CVE-2016-4461 in Struts
Summary
by MITRE
Apache Struts 2.x before 2.3.29 allows remote attackers to execute arbitrary code via a "%{}" sequence in a tag attribute, aka forced double OGNL evaluation. NOTE: this vulnerability exists because of an incomplete fix for CVE-2016-0785.
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Analysis
by VulDB Data Team • 01/17/2021
Apache Struts 2 versions prior to 2.3.29 contain a critical remote code execution vulnerability that stems from an incomplete remediation of CVE-2016-0785, creating a persistent security gap in web applications built on this framework. This vulnerability specifically targets the tag attribute processing mechanism where attackers can inject malicious OGNL (Object-Graph Navigation Language) expressions through carefully crafted "%{}" sequences within tag attributes. The flaw operates by exploiting the framework's forced double OGNL evaluation process, which occurs when the Struts framework processes user input through tag attributes and subsequently evaluates the same expressions twice, allowing attackers to execute arbitrary code on the server with the privileges of the application server.
The technical exploitation of this vulnerability leverages the OGNL expression evaluation engine within Apache Struts, which is designed to provide dynamic access to object properties and methods. When a "%{}" sequence is present in a tag attribute, the framework's processing logic performs an initial evaluation of the expression followed by a second forced evaluation, creating a window where malicious code can be injected and executed. This double evaluation process bypasses normal input sanitization mechanisms and allows attackers to construct payloads that execute commands on the server, potentially leading to complete system compromise. The vulnerability is particularly dangerous because it can be triggered through various tag attributes across different Struts components, making it difficult to fully mitigate without proper patching.
The operational impact of CVE-2016-4461 extends far beyond simple remote code execution, as it provides attackers with the capability to gain persistent access to affected systems and potentially escalate privileges within the application environment. Attackers can leverage this vulnerability to execute arbitrary commands, access sensitive data, modify application behavior, and establish backdoors for continued access. The vulnerability affects organizations running Apache Struts 2 applications that have not upgraded to version 2.3.29 or later, making it a significant concern for enterprise environments where multiple applications depend on this framework. The incomplete fix for CVE-2016-0785 demonstrates a pattern of security remediation gaps that can leave systems vulnerable to continued exploitation, particularly in complex frameworks where multiple evaluation contexts interact. Organizations may face regulatory compliance issues, data breaches, and operational disruptions if they fail to address this vulnerability promptly.
Organizations should immediately upgrade to Apache Struts 2.3.29 or later versions to remediate this vulnerability, as this represents the complete fix for both CVE-2016-4461 and its predecessor CVE-2016-0785. Security teams should implement network-based mitigations such as web application firewalls and input validation rules to filter out suspicious "%{}" sequences, though these should be considered temporary measures until proper patching occurs. The vulnerability aligns with CWE-94, which describes improper control of generation of code, and maps to ATT&CK technique T1059.007 for command and script interpreter execution. Additionally, this vulnerability demonstrates characteristics of privilege escalation and persistence mechanisms, making it a high-value target for attackers who may use it as a stepping stone for more extensive compromise of enterprise networks. Regular security assessments and vulnerability scanning should be implemented to identify other potential instances of this vulnerability across the organization's application portfolio, particularly in legacy systems that may not receive regular updates.