CVE-2021-43784

Overflow in netlink bytemsg length field allows attacker to override netlink-based container configuration in RunC

In runc, netlink is used internally as a serialization system for specifying the relevant container configuration to the C portion of our code (responsible for the based namespace setup of containers). In all versions of runc prior to 1.0.3, the encoder did not handle the possibility of an integer overflow in the 16-bit length field for the byte array attribute type, meaning that a large enough malicious byte array attribute could result in the length overflowing and the attribute contents being parsed as netlink messages for container configuration.

This vulnerability requires the attacker to have some control over the configuration of the container and would allow the attacker to bypass the namespace restrictions of the container by simply adding their own netlink payload which disables all namespaces.

Prior to 9c444070ec7bb83995dbc0185da68284da71c554, in practice it was fairly difficult to specify an arbitrary-length netlink message with most container runtimes. The only user-controlled byte array was the namespace paths attributes which can be specified in runc's config.json, but as far as we can tell no container runtime gives raw access to that configuration setting -- and having raw access to that setting would allow the attacker to disable namespace protections entirely anyway (setting them to /proc/1/ns/... for instance). In addition, each namespace path is limited to 4096 bytes (with only 7 namespaces supported by runc at the moment) meaning that even with custom namespace paths it appears an attacker still cannot shove enough bytes into the netlink bytemsg in order to overflow the uint16 counter.

However, out of an abundance of caution (given how old this bug is) we decided to treat it as a potentially exploitable vulnerability with a low severity. After 9c444070ec7bb83995dbc0185da68284da71c554 (which was not present in any release of runc prior to the discovery of this bug), all mount paths are included as a giant netlink message which means that this bug becomes significantly more exploitable in more reasonable threat scenarios.

The main users impacted are those who allow untrusted images with untrusted configurations to run on their machines (such as with shared cloud infrastructure), though as mentioned above it appears this bug was not practically exploitable on any released version of runc to date.

An arithmetic operation produces a value that exceeds the integer type's maximum, causing it to wrap to an unexpected small value. Typical impact: incorrect size calculations leading to heap overflows or logic errors.

CVE-2021-43784 has a CVSS score of 6.0 (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 (1.0.3); upgrading removes the vulnerable code path.

Kodem intelligence

Already deployed Kodem?

The patch for this is d72d057ba794164c3cce9451a00b72a78b25e1ae and runc 1.0.3 was released with this bug fixed.

1. What is CVE-2021-43784? CVE-2021-43784 is a medium-severity integer overflow or wraparound vulnerability in github.com/opencontainers/runc (go), affecting versions < 1.0.3. It is fixed in 1.0.3. An arithmetic operation produces a value that exceeds the integer type's maximum, causing it to wrap to an unexpected small value.
2. How severe is CVE-2021-43784? CVE-2021-43784 has a CVSS score of 6.0 (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/opencontainers/runc are affected by CVE-2021-43784? github.com/opencontainers/runc (go) versions < 1.0.3 is affected.
4. Is there a fix for CVE-2021-43784? Yes. CVE-2021-43784 is fixed in 1.0.3. Upgrade to this version or later.
5. Is CVE-2021-43784 exploitable, and should I be worried? Whether CVE-2021-43784 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-2021-43784 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-2021-43784? Upgrade github.com/opencontainers/runc to 1.0.3 or later.

Other vulnerabilities in github.com/opencontainers/runc