CVE-2024-29018

Moby is an open source container framework originally developed by Docker Inc. as Docker. It is a key component of Docker Engine, Docker Desktop, and other distributions of container tooling or runtimes. As a batteries-included container runtime, Moby comes with a built-in networking implementation that enables communication between containers, and between containers and external resources.

Moby's networking implementation allows for creating and using many networks, each with their own subnet and gateway. This feature is frequently referred to as custom networks, as each network can have a different driver, set of parameters, and thus behaviors. When creating a network, the --internal flag is used to designate a network as internal. The internal attribute in a docker-compose.yml file may also be used to mark a network internal, and other API clients may specify the internal parameter as well.

When containers with networking are created, they are assigned unique network interfaces and IP addresses (typically from a non-routable RFC 1918 subnet). The root network namespace (hereafter referred to as the 'host') serves as a router for non-internal networks, with a gateway IP that provides SNAT/DNAT to/from container IPs.

Containers on an internal network may communicate between each other, but are precluded from communicating with any networks the host has access to (LAN or WAN) as no default route is configured, and firewall rules are set up to drop all outgoing traffic. Communication with the gateway IP address (and thus appropriately configured host services) is possible, and the host may communicate with any container IP directly.

In addition to configuring the Linux kernel's various networking features to enable container networking, dockerd directly provides some services to container networks. Principal among these is serving as a resolver, enabling service discovery (looking up other containers on the network by name), and resolution of names from an upstream resolver.

When a DNS request for a name that does not correspond to a container is received, the request is forwarded to the configured upstream resolver (by default, the host's configured resolver). This request is made from the container network namespace: the level of access and routing of traffic is the same as if the request was made by the container itself.

As a consequence of this design, containers solely attached to internal network(s) will be unable to resolve names using the upstream resolver, as the container itself is unable to communicate with that nameserver. Only the names of containers also attached to the internal network are able to be resolved.

Many systems will run a local forwarding DNS resolver, typically present on a loopback address (127.0.0.0/8), such as systemd-resolved or dnsmasq. Common loopback address examples include 127.0.0.1 or 127.0.0.53. As the host and any containers have separate loopback devices, a consequence of the design described above is that containers are unable to resolve names from the host's configured resolver, as they cannot reach these addresses on the host loopback device.

To bridge this gap, and to allow containers to properly resolve names even when a local forwarding resolver is used on a loopback address, dockerd will detect this scenario and instead forward DNS requests from the host/root network namespace. The loopback resolver will then forward the requests to its configured upstream resolvers, as expected.

Workarounds

• Run containers intended to be solely attached to internal networks with a custom upstream address (--dns argument to docker run, or API equivalent), which will force all upstream DNS queries to be resolved from the container network namespace.

Background

• yair zak originally reported this issue to the Docker security team.
• PR https://github.com/moby/moby/pull/46609 was opened in public to fix this issue, as it was not originally considered to have a security implication.
• The official documentation claims that "the --internal flag that will completely isolate containers on a network from any communications external to that network," which necessitated this advisory and CVE.

Because dockerd will forward DNS requests to the host loopback device, bypassing the container network namespace's normal routing semantics entirely, internal networks can unexpectedly forward DNS requests to an external nameserver.

By registering a domain for which they control the authoritative nameservers, an attacker could arrange for a compromised container to exfiltrate data by encoding it in DNS queries that will eventually be answered by their nameservers. For example, if the domain evil.example was registered, the authoritative nameserver(s) for that domain could (eventually and indirectly) receive a request for this-is-a-secret.evil.example.

Docker Desktop is not affected, as Docker Desktop always runs an internal resolver on a RFC 1918 address.

CVE-2024-29018 has a CVSS score of 5.9 (Medium). The vector is network-reachable, no 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 (26.0.0-rc3, 25.0.5, 23.0.11); upgrading removes the vulnerable code path.

Kodem intelligence

Moby releases 26.0.0-rc3, 25.0.5 (released) and 23.0.11 (to be released) are patched to prevent forwarding DNS requests from internal networks.

1. What is CVE-2024-29018? CVE-2024-29018 is a medium-severity security vulnerability in github.com/docker/docker (go), affecting versions >= 26.0.0-rc1, < 26.0.0-rc3. It is fixed in 26.0.0-rc3, 25.0.5, 23.0.11.
2. How severe is CVE-2024-29018? CVE-2024-29018 has a CVSS score of 5.9 (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/docker/docker are affected by CVE-2024-29018? github.com/docker/docker (go) versions >= 26.0.0-rc1, < 26.0.0-rc3 is affected.
4. Is there a fix for CVE-2024-29018? Yes. CVE-2024-29018 is fixed in 26.0.0-rc3, 25.0.5, 23.0.11. Upgrade to this version or later.
5. Is CVE-2024-29018 exploitable, and should I be worried? Whether CVE-2024-29018 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-2024-29018 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-2024-29018?
   • Upgrade github.com/docker/docker to 26.0.0-rc3 or later
   • Upgrade github.com/docker/docker to 25.0.5 or later
   • Upgrade github.com/docker/docker to 23.0.11 or later