QSB-124: Partial filtering bypass in split-gpg2

We have published Qubes Security Bulletin (QSB) 124: Partial filtering bypass in split-gpg2. 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 124

             ---===[ Qubes Security Bulletin 124 ]===---

                             2026-11-13

                Partial filtering bypass in split-gpg2

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

Continue to update normally via the Qubes Update tool or its
command-line equivalents [1] in order to apply the security updates
described in the "Patching" section below.

Summary
--------

Split-gpg2 filters incoming gpg-agent commands by parsing them and then
reconstructing allowed commands, in some cases by removing or replacing
some of their parts. Due to a bug, it is possible for lines to be sent
that exceed the line length limit of the protocol. Combined with GnuPG's
handling of such overlong lines, this allows commands to be injected
that split-gpg2's filtering would normally not allow.

Impact
-------

In the supported split-gpg2 configuration, a malicious qube can cause
the gpg-agent to display a password prompt with arbitrary text and
receive the response that the user enters. Such a prompt would have the
window borders of the backend (for example, vault) qube, so an attacker
could use this to try to trick the user into entering a passphrase.

However, if the user's PGP key does not have a passphrase, then such a
request might not make sense (e.g., the user might be confused about the
passphrase being requested and may not have one to provide). Moreover,
in the supported configuration, the vulnerability under discussion does
not allow for extracting or manipulating secret keys, since split-gpg2
uses an "extra" gpg-agent socket that has limited capabilities.
Therefore, even if the user's PGP key does have a passphrase, and even
if the attacker manages to steal that passphrase, the passphrase should
be useless to the attacker without the secret key.

In theory, this vulnerability could also be used to exploit some further
hypothetical GnuPG vulnerability that normally would have been prevented
by split-gpg2's filtering.

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

All supported Qubes OS releases are affected. Only users of split-gpg2
(i.e., the `split-gpg2` package) are affected. All recent versions of
split-gpg2 are affected.

Note: split-gpg2 is a distinct component from split-gpg1 (i.e., the
`qubes-gpg-split` package).

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

When checking incoming "D" commands in response to an "INQUIRY" from the
gpg-agent, split-gpg2 allows unescaped "%" characters. When sending
these to the gpg-agent, split-gpg2 correctly escapes them ("%25") as
required by the protocol. This increases the lengths of lines containing
such characters. Therefore, split-gpg2 can send lines that exceed the
line length limit (which it correctly checks on incoming lines).

When receiving overlong lines, the behavior of GnuPG's gpg-agent
implementation depends on the context. For normal commands, they are
treated as fatal errors. However, for "INQUIRY" responses, the command
that initiated the "INQUIRY" is simply aborted with an error response.
The part of the line exceeding the limit is then treated as a new line
that is parsed normally as a command.

While the vulnerability under discussion does not allow an attacker to
extract secret keys in the supported configuration, it is possible for
the user to manually set up an unsupported configuration that does allow
an attacker to extract secret keys. In particular, the user can enable
the `allow_keygen` option in the split-gpg2 config, as documented in the
README [5]:

    ## Allow key generation

    By setting `allow_keygen = yes` in `qubes-split-gpg2.conf` you can
    allow the client to generate new keys. Normal usage should not need
    this.

    **Warning**: This feature is new and not much tested. Therefore it's
    not security supported!

Again, as stated in the quoted text above, this configuration is not
security supported. It is mentionned here only for the sake of
completeness.

Patching
---------

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

  For Qubes 4.3, in templates used for split-gpg2:
  - split-gpg2 package, version 1.1.16

These packages will migrate from the security-testing repositories to
the current (stable) repositories after being tested by the community.
This testing period is intended to catch bugs affecting Qubes OS
stability before the packages reach the broader userbase. By default,
the security-testing repositories are disabled in both dom0 and
templates. If you wish to receive security updates faster at the risk of
encountering more bugs, you can enable the security-testing repositories
either temporarily (for a single update) or permanently. [2] Dom0,
templates, and standalones each have their own security-testing
repositories, which can be enabled independently of one another. [3][4]
If you do not have the relevant security-testing repositories enabled
while security updates are in security-testing, you must wait until they
migrate to the current (stable) repositories, which generally takes one
to two weeks but may be expedited in urgent cases. Once the packages are
available to your Qubes OS system, you should install them as described
in the "User action" section above.

After you have installed the packages listed above, you must shut down
the relevant templates and restart all split-gpg2 backend qubes.

Credits
--------

This issue was reported by Atrate.

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#updates
[3] https://doc.qubes-os.org/en/latest/user/advanced-topics/how-to-install-software-in-dom0.html#testing-repositories
[4] https://doc.qubes-os.org/en/latest/user/how-to-guides/how-to-install-software.html#testing-repositories
[5] https://github.com/QubesOS/qubes-app-linux-split-gpg2/blob/main/README.md

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

Source: qsb-124-2026.txt

Marek Marczykowski-Górecki’s PGP signature

Source: qsb-124-2026.txt.sig.marmarek

Simon Gaiser (aka HW42)’s PGP signature

Source: qsb-124-2026.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-124), the commands are:

$ gpg --verify qsb-124-2026.txt.sig.marmarek qsb-124-2026.txt
$ gpg --verify qsb-124-2026.txt.sig.simon qsb-124-2026.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-124 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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