CVE-2017-9802 in Sling
Summary
by MITRE
The Javascript method Sling.evalString() in Apache Sling Servlets Post before 2.3.22 uses the javascript 'eval' function to parse input strings, which allows for XSS attacks by passing specially crafted input strings.
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Analysis
by VulDB Data Team • 12/15/2022
The vulnerability identified as CVE-2017-9802 resides within Apache Sling Servlets Post component version 2.3.22 and earlier, where the Sling.evalString() javascript method demonstrates a critical security flaw through its reliance on the javascript eval function for input string parsing. This method operates within the server-side javascript execution environment of Apache Sling, which is commonly used for building dynamic web applications and content management systems. The vulnerability represents a classic cross-site scripting attack vector that exploits the dangerous nature of the eval function when processing untrusted user input, creating a pathway for malicious actors to inject and execute arbitrary javascript code within the context of affected web applications.
The technical implementation of this vulnerability stems from the improper handling of user-supplied input within the Sling.evalString() method, which directly feeds this input into the javascript eval function without adequate sanitization or validation. When the servlet processes POST requests containing specially crafted input strings, the eval function interprets these strings as executable javascript code rather than simple data, allowing attackers to inject malicious scripts that can execute in the browser context of legitimate users. This flaw aligns with CWE-94, which categorizes improper control of generation of code, specifically highlighting the dangerous use of eval functions in web applications. The vulnerability exists because the system fails to properly escape or validate input before passing it to the javascript interpreter, creating a direct code injection pathway that bypasses normal security controls.
The operational impact of CVE-2017-9802 extends beyond simple cross-site scripting, as successful exploitation can enable attackers to perform a wide range of malicious activities including session hijacking, data theft, redirection to malicious sites, and complete compromise of user sessions. Attackers can leverage this vulnerability to steal cookies, modify application behavior, access sensitive data, and potentially escalate privileges within the affected system. The vulnerability affects any web application built on Apache Sling that utilizes the Servlets Post component for processing user input, making it particularly dangerous in enterprise environments where such applications often handle sensitive business data and user authentication information. This flaw can be exploited through various attack vectors including web application penetration testing, social engineering campaigns, or by leveraging other vulnerabilities that allow initial access to the application's POST endpoints, aligning with ATT&CK technique T1059.007 for execution through scripting and T1566.001 for initial access through web application attacks.
Mitigation strategies for CVE-2017-9802 primarily focus on upgrading to Apache Sling Servlets Post version 2.3.22 or later, which includes proper input validation and sanitization mechanisms that prevent the dangerous use of eval functions. Organizations should also implement comprehensive input validation at multiple layers including client-side and server-side controls, employ Content Security Policy headers to limit script execution, and utilize web application firewalls to detect and block malicious input patterns. The remediation process should include thorough code reviews to identify any other instances where eval functions might be used inappropriately, and implementing proper sandboxing mechanisms for dynamic code execution. Security teams should also consider implementing automated vulnerability scanning tools that can detect similar patterns in custom applications and establish secure coding practices that eliminate the use of eval functions for processing untrusted input, following industry best practices outlined in OWASP Top 10 and secure coding guidelines that specifically address the risks associated with dynamic code evaluation and script injection vulnerabilities.