QSB-119: Potential attacker-controlled format string in qvm-open-in-vm

We have published Qubes Security Bulletin (QSB) 119: Potential attacker-controlled format string in qvm-open-in-vm. The text of this QSB and its accompanying cryptographic signatures are reproduced below, followed by a general explanation of this announcement and authentication instructions.

Qubes Security Bulletin 119

             ---===[ Qubes Security Bulletin 119 ]===---

                              2026-09-15

     Potential attacker-controlled format string in qvm-open-in-vm

User action
------------

Continue to update normally [1] in order to receive the security updates
described in the "Patching" section below. No other user action is
required in response to this QSB.

Summary
--------

Under certain circumstances (see "Technical details" below), if the user
invokes qvm-open-in-vm (either directly or through the "Edit in
disposable qube" GUI integration) on a file with an attacker-controlled
filename or path, the attacker might be able to execute code in the qube
in which the user invoked qvm-open-in-vm.

Impact
-------

An attacker who successfully exploits this vulnerability can take
control over the qube in which the user invoked qvm-open-in-vm.

Affected systems
-----------------

All supported Qubes OS releases are affected. Among official Qubes OS
templates, only Debian templates are affected.

Technical details
------------------

When qvm-open-in-vm is invoked with a file (not an URI), the source-side
part of the qrexec call is handled by the qopen-in-vm helper program.
After sending the file content to the other side, qopen-in-vm tries to
open a temporary file alongside the original file in order to save the
response in case the user has edited the file in the target qube. If
creating this file fails (e.g., because the directory is read-only),
qopen-in-vm creates a temporary file in /tmp and shows an error message
to the user. When printing this error message, it incorrectly invokes
the gui_nonfatal function, such that the original filename ends up in a
printf format string, which is unsafe.

For this vulnerability to be exploitable, multiple conditions must be
fulfilled:

 1. The attacker must trick the user into using qvm-open-in-vm to open a
    file that either
    a. has a filename (i.e., the last component of the file's path)
       longer than 248 bytes or
    b. is located in a directory that is not writable.

 2. The path (but not necessarily its last component) must
    contain printf-style conversion specifiers (such as '%n').

 3. qvm-open-in-vm must not have been invoked with the --view-only flag.

 4. The target qube must either
    a. send back the file content because its mtime changed (which
       usually happens because the user saved the file being edited,
       even if merely by overwriting it with the same content) or
    b. be compromised by the attacker.

 5. The build of qvm-open-in-vm must have been compiled without
    _FORTIFY_SOURCE hardening enabled (or set to a level below 2). Due
    to an unfortunate interaction between our build script and the way
    Debian handles this option, this hardening was not enabled for our
    Debian packages. (This problem did not affect our Fedora packages,
    for which the hardening was correctly enabled.) With enabled
    hardening, the program safely aborts with an error, making the bug
    unexploitable.

The first person to report this vulnerability tested its exploitability
on his system. He found that it was successfully exploited in less than
3 % of attempts. Moreover, his demonstration required a very long path
with multiple nested directories and that contained many format
specifiers. In the real world, the attacker may control only the last
part of the file name in most cases. And even when they do control the
full path, many users would likely find such a path suspicious, which
would likely make it more difficult for an attacker to successfully
trick the user into opening such a path.

Discussion
-----------

Normally, we do not issue Qubes security bulletins for purely "in-VM"
vulnerabilities (like this one) that do not involve crossing the VM
security boundary. However, we have decided to make an exception in this
case, since this is a bug in our code and since qvm-open-in-vm is
specifically intended to operate on untrusted input.

While qvm-open-in-vm is designed to be hardened, keep in mind that
handling less trusted data in a more trusted VM still can be risky for
other reasons, for example:

 - Unsafe handling of attacker-controlled paths is an easy mistake to
   make in the shell.
 - A file explorer could render the thumbnail of an untrusted file with
   a buggy parser.
 - You might accidentally open the file with an application other than
   qvm-open-in-vm, exposing the full attack surface of that application.

