QSB-118: Dom0 arbitrary code execution in qvm-copy-to-vm error reporting

We have published Qubes Security Bulletin (QSB) 118: Dom0 arbitrary code execution in qvm-copy-to-vm error reporting. 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 118

             ---===[ Qubes Security Bulletin 118 ]===---

                              2026-08-28

    Dom0 arbitrary code execution in qvm-copy-to-vm error reporting

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
--------

If `qvm-copy-to-vm` is used to copy a file from dom0 to a malicious
qube, that qube can inject an arbitrary command into dom0.

Impact
-------

If an attacker has compromised a qube, and if the user initiates a
`qvm-copy-to-vm` call from dom0 to the compromised qube, then the
attacker can exploit this vulnerability in order to inject an arbitrary
command into dom0, which allows the attacker to take control of
Qubes OS.

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

The `qvm-copy-to-vm` tool allows copying files from dom0 to a specified
qube. It uses the "qfile" protocol, which is a simplified archive
format, including simple file metadata (much simpler than `tar` or
`cpio`). The protocol also includes transfer confirmation at the end,
which is sent by the target back to the source. This confirmation
includes a checksum of all the transferred files, an error code (if
any), and the name of the last received file. In the case of an error,
as reported by the error code field, dom0 displays a GUI message that
includes the error information and the name of the affected file, as
reported by the target qube. The vulnerability exists in the processing
of that file name:

1. The `wait_for_result()` function calls `sanitize_remote_filename()`
on the received name before passing it to the error handler:

linux-utils/qrexec-lib/pack.c:

     55 static void sanitize_remote_filename(char *untrusted_filename)
     56 {
     57     for (; *untrusted_filename; ++untrusted_filename) {
     58         if (*untrusted_filename < ' ' ||
     59             *untrusted_filename > '~' ||
     60             *untrusted_filename == '"')
     61             *untrusted_filename = '_';
     62     }
     63 }
     64
     65 void wait_for_result(void)
     66 {
    ...
     98     /* sanitize the remote filename */
     99     sanitize_remote_filename(last_filename);
    100
    101     errno = hdr.error_code;
    102     if (hdr.error_code != 0) {
    103         switch (hdr.error_code) {
    104             case EEXIST:
    105                 call_error_handler("A file named \"%s\" already exists in QubesIncoming dir", last_filename);
    106                 break;
    ...

2. Then, `call_error_handler()` calls the dom0 variant of the error
reporting function -- `gui_fatal()` -> `display_error()`, which uses
`system()` to launch the actual error dialog:

core-admin-linux/file-copy-vm/qfile-dom0-agent.c:

     15 void display_error(const char *fmt, va_list args) {
     16     char *dialog_cmd;
     17     char buf[1024];
     18     struct stat st_buf;
     19     int ret;
     20
     21     (void) vsnprintf(buf, sizeof(buf), fmt, args);
     22     ret = stat("/usr/bin/kdialog", &st_buf);
     23 #define KDIALOG_CMD "kdialog --title 'File copy/move error' --sorry "
     24 #define ZENITY_CMD "zenity --title 'File copy/move error' --warning --text "
     25     if (asprintf(&dialog_cmd, "%s '%s: %s (error type: %s)'",
     26                 ret==0 ? KDIALOG_CMD : ZENITY_CMD,
     27                 program_invocation_short_name, buf, strerror(errno)) < 0) {
     28         fprintf(stderr, "Failed to allocate memory for error message :(\n");
     29         return;
     30     }
     31 #undef KDIALOG_CMD
     32 #undef ZENITY_CMD
     33     fprintf(stderr, "%s\n", buf);
     34     system(dialog_cmd);
     35 }
     36
     37 _Noreturn void gui_fatal(const char *fmt, ...) {
     38     va_list args;
     39     va_start(args, fmt);
     40     display_error(fmt, args);
     41     va_end(args);
     42     exit(1);
     43 }

The problem is that `sanitize_remote_filename()` removes only non-ASCII
characters (and double quotation marks) but leaves shell meta-characters
in place. Then, `system()` runs the constructed command, including the
attacker-controlled name via the shell.

Note that the VM variant of `qvm-copy-to-vm` is not affected, as its
version of the error reporting function does not use `system()`:

core-agent-linux/qubes-rpc/gui-fatal.c:

     16 static void produce_message(const char *type, const char *fmt, va_list args)
     17 {
    ...
     31     if (progress_type && !strcmp(progress_type, "gui"))
     32     {
     33         switch (fork())
     34         {
     35         case -1:
     36             exit(1); // what else
     37         case 0:
     38             if (geteuid() == 0) {
     39                 if (setuid(getuid()) != 0) {
     40                     perror("setuid failed, not calling zenity/kdialog");
     41                     exit(1);
     42                 }
     43             }
     44             fix_display();
     45             execlp("/usr/bin/zenity", "zenity", "--error", "--text", dialog_msg, NULL);
     46             execlp("/usr/bin/kdialog", "kdialog", "--sorry", dialog_msg, NULL);
     47             exit(1);
     48         default:;
     49         }
     50     }
     51     free(dialog_msg);
     52 }
     53
     54 void gui_fatal(const char *fmt, ...)
     55 {
     56     va_list args;
     57     va_start(args, fmt);
     58     produce_message("Fatal error", fmt, args);
     59     va_end(args);
     60     exit(1);
     61 }

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

All Qubes OS releases are affected.

Patching
---------

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

  For Qubes 4.3, in dom0:
  - qubes-core-dom0-linux, version 4.3.22

This package will migrate from the security-testing repository to the
current (stable) repository after a short period of testing by the
community. [2] Once available, the package should be installed via the
Qubes Update tool or its command-line equivalents. [1]

Credits
--------

The vulnerability was discovered by Tim C.

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-118-2026.txt

Marek Marczykowski-Górecki’s PGP signature

Source: qsb-118-2026.txt.sig.marmarek

Simon Gaiser (aka HW42)’s PGP signature

Source: qsb-118-2026.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-118), the commands are:

$ gpg --verify qsb-118-2026.txt.sig.marmarek qsb-118-2026.txt
$ gpg --verify qsb-118-2026.txt.sig.simon qsb-118-2026.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-118 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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