CVE-2016-6798 in Sling
Summary
by MITRE
In the XSS Protection API module before 1.0.12 in Apache Sling, the method XSS.getValidXML() uses an insecure SAX parser to validate the input string, which allows for XXE attacks in all scripts which use this method to validate user input, potentially allowing an attacker to read sensitive data on the filesystem, perform same-site-request-forgery (SSRF), port-scanning behind the firewall or DoS the application.
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Analysis
by VulDB Data Team • 01/01/2021
The vulnerability identified as CVE-2016-6798 resides within the XSS Protection API module of Apache Sling, specifically affecting versions prior to 1.0.12. This issue demonstrates a critical flaw in how the system handles XML input validation, creating a pathway for attackers to exploit XML External Entity vulnerabilities. The affected component is part of Apache Sling's security framework, which is designed to protect against cross-site scripting attacks, making this a particularly concerning weakness as it undermines the very security measures intended to safeguard the application.
The technical flaw stems from the XSS.getValidXML() method's reliance on an insecure SAX parser implementation that fails to properly sanitize XML input. This parser does not adequately restrict external entity references, allowing malicious actors to craft XML payloads that can trigger XXE attacks when processed by the vulnerable method. The insecure parsing mechanism bypasses normal security controls and enables attackers to manipulate the XML processing behavior through carefully constructed input that references external resources. This vulnerability operates at the XML parser level, where the system's attempt to validate input inadvertently creates an attack surface that can be exploited to bypass intended security boundaries.
The operational impact of this vulnerability extends far beyond simple data exposure, creating multiple attack vectors that can be leveraged by malicious actors. Successful exploitation can result in unauthorized access to sensitive files on the application server's filesystem, enabling attackers to read configuration files, database credentials, or other confidential information. The vulnerability also permits server-side request forgery attacks that can be used to probe internal networks, perform port scanning behind firewalls, or launch denial-of-service attacks against internal systems. These capabilities make the vulnerability particularly dangerous in enterprise environments where applications may have access to sensitive internal resources and where attackers could potentially use the compromised system as a pivot point for broader network infiltration.
Organizations affected by this vulnerability should immediately upgrade to Apache Sling version 1.0.12 or later, which contains the necessary fixes to address the insecure SAX parser implementation. Additionally, administrators should implement comprehensive input validation at multiple layers of the application architecture, ensuring that XML processing is performed using secure parsers that properly disable external entity resolution. Network segmentation and firewall rules should be reviewed to limit potential attack surfaces, while monitoring systems should be enhanced to detect anomalous XML processing patterns that might indicate exploitation attempts. This vulnerability aligns with CWE-611 (Improper Restriction of XML External Entity Reference) and maps to ATT&CK techniques involving server-side request forgery and privilege escalation through insecure XML processing.
The broader implications of this vulnerability highlight the critical importance of secure XML processing in web applications and demonstrate how security controls designed to prevent one type of attack can inadvertently create vulnerabilities for different attack vectors. This case underscores the necessity of thorough security testing across all components of web applications, particularly those handling user input, and emphasizes the need for security measures to be implemented consistently throughout the application lifecycle rather than as afterthoughts. Organizations should conduct comprehensive vulnerability assessments to identify similar insecure XML processing patterns in other components and ensure that all XML parsing operations are performed using secure configurations that disable external entity references and other potentially dangerous features.