers will try to open the file based on MIME-type associations
(where the MIME-type is inferred either from the file name extension or
from the file content). Thunar, PCManFM, PCManFM-Qt were found to
exhibit this behavior.

For Thunar, this behavior has been fixed in v4.16.7 and v4.17.2 [7].

We can use a visually confusable file name such as REPORT.ΡDF (notice
the non-ASCII first letter in the extension) in order to trick the user
into thinking he is opening a "safe" file type while disabling MIME-type
detection based on the file name extension.

Moreover, in Firefox and Thunderbird [8], we can corrupt the file
association database (handlers.json) in order to display a bogus file
type description associated with the inode/directory or x-scheme-
handler/trash MIME type. This is done by first serving a "safe" file
type (such as a PDF) with this MIME type.

This begs several questions about file associations:

* Is it legitimate to register file associations for programs
   which can exbibit arbitrary code execution such as unsandboxed
   program interpreters?
* When a program (such as a file manager) is called with a regular file
   it does not handle, should it spawn a new program for handling the
   file without user confirmation (as it may be exploited for file type
* Should a client program reject special/bogus MIME types such as
   inode/* and x-scheme-handler/* as they are not expected to be
   used in this context (and it may be exploited for file type spoofing)?

I would consider the following behaviors to be vulnerabilities:

* Association of the Mono interpreter with a MIME type in the
   Debian/Ubuntu packages;
* Thunar delegates to MIME type associations when opened with a regular
   file (CVE-2021-32563);
* PCManFM delegates to MIME type associations when opened with a regular
* PCManFM-Qt delegates to MIME type associations when opened with a
   regular file;
* Firefox and Thunderbird accept "special" MIME types (inode/* and
   x-scheme-handler/*) from remote servers;
* File type spoofing by corrupting the Firefox and Thunderbird
   handlers.json database.


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