Patching
---------

The following package contains the security update that addresses the
vulnerability described in this bulletin:

  For Qubes 4.3, in affected templates and standalones:
  - qubes-core-agent version 4.3.48

This package will migrate from the security-testing repository to the
current (stable) repository over the next two weeks after being tested
by the community. [2] Once available, the package should be installed
via the Qubes Update tool or its command-line equivalents. [1]

In order for this security update to take effect, all affected templates
must be updated with the package above, then shut down. Afterward, all
qubes based on these templates (including disposables) must be
restarted. All affected standalones must be updated with the package
above.

Credits
--------

This vulnerability was first reported by Giulio Berra. Shortly after, it
was independently reported by Rafal Wojtczuk.

References
-----------

[1] https://doc.qubes-os.org/en/latest/user/how-to-guides/how-to-update.html
[2] https://doc.qubes-os.org/en/latest/user/downloading-installing-upgrading/testing.html

--
The Qubes Security Team
https://www.qubes-os.org/security/

Source: qsb-119-2026.txt

Marek Marczykowski-Górecki’s PGP signature

Source: qsb-119-2026.txt.sig.marmarek

Simon Gaiser (aka HW42)’s PGP signature

Source: qsb-119-2026.txt.sig.simon

What is the purpose of this announcement?

The purpose of this announcement is to inform the Qubes community that a new Qubes security bulletin (QSB) has been published.

What is a Qubes security bulletin (QSB)?

A Qubes security bulletin (QSB) is a security announcement issued by the Qubes security team. A QSB typically provides a summary and impact analysis of one or more recently-discovered software vulnerabilities, including details about patching to address them.

Why should I care about QSBs?

QSBs tell you what actions you must take in order to protect yourself from recently-discovered security vulnerabilities. In most cases, security vulnerabilities are addressed by updating normally. However, in some cases, special user action is required. In all cases, the required actions are detailed in QSBs.

What are the PGP signatures that accompany QSBs?

A PGP signature is a cryptographic digital signature made in accordance with the OpenPGP standard. PGP signatures can be cryptographically verified with programs like GNU Privacy Guard (GPG). The Qubes security team cryptographically signs all QSBs so that Qubes users have a reliable way to check whether QSBs are genuine. The only way to be certain that a QSB is authentic is by verifying its PGP signatures.

Why should I care whether a QSB is authentic?

A forged QSB could deceive you into taking actions that adversely affect the security of your Qubes OS system, such as installing malware or making configuration changes that render your system vulnerable to attack. Falsified QSBs could sow fear, uncertainty, and doubt about the security of Qubes OS or the status of the Qubes OS Project.

How do I verify the PGP signatures on a QSB?

The following command-line instructions assume a Linux system with git and gpg installed. (For Windows and Mac options, see OpenPGP software.)

Obtain the Qubes Master Signing Key (QMSK), e.g.:

$ gpg --fetch-keys https://keys.qubes-os.org/keys/qubes-master-signing-key.asc
gpg: directory '/home/user/.gnupg' created
gpg: keybox '/home/user/.gnupg/pubring.kbx' created
gpg: requesting key from 'https://keys.qubes-os.org/keys/qubes-master-signing-key.asc'
gpg: /home/user/.gnupg/trustdb.gpg: trustdb created
gpg: key DDFA1A3E36879494: public key "Qubes Master Signing Key" imported
gpg: Total number processed: 1
gpg:               imported: 1

(For more ways to obtain the QMSK, see How to import and authenticate the Qubes Master Signing Key.)

View the fingerprint of the PGP key you just imported. (Note: gpg> indicates a prompt inside of the GnuPG program. Type what appears after it when prompted.)

$ gpg --edit-key 0x427F11FD0FAA4B080123F01CDDFA1A3E36879494
gpg (GnuPG) 2.2.27; Copyright (C) 2021 Free Software Foundation, Inc.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
   
   
pub  rsa4096/DDFA1A3E36879494
     created: 2010-04-01  expires: never       usage: SC
     trust: unknown       validity: unknown
[ unknown] (1). Qubes Master Signing Key
   
gpg> fpr
pub   rsa4096/DDFA1A3E36879494 2010-04-01 Qubes Master Signing Key
 Primary key fingerprint: 427F 11FD 0FAA 4B08 0123  F01C DDFA 1A3E 3687 9494

Important: At this point, you still don’t know whether the key you just imported is the genuine QMSK or a forgery. In order for this entire procedure to provide meaningful security benefits, you must authenticate the QMSK out-of-band. Do not skip this step! The standard method is to obtain the QMSK fingerprint from multiple independent sources in several different ways and check to see whether they match the key you just imported. For more information, see How to import and authenticate the Qubes Master Signing Key.

