CVE-2016-5394 in Sling
Summary
by MITRE
In the XSS Protection API module before 1.0.12 in Apache Sling, the encoding done by the XSSAPI.encodeForJSString() method is not restrictive enough and for some input patterns allows script tags to pass through unencoded, leading to potential XSS vulnerabilities.
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Analysis
by VulDB Data Team • 10/27/2019
The vulnerability CVE-2016-5394 affects Apache Sling's XSS Protection API module, specifically targeting the XSSAPI.encodeForJSString() method implementation prior to version 1.0.12. This issue represents a critical security flaw in web application input sanitization that directly impacts the protection mechanisms designed to prevent cross-site scripting attacks. The vulnerability stems from insufficient encoding logic that fails to adequately sanitize user input when embedded within JavaScript contexts, creating a pathway for malicious scripts to execute in victim browsers.
The technical flaw manifests in the inadequate implementation of JavaScript string encoding within the XSS Protection API. When user-supplied data is processed through the encodeForJSString() method, certain input patterns bypass the encoding mechanism entirely, allowing raw script tags to remain unescaped and executable. This weakness specifically affects how the API handles special characters and escape sequences when preparing data for insertion into JavaScript string contexts. The vulnerability is categorized under CWE-79 as a cross-site scripting flaw, and more specifically aligns with CWE-116 which addresses improper encoding or escaping of output. The flaw demonstrates a failure in the principle of least privilege and proper input validation, where the system assumes that its encoding functions provide sufficient protection without accounting for edge cases in character handling.
The operational impact of this vulnerability extends beyond simple script execution, potentially enabling attackers to perform session hijacking, defacement, data theft, and other malicious activities through compromised web applications. When exploited, the vulnerability allows attackers to inject malicious JavaScript code that executes in the context of authenticated users, leading to privilege escalation and unauthorized access to sensitive data. The attack vector typically involves submitting malicious input through web forms, URL parameters, or API endpoints that are subsequently processed by the vulnerable Sling application. This vulnerability directly maps to ATT&CK technique T1059.007 for JavaScript execution and T1566 for credential access through web application attacks. The exposure creates a persistent threat vector that can be exploited across multiple applications built on the Apache Sling framework, particularly those handling user-generated content or dynamic data processing.
Mitigation strategies for CVE-2016-5394 require immediate patching of affected Apache Sling installations to version 1.0.12 or later, which includes the corrected encoding implementation. Organizations should also implement additional defensive measures such as content security policy headers, proper input validation at multiple layers, and regular security scanning of web applications. The fix addresses the core encoding deficiency by implementing more comprehensive escape sequence handling and ensuring that all potentially dangerous characters are properly encoded for JavaScript contexts. Security teams should conduct thorough vulnerability assessments of all Sling-based applications, review input handling mechanisms, and implement proper security monitoring to detect potential exploitation attempts. Additionally, developers should adopt secure coding practices that emphasize the use of established security libraries and frameworks rather than custom encoding implementations, and implement comprehensive testing including fuzzing and penetration testing to identify similar encoding vulnerabilities in other components of the application stack.