CVE-2020-17519: Apache Flink Improper Access Control Vulnerability

CVE-2020-17519 is an improper access control vulnerability in Apache Flink. It allows an attacker to read any file on the local filesystem of the JobManager through its REST interface. This matters because JobManagers often hold configuration files, credentials, keys, or other sensitive operational data; unauthorized reads can give an attacker a foothold for further compromise of stream-processing or analytics environments.

Defenders should treat exposure of the JobManager REST interface as high priority and confirm exact impact and fixed releases against the vendor advisory.

How it works

The weakness is classified as CWE-552 (Files or Directories Accessible to External Parties). In this case, Apache Flink’s JobManager REST interface does not properly restrict file-access operations. An unauthenticated or insufficiently authorized remote attacker who can reach the REST endpoint can request arbitrary local filesystem paths and retrieve their contents.

No exploit code or precise request format is provided here; the practical abuse path is simply that the REST interface becomes a read oracle for any file the JobManager process can open. Confirm the exact attack surface and any authentication requirements against the vendor advisory.

Am I affected? How to find it in your systems

Apache Flink is commonly deployed for stream and batch processing in data platforms, often as a cluster with one or more JobManagers and TaskManagers. The JobManager’s REST interface is typically exposed on a management port for job submission, monitoring, and administration.

• Inventory all hosts and containers running Apache Flink processes or packages; check orchestration manifests, configuration management databases, and package inventories.
• Identify any JobManager REST endpoints that are reachable from untrusted networks or from other internal segments that should not have management access.
• Confirm whether the installed Flink release is affected by comparing it to the versions listed in the vendor advisory; do not rely on version numbers stated outside that advisory.
• Review access logs, reverse-proxy logs, or network telemetry for unexpected GET or similar requests targeting the JobManager REST paths, especially those that appear to reference filesystem locations. Unusual volume or source IPs contacting the management port are also useful indicators.

If public detail on exact log signatures is limited, treat any unexplained REST activity against JobManagers as suspicious until investigated.

How to remediate

Apply the vendor-supplied update or mitigations first. CISA’s required action is to apply mitigations per vendor instructions or discontinue use of the product if mitigations are unavailable. Confirm the precise patch, configuration change, or upgrade path against the official Apache Flink advisory for CVE-2020-17519.

After patching, harden the deployment for this class of flaw:

• Bind the JobManager REST interface only to trusted management networks or localhost where operationally feasible.
• Enforce authentication and authorization on all management endpoints; remove any anonymous access.
• Apply least-privilege file-system permissions so the Flink process cannot read unnecessary sensitive files even if a similar flaw reappears.
• Document and restrict which operators or automation systems may reach the REST interface.

If you can't patch immediately

Until the vendor fix can be applied, reduce exposure with compensating controls:

• Network segmentation or firewall rules that block untrusted sources from reaching the JobManager REST port.
• Temporary disablement of the REST interface if the operational model allows job submission and monitoring by other means.
• Virtual patching or WAF rules that reject requests attempting to access filesystem-related paths through the REST API (validate any rule against your traffic patterns to avoid breaking legitimate use).
• Heightened monitoring and alerting on the management port, including source IP allow-listing and anomaly detection for file-read patterns.
• If no effective mitigation is available, plan to discontinue use of the affected Flink deployment until it can be secured.

These steps lower risk but do not replace the vendor update.

If your data may have been exposed

Actively exploited vulnerabilities of this type can lead to breaches in which configuration files, credentials, or other local data are exfiltrated. Review JobManager hosts for signs of unauthorized file access and rotate any secrets that may have been readable. You can also run a free exposure scan of your email addresses against known breach data to check whether related accounts appear in public breach corpora.