Update: Please note that QSB-071 was updated on 2021-09-17. See below for the updated version, including the changelog.
We have just published Qubes Security Bulletin (QSB) 071: Fatal options filtering flaw in Split GPG. The text of this QSB is reproduced below. This QSB and its accompanying signatures will always be available in the Qubes Security Pack (qubes-secpack).
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---===[ Qubes Security Bulletin 071 ]===--- 2021-09-09 Fatal options filtering flaw in Split GPG Changelog ---------- 2021-09-09: Original QSB published 2021-09-17: Updated impact section User action required --------------------- Users must install the following specific packages in order to address the issues discussed in this bulletin: For Qubes 4.0, in templates and standalones: - qubes-gpg-split 2.0.53 For Qubes 4.1, in templates and standalones: - qubes-gpg-split 2.0.53 Due to the ease with which this flaw can be exploited, we are immediately migrating these packages to the current (stable) repository, bypassing the usual testing period. These packages are to be installed via the Qubes Update tool or its command-line equivalents.  Summary -------- Split GPG  is designed to isolate private keys from the application using them in order to protect them from being extracted and to allow the user to retain control over when they are used. This isolation is implemented by forwarding calls to `gpg` into a backend qube that holds the private keys and allowing only specific `gpg` options. This option filtering mechanism rejects options like `--export-secret-keys` and others that might leak private keys to the frontend qube. Unfortunately, several options were declared incorrectly, which allowed this filtering mechanism to be bypassed. Impact ------- An attacker controlling a frontend qube (where `qubes-gpg-client` is executed) can execute arbitrary commands and extract arbitrary files (including secret keys) from the backend qube. Discussion ----------- Several `gpg` options were declared incorrectly in Split GPG, which resulted in Split GPG interpreting them differently than `gpg`. If Split GPG interpreted one option as an argument to another option, Split GPG would allow it, since option filtering is performed at the level of the options themselves, not their arguments. This would allow options misinterpreted as arguments to bypass the filtering mechanism. Specifically: - All `--s2k-*` options were declared as not taking arguments when in fact they do take arguments. - `--export-ssh-key` was declared as taking an argument when it doesn't take one directly; it does change the meanings of positional arguments, however. - `--with-colons` was aliased with `-k`, which differs in its argument requirements. - `--default-recipient`, which takes an argument, was interpreted as `--default-recipient-self`, which does not take an argument. - `--display` was interpreted as `--display-charset`, which resulted in `--display` being allowed when it should have been denied. For our immediate, initial response, we have corrected all of these inconsistencies and added automated testing to verify that GnuPG and Split GPG both understand the options in the same way. More generally, we will prioritize finishing Split GPG 2 , which does not rely on option filtering at all. Instead, it uses `gpg-agent`'s protocol to delegate only secret key processing to the backend qube. In addition to obviating the need for fragile option filtering, this dramatically reduces the attack surface, as most of the untrusted data processing is done in the frontend qube and never reaches the backend qube. Credits -------- This issue was discovered by Demi Marie Obenour. References -----------  https://www.qubes-os.org/doc/updating-qubes-os/  https://www.qubes-os.org/doc/split-gpg/  https://github.com/QubesOS/qubes-issues/issues/474 -- The Qubes Security Team https://www.qubes-os.org/security/
This is a companion discussion topic for the original entry at https://www.qubes-os.org/news/2021/09/09/qsb-071/