# CVE-2026-12039: Docker Sandboxes network egress allowlist bypass via unfiltered DNS resolution

> Untrusted workloads in Docker Sandboxes can bypass network egress allowlists and exfiltrate data via DNS covert channels due to unfiltered DNS resolution.

- **Severity:** Medium 5.7 (`CVSS:4.0/AV:L/AC:L/AT:P/PR:L/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 18 Jun 2026
- **Updated:** 13 Aug 2026
- **Also known as:** GHSA-4crj-qc6g-g3xj

## What is CVE-2026-12039?

CVE-2026-12039 is a medium-severity vulnerability in Docker Sandboxes, published on 18 Jun 2026. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

Docker Sandboxes (sbx) enforces an HTTP/S-only egress allowlist but does not apply it to DNS resolution: the per-network embedded DNS server forwards any queried name to the host resolver whenever the network is internet-connected, without consulting the policy. A workload inside a sandbox, which the threat model treats as untrusted, can therefore encode data into DNS labels for an attacker-controlled domain and exfiltrate it through a DNS covert channel, bypassing the configured allowlist.

Weakness: CWE-923 (Improper Restriction of Communication Channel to Intended Endpoints)

## Which versions are affected by CVE-2026-12039?


## How do I fix CVE-2026-12039?

No fix has been published for CVE-2026-12039 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-12039 being exploited?

There is no known exploitation of CVE-2026-12039 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-12039 and fix it if it is affected.

What it is: CVE-2026-12039 is a medium-severity vulnerability in Docker Sandboxes, published on 18 Jun 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-12039 in the wild and no public exploit code.

Affected product: Docker Sandboxes. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-12039/ (Markdown: https://insights.sbomify.com/CVE-2026-12039/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-12039
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-12039
- OSV: https://osv.dev/vulnerability/CVE-2026-12039

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-12039/
Summaries are generated and may contain mistakes; the official description above is the record.