CVE-2021-32783 – This vulnerability in Contour Kubernetes ingres... (How to Fix)

Q: What is the severity of CVE-2021-32783?

CVE-2021-32783 has a CVSS score of 8.5 (HIGH). This vulnerability in Contour Kubernetes ingress controller allows attackers to access Envoy's admin interface via specially crafted ExternalName Services. This enables remote denial of service, traffic manipulation, and exposure of secret metadata (though not secret content). All Contour users running vulnerable versions are affected.

📋 TL;DR

This vulnerability in Contour Kubernetes ingress controller allows attackers to access Envoy's admin interface via specially crafted ExternalName Services. This enables remote denial of service, traffic manipulation, and exposure of secret metadata (though not secret content). All Contour users running vulnerable versions are affected.

💻 Affected Systems

Products:

Contour (Kubernetes ingress controller)

Versions: All versions before 1.17.1

Operating Systems: Any OS running Contour

Default Config Vulnerable: ⚠️ Yes

Notes: Only affects deployments using ExternalName Services. Contour deployments without ExternalName Services are not vulnerable.

📦 What is this software?

Contour by Projectcontour

View all CVEs affecting Contour →

⚠️ Risk & Real-World Impact

🔴

Worst Case

Complete shutdown of Envoy proxies causing service disruption, exposure of TLS certificate metadata, and potential traffic routing manipulation across the cluster.

🟠

Likely Case

Targeted denial of service against specific Envoy instances, exposure of secret metadata that could aid further attacks, and potential traffic disruption.

🟢

If Mitigated

Limited impact with proper network segmentation and access controls, though the vulnerability still exists in the software.

🌐 Internet-Facing: HIGH - Contour typically handles external traffic, making internet-facing deployments particularly vulnerable to exploitation.

🏢 Internal Only: MEDIUM - Internal attackers or compromised internal systems could exploit this, but requires Kubernetes cluster access.

🎯 Exploit Status

Public PoC: ✅ No

Weaponized: LIKELY

Unauthenticated Exploit: ⚠️ Yes

Complexity: LOW

Exploitation requires creating or modifying ExternalName Services, which typically requires Kubernetes RBAC permissions. The attack technique is documented in the advisory.

🛠️ Fix & Mitigation

✅ Official Fix

Patch Version: 1.17.1 (cherry-pick) and 1.18.0

Vendor Advisory: https://github.com/projectcontour/contour/security/advisories/GHSA-5ph6-qq5x-7jwc

Restart Required: Yes

Instructions:

1. Update Contour to version 1.17.1 or later. 2. Update Contour deployment manifests. 3. Restart Contour pods. 4. Verify Envoy pods restart with new configuration.

🔧 Temporary Workarounds

Restrict ExternalName Service Creation

all

Use Kubernetes RBAC to prevent creation/modification of ExternalName Services in namespaces where Contour operates.

kubectl create clusterrole deny-externalname --verb=create,update,patch --resource=services --resource-name=ExternalName
kubectl create clusterrolebinding deny-externalname-binding --clusterrole=deny-externalname --group=system:authenticated

Network Policy Isolation

all

Implement network policies to restrict access to Envoy admin interface (port 9001 by default) from unauthorized sources.

kubectl apply -f network-policy.yaml (with appropriate rules blocking external access to Envoy admin port)

🧯 If You Can't Patch

Implement strict RBAC controls to prevent unauthorized users from creating or modifying ExternalName Services
Deploy network policies to isolate Envoy admin interface and monitor for unauthorized access attempts

🔍 How to Verify

Check if Vulnerable:

Check Contour version: kubectl get deployment contour -n projectcontour -o jsonpath='{.spec.template.spec.containers[0].image}' | grep -o ':[0-9.]*'

Check Version:

kubectl get deployment contour -n projectcontour -o jsonpath='{.spec.template.spec.containers[0].image}'

Verify Fix Applied:

Verify Contour version is 1.17.1 or later and test that ExternalName Services cannot access Envoy admin interface

📡 Detection & Monitoring

Log Indicators:

Unauthorized access attempts to Envoy admin interface (port 9001)
Unexpected ExternalName Service creation/modification events in Kubernetes audit logs

Network Indicators:

Traffic to Envoy admin port (default 9001) from unexpected sources
ExternalName Service DNS resolution patterns matching attack vectors

SIEM Query:

source="kubernetes-audit" AND (resource="services" AND verb IN ("create", "update", "patch") AND objectRef.subresource="externalName") OR (destination.port=9001 AND NOT source.ip IN (allowed_admin_ips))

📊 Metadata

CVE ID: CVE-2021-32783

CVSS v3 Score: 8.5 (HIGH)

CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:L/I:N/A:H

CWE: CWE-441

Published: July 23, 2021

Last Updated: November 21, 2024

🔗 References

https://github.com/projectcontour/contour/commit/b53a5c4fd927f4ea2c6cf02f1359d8e28bef852e Patch,Third Party Advisory
https://github.com/projectcontour/contour/releases/tag/v1.17.1 Release Notes,Third Party Advisory
https://github.com/projectcontour/contour/security/advisories/GHSA-5ph6-qq5x-7jwc Third Party Advisory
https://github.com/projectcontour/contour/commit/b53a5c4fd927f4ea2c6cf02f1359d8e28bef852e Patch,Third Party Advisory
https://github.com/projectcontour/contour/releases/tag/v1.17.1 Release Notes,Third Party Advisory
https://github.com/projectcontour/contour/security/advisories/GHSA-5ph6-qq5x-7jwc Third Party Advisory

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