Tip: After you have authenticated the QMSK out-of-band to your satisfaction, record the QMSK fingerprint in a safe place (or several) so that you don’t have to repeat this step in the future.

Once you are satisfied that you have the genuine QMSK, set its trust level to 5 (“ultimate”), then quit GnuPG with q.

gpg> trust
pub  rsa4096/DDFA1A3E36879494
     created: 2010-04-01  expires: never       usage: SC
     trust: unknown       validity: unknown
[ unknown] (1). Qubes Master Signing Key
   
Please decide how far you trust this user to correctly verify other users' keys
(by looking at passports, checking fingerprints from different sources, etc.)
   
  1 = I don't know or won't say
  2 = I do NOT trust
  3 = I trust marginally
  4 = I trust fully
  5 = I trust ultimately
  m = back to the main menu
   
Your decision? 5
Do you really want to set this key to ultimate trust? (y/N) y
   
pub  rsa4096/DDFA1A3E36879494
     created: 2010-04-01  expires: never       usage: SC
     trust: ultimate      validity: unknown
[ unknown] (1). Qubes Master Signing Key
Please note that the shown key validity is not necessarily correct
unless you restart the program.
   
gpg> q

Use Git to clone the qubes-secpack repo.

$ git clone https://github.com/QubesOS/qubes-secpack.git
Cloning into 'qubes-secpack'...
remote: Enumerating objects: 4065, done.
remote: Counting objects: 100% (1474/1474), done.
remote: Compressing objects: 100% (742/742), done.
remote: Total 4065 (delta 743), reused 1413 (delta 731), pack-reused 2591
Receiving objects: 100% (4065/4065), 1.64 MiB | 2.53 MiB/s, done.
Resolving deltas: 100% (1910/1910), done.

Import the included PGP keys. (See our PGP key policies for important information about these keys.)

$ gpg --import qubes-secpack/keys/*/*
gpg: key 063938BA42CFA724: public key "Marek Marczykowski-Górecki (Qubes OS signing key)" imported
gpg: qubes-secpack/keys/core-devs/retired: read error: Is a directory
gpg: no valid OpenPGP data found.
gpg: key 8C05216CE09C093C: 1 signature not checked due to a missing key
gpg: key 8C05216CE09C093C: public key "HW42 (Qubes Signing Key)" imported
gpg: key DA0434BC706E1FCF: public key "Simon Gaiser (Qubes OS signing key)" imported
gpg: key 8CE137352A019A17: 2 signatures not checked due to missing keys
gpg: key 8CE137352A019A17: public key "Andrew David Wong (Qubes Documentation Signing Key)" imported
gpg: key AAA743B42FBC07A9: public key "Brennan Novak (Qubes Website & Documentation Signing)" imported
gpg: key B6A0BB95CA74A5C3: public key "Joanna Rutkowska (Qubes Documentation Signing Key)" imported
gpg: key F32894BE9684938A: public key "Marek Marczykowski-Górecki (Qubes Documentation Signing Key)" imported
gpg: key 6E7A27B909DAFB92: public key "Hakisho Nukama (Qubes Documentation Signing Key)" imported
gpg: key 485C7504F27D0A72: 1 signature not checked due to a missing key
gpg: key 485C7504F27D0A72: public key "Sven Semmler (Qubes Documentation Signing Key)" imported
gpg: key BB52274595B71262: public key "unman (Qubes Documentation Signing Key)" imported
gpg: key DC2F3678D272F2A8: 1 signature not checked due to a missing key
gpg: key DC2F3678D272F2A8: public key "Wojtek Porczyk (Qubes OS documentation signing key)" imported
gpg: key FD64F4F9E9720C4D: 1 signature not checked due to a missing key
gpg: key FD64F4F9E9720C4D: public key "Zrubi (Qubes Documentation Signing Key)" imported
gpg: key DDFA1A3E36879494: "Qubes Master Signing Key" not changed
gpg: key 1848792F9E2795E9: public key "Qubes OS Release 4 Signing Key" imported
gpg: qubes-secpack/keys/release-keys/retired: read error: Is a directory
gpg: no valid OpenPGP data found.
gpg: key D655A4F21830E06A: public key "Marek Marczykowski-Górecki (Qubes security pack)" imported
gpg: key ACC2602F3F48CB21: public key "Qubes OS Security Team" imported
gpg: qubes-secpack/keys/security-team/retired: read error: Is a directory
gpg: no valid OpenPGP data found.
gpg: key 4AC18DE1112E1490: public key "Simon Gaiser (Qubes Security Pack signing key)" imported
gpg: Total number processed: 17
gpg:               imported: 16
gpg:              unchanged: 1
gpg: marginals needed: 3  completes needed: 1  trust model: pgp
gpg: depth: 0  valid:   1  signed:   6  trust: 0-, 0q, 0n, 0m, 0f, 1u
gpg: depth: 1  valid:   6  signed:   0  trust: 6-, 0q, 0n, 0m, 0f, 0u

