Date: Thu, 5 Jan 2023 11:02:50 -0500 From: Demi Marie Obenour <demi@...isiblethingslab.com> To: oss-security@...ts.openwall.com Subject: Re: Code execution through MIME-type association of Mono interpreter and security expectations of MIME type associations On Wed, Jan 04, 2023 at 11:47:12PM +0100, Gabriel Corona wrote: > On Debian and derivatives, the mono-runtime-common package associates > the application/x-ms-dos-executable MIME type with the Mono CLR > interpreter . This makes it very easy for an attacker to trigger > arbitrary code execution through programs such as Chromium , Firefox >  and Thunderbird  when the Mono packages are installed. > > This has been fixed in package 126.96.36.199+dfsg-3.3  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 . > > 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 , 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? No. Failure to do this is a major cause of security problems in Microsoft Windows. > * 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 > spoofing)? No, it should not. > * 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)? Yes, and there needs to be a database of such types. > I would consider the following behaviors to be vulnerabilities: > > * Association of the Mono interpreter with a MIME type in the > Debian/Ubuntu packages; I agree. > * Thunar delegates to MIME type associations when opened with a regular > file (CVE-2021-32563); I agree. > * PCManFM delegates to MIME type associations when opened with a regular > file; I agree. > * PCManFM-Qt delegates to MIME type associations when opened with a > regular file; I agree. > * Firefox and Thunderbird accept "special" MIME types (inode/* and > x-scheme-handler/*) from remote servers; Not sure what you mean by “accept”. Do you mean that download should be aborted? > * File type spoofing by corrupting the Firefox and Thunderbird > handlers.json database. I agree. >  https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=972146 >  https://www.gabriel.urdhr.fr/videos/chromium-filetype-spoofing-poc.ogv >  https://www.gabriel.urdhr.fr/videos/firefox-filetype-spoofing-poc.ogv >  https://www.gabriel.urdhr.fr/videos/thunderbird-filetype-spoofing-poc.ogv >  https://packages.debian.org/buster/mono-runtime-common >  https://packages.ubuntu.com/search?keywords=mono-runtime-common&searchon=names&suite=all§ion=all >  https://nvd.nist.gov/vuln/detail/CVE-2021-32563 >  https://www.gabriel.urdhr.fr/videos/firefox-filetype-spoofing-poc2.ogv Qubes OS should probably register a catchall handler for special MIME types that does nothing. -- Sincerely, Demi Marie Obenour (she/her/hers) Invisible Things Lab Download attachment "signature.asc" of type "application/pgp-signature" (834 bytes)
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