Bug 2254825 (CVE-2023-6683)

Summary: CVE-2023-6683 QEMU: VNC: NULL pointer dereference in qemu_clipboard_request()
Product: [Other] Security Response Reporter: Mauro Matteo Cascella <mcascell>
Component: vulnerabilityAssignee: Product Security <prodsec-ir-bot>
Status: NEW --- QA Contact:
Severity: medium Docs Contact:
Priority: medium    
Version: unspecifiedCC: ailan, ddepaula, jen, jferlan, jmaloy, kkiwi, knoel, mrezanin, mst, nilal, pbonzini, security-response-team, ymankad
Target Milestone: ---Keywords: Security
Target Release: ---   
Hardware: All   
OS: Linux   
Fixed In Version: qemu-kvm 9.0.0 Doc Type: ---
Doc Text:
A flaw was found in the QEMU built-in VNC server while processing ClientCutText messages. The qemu_clipboard_request() function can be reached before vnc_server_cut_text_caps() was called and had the chance to initialize the clipboard peer, leading to a NULL pointer dereference. This could allow a malicious authenticated VNC client to crash QEMU and trigger a denial of service.
Story Points: ---
Clone Of: Environment:
Last Closed: Type: ---
Regression: --- Mount Type: ---
Documentation: --- CRM:
Verified Versions: Category: ---
oVirt Team: --- RHEL 7.3 requirements from Atomic Host:
Cloudforms Team: --- Target Upstream Version:
Bug Depends On: 2258118    
Bug Blocks: 2254016    

Description Mauro Matteo Cascella 2023-12-16 17:33:41 UTC
It can be that a client sends a VNC_MSG_CLIENT_CUT_TEXT message before sending a VNC_MSG_CLIENT_SET_ENCODINGS message with VNC_ENCODING_CLIPBOARD_EXT for configuring the clipboard extension.

This means that qemu_clipboard_request() can be reached (via vnc_client_cut_text_ext()) before vnc_server_cut_text_caps() was called and had the chance to initialize the clipboard peer. In that case, info->owner->request is NULL instead of a function and so attempting to call it in qemu_clipboard_request() results in a segfault.

In particular, this can happen when using the KRDC (22.12.3) VNC client on Wayland. A malicious or misbehaving VNC client can crash QEMU - only after successful authentication.

Comment 2 Mauro Matteo Cascella 2023-12-16 17:39:54 UTC
Red Hat Product Security would like to thank Markus Frank (Proxmox) and Fiona Ebner (Proxmox) for reporting this issue.

Comment 5 Mauro Matteo Cascella 2024-01-12 18:48:54 UTC
Upstream patch:

Comment 6 Mauro Matteo Cascella 2024-01-12 18:49:44 UTC
Created qemu tracking bugs for this issue:

Affects: fedora-all [bug 2258118]

Comment 7 Mauro Matteo Cascella 2024-04-08 06:57:55 UTC
Upstream commit:

Comment 8 errata-xmlrpc 2024-04-30 09:35:44 UTC
This issue has been addressed in the following products:

  Red Hat Enterprise Linux 9

Via RHSA-2024:2135 https://access.redhat.com/errata/RHSA-2024:2135