CVE-2017-9804 in Struts
Summary
by MITRE
In Apache Struts 2.3.7 through 2.3.33 and 2.5 through 2.5.12, if an application allows entering a URL in a form field and built-in URLValidator is used, it is possible to prepare a special URL which will be used to overload server process when performing validation of the URL. NOTE: this vulnerability exists because of an incomplete fix for S2-047 / CVE-2017-7672.
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Analysis
by VulDB Data Team • 01/11/2021
Apache Struts 2 represents a widely adopted Java web application framework that has been the target of numerous security vulnerabilities over its lifecycle. The vulnerability described in CVE-2017-9804 specifically affects versions ranging from 2.3.7 through 2.3.33 and 2.5 through 2.5.12, highlighting a critical flaw in the framework's URL validation mechanism that can be exploited to cause denial of service conditions. This vulnerability emerges from an incomplete remediation of a previous security issue, S2-047 or CVE-2017-7672, which demonstrates the complexity of addressing security flaws in web application frameworks where remediation efforts can sometimes introduce new attack vectors.
The technical flaw manifests when applications utilize the built-in URLValidator component and permit users to input URL values through form fields. The vulnerability exploits a specific URL format that, when processed by the validation logic, causes the server process to become overloaded. This occurs because the validation mechanism fails to properly handle certain malformed URL structures, leading to excessive resource consumption during the validation process. The flaw essentially creates a condition where an attacker can craft a URL that triggers an infinite loop or excessive computational overhead within the validation routine, consuming server resources and potentially causing system instability or complete service disruption.
The operational impact of this vulnerability extends beyond simple denial of service, as it represents a significant risk to application availability and system integrity. When exploited, the vulnerability can cause servers to consume excessive CPU cycles and memory resources, potentially leading to complete system crashes or the unavailability of critical business applications. This is particularly concerning for enterprise environments where Apache Struts applications may be handling sensitive transactions or serving high-traffic user bases. The vulnerability's persistence across multiple version ranges indicates a fundamental flaw in the validation logic that was not adequately addressed in the previous patch for CVE-2017-7672, creating a window of opportunity for attackers to exploit this weakness.
Organizations affected by this vulnerability should prioritize immediate remediation through version upgrades to Apache Struts 2.5.13 or later, which contain the proper fix for this issue. The remediation process should include thorough testing of applications to ensure that the upgrade does not introduce compatibility issues with existing functionality. Security teams should also implement network-level monitoring to detect unusual resource consumption patterns that might indicate exploitation attempts. From a security controls perspective, this vulnerability aligns with CWE-400, which addresses "Uncontrolled Resource Consumption" and represents a classic example of how incomplete security fixes can create new attack surfaces. The vulnerability also maps to ATT&CK technique T1499.004, "Endpoint Denial of Service," demonstrating how application-level flaws can be leveraged to achieve system-level disruption.
The root cause of this vulnerability demonstrates the importance of comprehensive security testing and validation of security patches, particularly when addressing complex validation logic in web frameworks. It underscores the need for organizations to maintain current security awareness and to conduct thorough regression testing when applying security updates. The vulnerability's existence as a follow-on issue from CVE-2017-7672 highlights the interconnected nature of security flaws and the critical importance of ensuring that security patches are complete and thoroughly tested before deployment. Organizations should also consider implementing additional validation layers and input sanitization techniques to reduce the attack surface and provide defense-in-depth protection against similar vulnerabilities in their web applications.