What is CVE-2020-11091?

CVE-2020-11091 is a medium-severity security vulnerability in github.com/weaveworks/weave (go), affecting versions < 2.6.3. It is fixed in 2.6.3.

How severe is CVE-2020-11091?

CVE-2020-11091 has a CVSS score of 5.8 (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.

Which versions of github.com/weaveworks/weave are affected by CVE-2020-11091?

github.com/weaveworks/weave (go) versions < 2.6.3 is affected.

Is there a fix for CVE-2020-11091?

Yes. CVE-2020-11091 is fixed in 2.6.3. Upgrade to this version or later.

Is CVE-2020-11091 exploitable, and should I be worried?

Whether CVE-2020-11091 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.

What actually determines whether CVE-2020-11091 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.

How do I fix CVE-2020-11091?

Upgrade github.com/weaveworks/weave to 2.6.3 or later.

CVE-2020-11091 in github.com/weaveworks/weave

Q: How severe is CVE-2020-11091?

CVE-2020-11091 has a CVSS score of 5.8 (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.

Q: Which versions of github.com/weaveworks/weave are affected by CVE-2020-11091?

github.com/weaveworks/weave (go) versions < 2.6.3 is affected.

Q: Is there a fix for CVE-2020-11091?

Yes. CVE-2020-11091 is fixed in 2.6.3. Upgrade to this version or later.

Q: Is CVE-2020-11091 exploitable, and should I be worried?

Whether CVE-2020-11091 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.

Q: What actually determines whether CVE-2020-11091 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.

Q: How do I fix CVE-2020-11091?

Upgrade github.com/weaveworks/weave to 2.6.3 or later.

Summary

Weave Net clusters susceptible to MitM attacks via IPv6 rogue router advertisements

Workarounds

Users should not run containers with CAP_NET_RAW capability. This has been the advice from Weave Net for years.
https://www.weave.works/docs/net/latest/kubernetes/kube-addon/#-securing-the-setup

For more information

If you have any questions or comments about this advisory:

Open an issue in the Weave Net repo
Join the Weave Users Slack.

Impact

An attacker able to run a process as root in a container is able to respond to DNS requests from the host and thereby insert themselves as a fake service.

In a cluster with an IPv4 internal network, if IPv6 is not totally disabled on the host (via ipv6.disable=1 on the kernel cmdline), it will be either unconfigured or configured on some interfaces, but it’s pretty likely that ipv6 forwarding is disabled, ie /proc/sys/net/ipv6/conf//forwarding == 0. Also by default, /proc/sys/net/ipv6/conf//accept_ra == 1. The combination of these 2 sysctls means that the host accepts router advertisements and configure the IPv6 stack using them.

By sending “rogue” router advertisements, an attacker can reconfigure the host to redirect part or all of the IPv6 traffic of the host to the attacker controlled container.
Even if there was no IPv6 traffic before, if the DNS returns A (IPv4) and AAAA (IPv6) records, many HTTP libraries will try to connect via IPv6 first then fallback to IPv4, giving an opportunity to the attacker to respond.
If by chance you also have on the host a vulnerability like last year’s RCE in apt (CVE-2019-3462), you can now escalate to the host.

CVE-2020-11091 has a CVSS score of 5.8 (Medium). The vector is network-reachable, high 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 (2.6.3); upgrading removes the vulnerable code path.

Affected versions

github.com/weaveworks/weave (< 2.6.3)

Security releases

github.com/weaveworks/weave → 2.6.3 (go)

Kodem intelligence

Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.

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. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.

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Remediation advice

Weave Net version 2.6.3 (to be released soon) will disable the accept_ra option on the veth devices that it creates.

Frequently Asked Questions

What is CVE-2020-11091? CVE-2020-11091 is a medium-severity security vulnerability in github.com/weaveworks/weave (go), affecting versions < 2.6.3. It is fixed in 2.6.3.
How severe is CVE-2020-11091? CVE-2020-11091 has a CVSS score of 5.8 (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.
Which versions of github.com/weaveworks/weave are affected by CVE-2020-11091? github.com/weaveworks/weave (go) versions < 2.6.3 is affected.
Is there a fix for CVE-2020-11091? Yes. CVE-2020-11091 is fixed in 2.6.3. Upgrade to this version or later.
Is CVE-2020-11091 exploitable, and should I be worried? Whether CVE-2020-11091 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
What actually determines whether CVE-2020-11091 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.
How do I fix CVE-2020-11091? Upgrade github.com/weaveworks/weave to 2.6.3 or later.