Flannel has cross-node remote code execution via extension backend BackendData injectionGHSA-vchx-5pr6-ffx2
HIGHFix: flannel-io/flannel@08bc9a4GHSA-vchx-5pr6-ffx2 is a high-severity (CVSS 7.5) CWE-77 vulnerability in github.com/flannel-io/flannel. A fix is available for github.com/flannel-io/flannel — see the affected versions and patch details below.
Exploitation Status
No confirmed exploitation observed yet
- A successful exploit gives an attacker total control of the affected component, not partial access.
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for GHSA-vchx-5pr6-ffx2.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
Where this sits among everything scored
Real-World Exposure
github.com/flannel-io/flannelReal-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
Background
The Flannel project includes an experimental Extension backend that allows users to easily prototype new backend types. This backend uses shell commands stored in Kubernetes annotations to configure network connectivity on the node.
Note: consumers are only affected by this vulnerability if they use the experimental Extension backend. Other backends such as vxlan and wireguard are unaffected.
Vulnerability
This Extension backend is vulnerable to a command injection that allows an attacker who can set Kubernetes Node annotations to achieve root-level arbitrary command execution on every flannel node in the cluster.
The Extension backend's SubnetAddCommand and SubnetRemoveCommand receive attacker-controlled data via stdin (from the flannel.alpha.coreos.com/backend-data Node annotation). The content of this annotation is unmarshalled and piped directly to a shell command without checks.
Impact
Kubernetes clusters using Flannel with the Extension backend are affected by this vulnerability. Other backends such as vxlan and wireguard are unaffected.
Patches
This is fixed in version v0.28.2.
Workaround
If consumers cannot update to a patched version, then use Flannel with another backend such as vxlan or wireguard.
Credits
Flannel would like to thank Shachar Tal from Palo Alto Networks for reporting this vulnerability.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐹Go | github.com/flannel-io/flannel | all versions | 0.28.2go get github.com/flannel-io/flannel@v0.28.2 |
Affected Products
flannelflannel-ioDetection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/flannel-io/flannel, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update github.com/flannel-io/flannel to 0.28.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-vchx-5pr6-ffx2 is resolved across your whole dependency graph.
Workarounds
Stop passing untrusted input into the interpreter or shell: call the affected binary with an argument array rather than a composed command string, reject anything outside a strict allowlist of expected values, and run the component under an account that cannot reach beyond the work it legitimately does.
Frequently Asked Questions
Is GHSA-vchx-5pr6-ffx2 in your dependencies?
Find it across Go, including transitive dependencies.