CVE-2026-23954: Incus Path Traversal Vulnerability

CVE-2026-23954 Overview

CVE-2026-23954 is a path traversal vulnerability [CWE-22] in Incus, a system container and virtual machine manager maintained by the Linux Containers project. Versions 6.21.0 and below permit a user with rights to launch a container from a custom image to abuse the image templating functionality. The templating logic fails to validate source and target paths for symbolic links or directory traversal sequences. An attacker can leverage this flaw to read and write arbitrary files on the host, leading to arbitrary command execution. IncusOS deployments are also affected.

Critical Impact
A member of the incus group can escape the container boundary through crafted metadata.yaml templates and obtain full command execution on the host system.

Affected Products

Linux Containers Incus versions 6.21.0 and earlier
Linux Containers Incus 6.0.x LTS branch (fix planned for 6.0.6)
IncusOS distributions bundling vulnerable Incus releases

Discovery Timeline

2026-01-22 - CVE-2026-23954 published to NVD
2026-01-30 - Last updated in NVD database

Technical Details for CVE-2026-23954

Vulnerability Analysis

The vulnerability resides in the LXC driver code that processes container image templates. When Incus launches a container from a custom image, it parses the bundled metadata.yaml file. This file can declare template entries that map source template files to target locations inside the container filesystem. The driver expands these templates during instance creation.

Neither the source nor the target path receives validation against symbolic links or .. traversal sequences. A crafted image can therefore direct the template engine to read files outside the image bundle and write rendered output anywhere the Incus daemon can reach. Because the daemon runs with elevated privileges on the host, writes can target system locations such as /etc/cron.d, /root/.ssh/authorized_keys, or systemd unit directories.

Root Cause

The template handling functions in internal/server/instance/drivers/driver_lxc.go (referenced at lines 7215 and 7294) resolve user-supplied paths without canonicalization or jail enforcement. The code trusts paths declared in metadata.yaml and follows symbolic links during read and write operations.

Attack Vector

Exploitation requires local or adjacent network access and low-privilege membership in the incus group. The attacker builds a malicious image archive containing a metadata.yaml that references templates with traversal sequences or symlinks. Launching a container from this image triggers the templated reads and writes against host paths. Writing an executable payload to a directory that the host periodically invokes results in arbitrary command execution as the Incus daemon user, typically root.

A proof-of-concept script (template_arbitrary_write.sh) demonstrating the arbitrary write path is referenced in the upstream advisory.

Detection Methods for CVE-2026-23954

Indicators of Compromise

Custom container images containing metadata.yaml entries with .. sequences or symbolic links in source or target template fields
Unexpected file writes to host paths such as /etc/, /root/, /var/spool/cron/, or systemd unit directories originating from the Incus daemon process
New entries in authorized_keys, cron tables, or service units created during container creation events

Detection Strategies

Audit Incus daemon logs for instance create and template rendering operations that resolve to paths outside the instance rootfs
Inspect image metadata before import using tar -tf and review templates/ directories and metadata.yaml for traversal patterns
Monitor process lineage where incusd spawns writes to sensitive system directories

Monitoring Recommendations

Enable file integrity monitoring on /etc, /root/.ssh, /etc/systemd/system, and cron directories on hosts running Incus
Forward Incus daemon audit logs to a centralized analytics platform and alert on template operations referencing paths outside expected image bundles
Track membership changes to the incus and incus-admin groups

How to Mitigate CVE-2026-23954

Immediate Actions Required

Restrict membership of the incus group to trusted administrators only and treat group members as privileged users on the host
Disable launching of containers from untrusted custom images until patched releases are deployed
Review existing image stores and remove any images with suspicious metadata.yaml template definitions

Patch Information

Fixed releases are planned in Incus 6.0.6 and 6.21.0 but had not been released at the time the advisory was published. Track the GitHub Security Advisory GHSA-7f67-crqm-jgh7 for release availability. An interim patch file (templates_arbitrary_write.patch) is published by the maintainers for operators who build Incus from source.

Workarounds

Permit only signed or vendor-supplied images in image stores and block user-supplied custom images
Run Incus inside a dedicated host with no sensitive data and isolate the host on a management VLAN to limit blast radius
Apply mandatory access control profiles (AppArmor or SELinux) to the incusd process to restrict write access to non-container directories

bash

# Configuration example: remove untrusted users from the incus group
sudo gpasswd -d <username> incus
sudo gpasswd -d <username> incus-admin

# Audit installed images for suspicious template paths
for img in $(incus image list -f csv -c f); do
  incus image export "$img" /tmp/img_inspect
  tar -xOf /tmp/img_inspect.tar.xz metadata.yaml 2>/dev/null | grep -E '(\.\./|^/)'
done