Verify signed Git tags.

$ cd qubes-secpack/
$ git tag -v `git describe`
object 266e14a6fae57c9a91362c9ac784d3a891f4d351
type commit
tag marmarek_sec_266e14a6
tagger Marek Marczykowski-Górecki 1677757924 +0100
   
Tag for commit 266e14a6fae57c9a91362c9ac784d3a891f4d351
gpg: Signature made Thu 02 Mar 2023 03:52:04 AM PST
gpg:                using RSA key 2D1771FE4D767EDC76B089FAD655A4F21830E06A
gpg: Good signature from "Marek Marczykowski-Górecki (Qubes security pack)" [full]

The exact output will differ, but the final line should always start with gpg: Good signature from... followed by an appropriate key. The [full] indicates full trust, which this key inherits in virtue of being validly signed by the QMSK.

Verify PGP signatures, e.g.:

$ cd QSBs/
$ gpg --verify qsb-087-2022.txt.sig.marmarek qsb-087-2022.txt
gpg: Signature made Wed 23 Nov 2022 04:05:51 AM PST
gpg:                using RSA key 2D1771FE4D767EDC76B089FAD655A4F21830E06A
gpg: Good signature from "Marek Marczykowski-Górecki (Qubes security pack)" [full]
$ gpg --verify qsb-087-2022.txt.sig.simon qsb-087-2022.txt
gpg: Signature made Wed 23 Nov 2022 03:50:42 AM PST
gpg:                using RSA key EA18E7F040C41DDAEFE9AA0F4AC18DE1112E1490
gpg: Good signature from "Simon Gaiser (Qubes Security Pack signing key)" [full]
$ cd ../canaries/
$ gpg --verify canary-034-2023.txt.sig.marmarek canary-034-2023.txt
gpg: Signature made Thu 02 Mar 2023 03:51:48 AM PST
gpg:                using RSA key 2D1771FE4D767EDC76B089FAD655A4F21830E06A
gpg: Good signature from "Marek Marczykowski-Górecki (Qubes security pack)" [full]
$ gpg --verify canary-034-2023.txt.sig.simon canary-034-2023.txt
gpg: Signature made Thu 02 Mar 2023 01:47:52 AM PST
gpg:                using RSA key EA18E7F040C41DDAEFE9AA0F4AC18DE1112E1490
gpg: Good signature from "Simon Gaiser (Qubes Security Pack signing key)" [full]

Again, the exact output will differ, but the final line of output from each gpg --verify command should always start with gpg: Good signature from... followed by an appropriate key.

For this announcement (QSB-119), the commands are:

$ gpg --verify qsb-119-2026.txt.sig.marmarek qsb-119-2026.txt
$ gpg --verify qsb-119-2026.txt.sig.simon qsb-119-2026.txt

You can also verify the signatures directly from this announcement in addition to or instead of verifying the files from the qubes-secpack. Simply copy and paste the QSB-119 text into a plain text file and do the same for both signature files. Then, perform the same authentication steps as listed above, substituting the filenames above with the names of the files you just created.

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