QSB-120: Two CTAP proxy vulnerabilities

We have published Qubes Security Bulletin (QSB) 120: Two CTAP proxy vulnerabilities. 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 120

             ---===[ Qubes Security Bulletin 120 ]===---

                              2026-10-08

                   Two CTAP proxy vulnerabilities

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.

Background
-----------

The Client to Authenticator Protocol (CTAP) is a standard governing the
interoperation of authentication devices, such as hardware security
keys, with client platforms, such as laptops. Qubes OS has a feature
known as the Qubes CTAP proxy [3] that enables a USB authentication
device to be used in a given qube without exposing that qube to the full
USB stack, similar to the way the USB keyboard and mouse proxies [4]
work in Qubes. There are multiple versions of CTAP, different ways it
can be used, and various related components that can work with it, so
there's a variety of other terms that you might see associated with
CTAP, such as U2F, CTAP1, CTAP2, FIDO2, WebAuthn, and passkeys.

Qubes users can write RPC policies [5] to control the way CTAP requests
forwarded through the CTAP proxy are handled. There are various request
types that are handled by different qrexec services: ctap.ClientPin,
ctap.GetInfo, u2f.Authenticate, and u2f.Register. In addition, users can
limit the u2f.Authenticate service to specific credentials by allowing
only specific qrexec service arguments.

Summary
--------

This QSB describes two CTAP proxy vulnerabilities:

Vulnerability 1: CTAP requests are supposed to match their requested
service types, but the CTAP proxy fails to enforce this.

Vulnerability 2: When RPC policies limit a qube to sending requests with
specific service arguments, the u2f.Authenticate service incorrectly
allows that qube to request multiple assertions (used for
authentication) as long as any of them match the allowed service
argument.

Impact
-------

Vulnerability 1: A qube that is allowed access to any of the
CTAP-related qrexec services can send any request type. For example,
even if the policy allows a qube access only to ctap.GetInfo, that qube
can also send an authentication request. In realistic configurations,
this means that argument-based filtering of access to specific
credentials is ineffective, since a qube can send authentication
requests through one of its other allowed services. This vulnerability
affects all CTAP versions.

Vulnerability 2: A qube that is allowed to have access only to specific
credentials on a forwarded authentication device can use any credentials
for CTAP2 (but not CTAP1) assertion requests, thereby enabling it to use
those credentials for authentication even though it should not be
allowed to do so.

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

Since these vulnerabilities affect only the CTAP proxy, only users who
use the CTAP proxy on a supported Qubes release are affected. Users who
do not use the CTAP proxy are not affected.

In practice, the only systems that are affected are those in which the
RPC policies have been configured to restrict a qube's access to a
subset of the credentials available on an authentication device, since
otherwise the qube already (intentionally) has access to all
credentials on the authentication device.

These vulnerabilities were introduced in qubes-ctap package version
2.0.0. Older versions of the package (previously known as qubes-u2f) are
not affected.

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

The software (usually a web browser) accessing a hardware authentication
device (either a real one, or in this case an emulated one that forwards
requests through qrexec) must limit the requests it sends to the device.
For example, if you visit <https://example.org>, the browser must ensure
that that site can request authentication only for
<https://example.org>. Therefore, in the absence of an additional,
exploitable vulnerability in the browser, a malicious website cannot
exploit either of the vulnerabilities described in this QSB. However,
the intention of the CTAP proxy is (in addition to making CTAP
authentication devices usable in the first place) to restrict the access
that qubes have to authentication devices, such that even if a qube is
compromised, it cannot access credentials on the authentication device
that it has not been given permission to access. The vulnerabilities
described in this QSB affect this latter form of protection.

Note that the RPC policies themselves are correctly enforced, so these
are not vulnerabilities in qrexec. Rather, the problem lies in the CTAP
proxy's implementation of the qrexec services, which breaks the intended
semantics of the (correctly-enforced) RPC policies.

Patching
---------

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

  For Qubes 4.3, in template for USB qube(s):
  - qubes-ctap package, version 2.0.8

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 the update to take effect, all USB qubes must be restarted
after their respective templates have been updated and shut down.

Credits
--------

Vulnerability 2 was discovered by Giulio Berra of the Freedom of the
Press Foundation.

Vulnerability 1 was discovered internally by Piotr Bartman-Szwarc during
review of the previous report.

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
[3] https://doc.qubes-os.org/en/r4.3/user/security-in-qubes/ctap-proxy.html
[4] https://doc.qubes-os.org/en/r4.3/user/advanced-topics/usb-qubes.html
[5] https://doc.qubes-os.org/en/r4.3/user/advanced-topics/rpc-policy.html

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

Source: qsb-120-2026.txt

Marek Marczykowski-Górecki’s PGP signature

Source: qsb-120-2026.txt.sig.marmarek

Simon Gaiser (aka HW42)’s PGP signature

Source: qsb-120-2026.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-120), the commands are:

$ gpg --verify qsb-120-2026.txt.sig.marmarek qsb-120-2026.txt
$ gpg --verify qsb-120-2026.txt.sig.simon qsb-120-2026.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-120 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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