CVE-2026-12039: Docker Sandboxes network egress allowlist bypass via unfiltered DNS resolution
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.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Compensating control
Mitigate the Docker Sandbox egress allowlist bypass by blocking outbound DNS queries from sandbox networks (e.g., prevent DNS resolution/forwarding to an internet-connected host resolver), so DNS labels for attacker-controlled domains cannot be resolved for DNS covert-channel exfiltration.
Event History
Frequently Asked Questions
What is the severity of CVE-2026-12039?
CVE-2026-12039 is classified as medium severity with a score of 5.7 according to CVSS.
How does CVE-2026-12039 affect Docker Sandboxes?
CVE-2026-12039 allows a bypass of the egress allowlist by enabling unfiltered DNS resolution within Docker Sandboxes.
How do I fix CVE-2026-12039?
To address CVE-2026-12039, ensure that your Docker Sandboxes are updated to at least version 0.33.0 or apply relevant security patches.
What is the exploit mechanism of CVE-2026-12039?
The exploit in CVE-2026-12039 occurs when the embedded DNS server forwards queries to the host resolver without adhering to the configured egress policy.
What impacts can CVE-2026-12039 have on security?
CVE-2026-12039 can potentially lead to unauthorized network access and data exfiltration from workloads running in Docker Sandboxes.