CVE-2026-23953

A user with the ability to launch a container with a custom YAML configuration (e.g a member of the ‘incus’ group) can create an environment variable containing newlines, which can be used to add additional configuration items in the container’s lxc.conf due to the newline injection. This can allow adding arbitrary lifecycle hooks, ultimately resulting in arbitrary command execution on the host.

Details

When passing environment variables in the config block of a new container, values are not checked for the presence of newlines [1], which can result in newline injection inside the generated container lxc.conf. This can be used to set arbitrary additional configuration items, such as lxc.hook.pre-start. By exploiting this, a user with the ability to launch a container with an arbitrary config can achieve arbitrary command execution as root on the host.

Exploiting this issue on IncusOS requires a slight modification of the payload to change to a different writable directory for the validation step (e.g /tmp). This can be confirmed with a second container with /tmp mounted from the host (A privileged action for validation only).

[1] https://github.com/lxc/incus/blob/HEAD/internal/server/instance/drivers/driver_lxc.go#L1081

PoC

A proof-of-concept script exploiting this vulnerability can be found attached, named environment_newline_injection.sh, showing arbitrary command execution, which will write a file to the root filesystem (/newline_injection_command_exec_poc)

Manual Reproduction steps:

1. Launch a new container with a configuration file containing a multiline YAML string as an environment variable value, such as in the listing below.
2. Observe that the lxc.conf (/run/incus/user-1000_poc/lxc.conf in my case) contains an additional lxc.hook.pre-start item
3. Observe the creation of the file in the host root directory, with contents proving command execution as root.

incus launch images:alpine/edge --ephemeral poc << EOF
config:
  environment.FOO: |-
    abc
    lxc.hook.pre-start = /bin/sh -c "id > /newline_injection_command_exec_poc"
EOF

Attachments

environment_newline_injection.sh
environment_newline_injection.patch

A user with the ability to launch a container with a custom YAML configuration (e.g a member of the ‘incus’ group) can achieve arbitrary command execution on the host.

CVE-2026-23953 has a CVSS score of 8.7 (High). The vector is reachable from an adjacent network, low privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (6.21.0); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-23953? CVE-2026-23953 is a high-severity security vulnerability in github.com/lxc/incus/v6 (go), affecting versions < 6.21.0. It is fixed in 6.21.0.
2. How severe is CVE-2026-23953? CVE-2026-23953 has a CVSS score of 8.7 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of github.com/lxc/incus/v6 are affected by CVE-2026-23953? github.com/lxc/incus/v6 (go) versions < 6.21.0 is affected.
4. Is there a fix for CVE-2026-23953? Yes. CVE-2026-23953 is fixed in 6.21.0. Upgrade to this version or later.
5. Is CVE-2026-23953 exploitable, and should I be worried? Whether CVE-2026-23953 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-23953 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-23953? Upgrade github.com/lxc/incus/v6 to 6.21.0 or later.