QSB-111: xfce4-screensaver login bypass

We have published Qubes Security Bulletin (QSB) 111: xfce4-screensaver login bypass. 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 111

             ---===[ Qubes Security Bulletin 111 ]===---

                             2026-04-14

                     xfce4-screensaver login bypass

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
--------

When connecting or disconnecting a display or, in some cases, when simply
activating the xfce4-screensaver login prompt on a locked screen, there
is a very short window of time during which keyboard input is not
intercepted by xfce4-screensaver and is instead sent to the application
that was active before locking.

The issue has been fixed upstream in [2].

Impact
-------

The time window is normally too short for any real harm to occur when
physically typing on a keyboard. However, if an attacker is able to
generate input events in an automated way, the attacker can attempt to
deactivate the screensaver by sending a command quickly enough.

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

The default Qubes OS 4.3 configuration, which uses xfce4-screensaver and
the Xfce desktop environment, is affected. Other Qubes OS releases and
other screensavers are not affected.

Systems that permit an attacker physical access to a keyboard interface
are affected. Most systems permit such access in some way, since the
user's legitimate keyboard must connect to the system through such an
interface, but the degree of difficulty for the attacker varies
significantly depending on the nature of the interface.

 - The easiest method for an attacker is unrestricted USB keyboard input
   without a confirmation prompt, but this is disabled in Qubes OS by
   default. However, users can override this default configuration to
   allow unrestricted USB keyboard input, and many users do so because
   they have no other way to connect a legitimate keyboard to their
   systems. If the user has allowed unrestricted USB keyboard input,
   then an attacker can easily and quickly generate keyboard input
   events by connecting a USB device that emulates a keyboard.

 - An attacker could also use a non-USB keyboard input interface, such
   as a PS/2 port. However, such interfaces are less common nowadays.
   Nonetheless, if the system has such an interface, and if the attacker
   has physical access to it, then the system is affected, even if USB
   keyboard input is restricted.

 - Even when there is no keyboard input interface exposed on the outside
   of the machine as a convenient port, as in the case of a laptop with
   disabled USB keyboard input and no PS/2 port, it may still be
   possible for the attacker to physically modify the machine in order
   to gain access to such an interface. For example, built-in laptop
   keyboards must still be internally connected to the laptop's
   motherboard through such an interface in order to function properly.
   An attacker who can gain access to the internal components of the
   machine could disconnect the legitimate keyboard and attach their own
   malicious device to the same interface. However, this form of attack
   is significantly more complex than the foregoing attack vectors and
   requires extensive physical access, making it much more difficult to
   execute successfully.

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

In general, we consider most attacks that require physical access to the
system to fall outside the scope of the Qubes security model. Aside from
the general challenges of protecting against physical attacks, this is
primarily because physical protections depend heavily on the hardware
and firmware details of the system. Since Qubes OS is designed to run on
any x86 system that meets its system requirements [3], these hardware
and firmware details can vary significantly between systems and are
typically outside of Qubes' control. Nonetheless, we consider this
screensaver login bypass vulnerability to fall within the scope of the
Qubes security model, since Qubes OS does have control over keyboard
input interception on the lock screen.

Patching
---------

The following packages contain security updates that address the
vulnerabilities described in this bulletin:

  For Qubes 4.3, in dom0 (no GUI domain):
  - xfce4-screensaver 4.18.4-5

For systems with a GUI domain, a similar update will be provided by the
distribution of the template on which the GUI domain is based (e.g.,
Fedora or Debian).

These packages will migrate from the security-testing repository to the
current (stable) repository over the next two weeks after being tested
by the community. [4] Once available, the packages should be installed
via the Qubes Update tool or its command-line equivalents. [1]

Dom0's user session must be restarted afterward in order for the updates
to take effect. This can be accomplished by logging out of Xfce4, then
logging back in again, or by simply restarting the system.

Credits
--------

The issue was reported by Murat Altindis and Maria Kessler of AWARE7
GmbH.

Qubes issue #10720 [5] is likely a report of the same problem. This
issue was incorrectly identified as a duplicate of another issue that
sounds similar but that is not relevant to the security of Qubes OS.

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

[1] https://doc.qubes-os.org/en/latest/user/how-to-guides/how-to-update.html
[2] https://gitlab.xfce.org/apps/xfce4-screensaver/-/commit/4436087c6af5e915a2438d7c1dd0fdc282f547f8
[3] https://doc.qubes-os.org/en/latest/user/hardware/system-requirements.html
[4] https://doc.qubes-os.org/en/latest/user/downloading-installing-upgrading/testing.html
[5] https://github.com/QubesOS/qubes-issues/issues/10720

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

Source: qsb-111-2026.txt

Marek Marczykowski-Górecki’s PGP signature

Source: qsb-111-2026.txt.sig.marmarek

Simon Gaiser (aka HW42)’s PGP signature

Source: qsb-111-2026.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-111), the commands are:

$ gpg --verify qsb-111-2026.txt.sig.marmarek qsb-111-2026.txt
$ gpg --verify qsb-111-2026.txt.sig.simon qsb-111-2026.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-111 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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