CVE-2026-12039: CWE-923: Improper Restriction of Communication Channel to Intended Endpoints in Docker Docker Sandboxes
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.
AI Analysis
Technical Summary
Docker Sandboxes enforce an HTTP/S-only egress allowlist to restrict outbound communication. However, this allowlist is not applied to DNS resolution. The embedded per-network DNS server forwards any DNS query to the host resolver when the network is internet-connected, bypassing the allowlist. This allows a sandboxed workload, considered untrusted, to encode data into DNS labels for attacker-controlled domains and exfiltrate data through a DNS covert channel, circumventing the configured network restrictions.
Potential Impact
An attacker controlling a workload inside a Docker Sandbox can bypass the HTTP/S-only egress allowlist by using DNS queries to exfiltrate data covertly. This undermines the intended network communication restrictions and may lead to unauthorized data leakage from the sandbox environment.
Mitigation Recommendations
Patch status is not yet confirmed — check the vendor advisory for current remediation guidance. No official fix or temporary mitigation has been published by Docker as of the information available. Users should monitor Docker advisories for updates and consider restricting DNS resolution or monitoring DNS traffic as interim controls.
CVE-2026-12039: CWE-923: Improper Restriction of Communication Channel to Intended Endpoints in Docker Docker Sandboxes
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.
CVSS v4.0
Score 5.7medium
Affected software
pkg:github/docker/sbx-releasesRun on your own infrastructure? Check whether these packages are installed with threat-finder — our free open-source scanner.
Weaknesses
AI-Powered Analysis
Machine-generated threat intelligence
Technical Analysis
Docker Sandboxes enforce an HTTP/S-only egress allowlist to restrict outbound communication. However, this allowlist is not applied to DNS resolution. The embedded per-network DNS server forwards any DNS query to the host resolver when the network is internet-connected, bypassing the allowlist. This allows a sandboxed workload, considered untrusted, to encode data into DNS labels for attacker-controlled domains and exfiltrate data through a DNS covert channel, circumventing the configured network restrictions.
Potential Impact
An attacker controlling a workload inside a Docker Sandbox can bypass the HTTP/S-only egress allowlist by using DNS queries to exfiltrate data covertly. This undermines the intended network communication restrictions and may lead to unauthorized data leakage from the sandbox environment.
Mitigation Recommendations
Patch status is not yet confirmed — check the vendor advisory for current remediation guidance. No official fix or temporary mitigation has been published by Docker as of the information available. Users should monitor Docker advisories for updates and consider restricting DNS resolution or monitoring DNS traffic as interim controls.
Technical Details
- Data Version
- 5.2
- Assigner Short Name
- Docker
- Date Reserved
- 2026-06-11T19:23:44.967Z
- Cvss Version
- 4.0
- State
- PUBLISHED
- Remediation Level
- null
Threat ID: 6a33ff00f198dc38c1f0d3c9
Added to database: 06/18/2026, 14:21:52 UTC
Last enriched: 06/30/2026, 21:00:21 UTC
Last updated: 07/31/2026, 19:22:57 UTC
Views: 70
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