CVE-2017-9791: Apache Struts 1 Improper Input Validation Vulnerability

CVE-2017-9791 is an improper input validation weakness in Apache Struts 1 that can allow remote code execution. According to CISA, the Struts 1 plugin in Apache Struts might allow remote code execution via a malicious field value passed in a raw message to the ActionMessage. For teams still running legacy Struts 1 applications, this matters because successful abuse can give an attacker control over the application process and the host it runs on.

Public detail is limited to the description above; exact affected builds, attack preconditions, and fixed releases must be confirmed against the vendor advisory. Treat any internet-facing or internally reachable Struts 1 instance as in scope until you verify otherwise.

How it works

The underlying weakness is CWE-20 (Improper Input Validation). In this case, the Struts 1 plugin does not adequately validate a field value that is supplied in a raw message destined for ActionMessage handling. An attacker who can submit such a crafted value may cause the framework to process it in an unsafe way, potentially leading to remote code execution in the context of the application server.

No further exploit mechanics, payloads, or preconditions are provided in the available facts. Defenders should assume that any untrusted input path that reaches ActionMessage processing is relevant and should not invent or rely on unconfirmed attack details. Confirm the precise trigger conditions and impact in the vendor advisory and your own code review of how messages are constructed and passed.

Am I affected? How to find it in your systems

Apache Struts 1 is a legacy Java web-application framework. It typically appears in older enterprise Java EE applications, often packaged as WAR/EAR files deployed on application servers such as Tomcat, JBoss/WildFly, WebLogic, or WebSphere. It may also be present as a transitive dependency inside custom or third-party applications that have not been modernized.

Practical inventory steps:

• Search artifact repositories, build files (pom.xml, build.gradle, Ivy), and deployed lib directories for Struts 1 packages or JARs (commonly named with “struts” and a 1.x identifier).
• Inspect running application servers for loaded Struts 1 classes or configuration files (struts-config.xml and related plugin descriptors).
• Review software bills of materials (SBOMs) and vulnerability scanner results for Apache Struts 1 components.
• Identify any application that still uses the Struts 1 plugin and ActionMessage handling paths reachable by untrusted users.

Because exact version ranges are not supplied here, compare every discovered Struts 1 instance against the vendor advisory. Telemetry signs of exploitation are not detailed in the facts; monitor application and server logs for anomalous input to message/ActionMessage handling, unexpected process creation, or unusual outbound connections from the application runtime. Absence of known signatures does not mean absence of compromise.

How to remediate

Patch first. CISA’s required action is to apply updates per vendor instructions. Obtain the fixed package or migration guidance directly from the Apache Struts project or your commercial distributor and apply it in accordance with the advisory. After patching, re-scan to confirm the vulnerable component is no longer present.

Additional hardening appropriate to this class of flaw:

• Remove or replace the Struts 1 plugin if the application no longer requires it.
• Enforce strict server-side validation and allow-listing of all data that flows into ActionMessage or equivalent message-construction APIs.
• Run the application with least privilege, separating the app-server identity from sensitive data stores and administrative interfaces.
• Prefer frameworks and libraries that are actively maintained; plan migration off Struts 1 where feasible.

If you can't patch immediately

Implement compensating controls while you arrange the vendor update:

• Segment the affected application: place it in a restricted network zone, limit inbound access to only necessary clients, and block unnecessary egress.
• Apply virtual patching via a web application firewall or reverse-proxy rules that reject suspicious or malformed field values targeting message-handling endpoints; tune rules carefully to avoid breaking legitimate traffic and confirm effectiveness against the advisory description.
• Disable or tightly restrict any unused Struts 1 plugin features and raw-message entry points if configuration allows.
• Increase monitoring: alert on anomalous request patterns to ActionMessage-related paths, unexpected child processes, and changes to application binaries or configuration.
• Ensure robust, tested backups and an incident-response path so that a successful compromise can be contained quickly.

These measures reduce risk but do not eliminate it; schedule the official update as soon as possible.

If your data may have been exposed

Actively exploited remote-code-execution vulnerabilities frequently lead to full application or host compromise and subsequent data theft. Known ransomware use is not documented for this CVE. If you have evidence of exploitation or cannot rule it out, follow your incident-response process: isolate affected systems, preserve logs and memory images, rotate credentials, and assess what data the application could access.

As a further check, you can run a free exposure scan of your email addresses against known breach data sets to see whether credentials or personal information associated with your organization have already appeared in public breach corpora. That check does not replace forensic investigation of the vulnerable systems themselves.