CVE-2016-8738 in Struts
Summary
by MITRE
In Apache Struts 2.5 through 2.5.5, if an application allows entering a URL in a form field and the 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.
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Analysis
by VulDB Data Team • 01/13/2021
The vulnerability identified as CVE-2016-8738 represents a critical server-side request forgery issue affecting Apache Struts versions 2.5 through 2.5.5. This flaw resides within the URLValidator component that applications commonly employ to validate user-provided URL inputs in web forms. The vulnerability stems from insufficient validation of URL schemes and their associated protocols, creating an avenue for malicious actors to manipulate the validation process through specially crafted URLs that trigger excessive server resource consumption. The technical implementation involves the URLValidator's handling of specific URL formats that cause the underlying validation logic to enter resource-intensive processing loops, effectively creating a denial of service condition.
The operational impact of this vulnerability extends beyond simple denial of service to potentially enable more sophisticated attack vectors when combined with other exploitation techniques. The flaw allows attackers to craft URLs that, when processed by the URLValidator, cause the server to consume excessive CPU cycles and memory resources during validation operations. This behavior creates a predictable pattern of resource exhaustion that can be exploited to disrupt legitimate service availability. The vulnerability operates at the application layer and can be triggered through any form field that utilizes the built-in URLValidator, making it particularly dangerous in web applications that process user input through Struts framework components. Security researchers have classified this vulnerability as a CWE-400 weakness, specifically related to excessive resource consumption, which aligns with the observed behavior of the validation process becoming unbounded under certain input conditions.
The exploitation of CVE-2016-8738 demonstrates a classic example of how seemingly benign input validation can become a vector for resource exhaustion attacks. When applications accept user-provided URLs and validate them using Apache Struts' URLValidator, malicious inputs can cause the validation process to enter infinite loops or consume disproportionate computational resources. This vulnerability can be leveraged by attackers to perform distributed denial of service attacks against targeted systems, as the resource consumption patterns are consistent and predictable. The attack surface includes any web application built on Apache Struts 2.5 through 2.5.5 that permits user input containing URLs and utilizes the URLValidator for validation purposes. Organizations implementing the affected Struts versions should consider the vulnerability's potential for being combined with other attack vectors, particularly those involving command injection or remote code execution, as the resource exhaustion can serve as a precursor to more severe exploitation attempts. The remediation strategy involves upgrading to Apache Struts 2.5.6 or later versions where the URLValidator has been patched to properly handle malformed URL inputs and prevent the excessive resource consumption patterns that enable this vulnerability.
The broader implications of this vulnerability highlight the importance of proper input validation and resource management in web application frameworks. Security practitioners should implement comprehensive monitoring for unusual resource consumption patterns in applications using affected Struts versions, as the vulnerability may be difficult to detect through traditional security scanning methods. The ATT&CK framework categorizes this vulnerability under the resource exhaustion technique, where attackers leverage application-level flaws to consume system resources. Organizations should consider implementing web application firewalls and input validation rules that can detect and block suspicious URL patterns before they reach the vulnerable validation logic. Additionally, regular security assessments of web applications should include verification of framework versions and proper configuration of validation components to prevent exploitation of similar vulnerabilities in other parts of the application stack.