CVE-2020-11091 is a medium-severity (CVSS 5.8) CWE-350 vulnerability in github.com/weaveworks/weave. O3 Security confirms whether CVE-2020-11091 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
Weave Net clusters susceptible to MitM attacks via IPv6 rogue router advertisements
Real-World Exposure
github.com/weaveworks/weaveReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects Go packages — download data is not available via public APIs for these ecosystems.
Description
In Weave Net before version 2.6.3, 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. Weave Net version 2.6.3 disables the accept_ra option on the veth devices that it creates.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐹Go | github.com/weaveworks/weave | all versions | 2.6.3 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/weaveworks/weave. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update github.com/weaveworks/weave to 2.6.3 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2020-11091 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether CVE-2020-11091 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to CVE-2020-11091. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2020-11091 in your dependencies?
O3 detects CVE-2020-11091 across Go dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.