CVE-2017-15717 in Sling XSS Protection API
Summary
by MITRE
A flaw in the way URLs are escaped and encoded in the org.apache.sling.xss.impl.XSSAPIImpl#getValidHref and org.apache.sling.xss.impl.XSSFilterImpl#isValidHref allows special crafted URLs to pass as valid, although they carry XSS payloads. The affected versions are Apache Sling XSS Protection API 1.0.4 to 1.0.18, Apache Sling XSS Protection API Compat 1.1.0 and Apache Sling XSS Protection API 2.0.0.
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Analysis
by VulDB Data Team • 12/22/2019
This vulnerability resides within the Apache Sling XSS Protection API implementation where improper URL escaping and encoding mechanisms fail to adequately validate hyperlink references. The flaw specifically affects the getValidHref and isValidHref methods in the XSSAPIImpl and XSSFilterImpl classes respectively, creating a pathway for malicious actors to craft specially formatted URLs that bypass security checks while still containing cross-site scripting payloads. The vulnerability impacts versions ranging from 1.0.4 through 1.0.18 of the core XSS Protection API, along with version 1.1.0 of the compatibility module and version 2.0.0 of the main API, representing a significant security gap in web application frameworks that handle user input validation.
The technical implementation flaw stems from inadequate sanitization of URL components during the validation process, particularly in how special characters and encoding sequences are processed. When URLs containing XSS payloads are submitted through the affected API methods, the validation logic fails to properly decode or escape certain URL-encoded sequences that should trigger security warnings. This allows attackers to embed malicious JavaScript code within URLs that are subsequently accepted as valid by the system, potentially leading to unauthorized code execution in the context of vulnerable web applications. The vulnerability operates at the input validation layer, making it particularly dangerous as it can bypass multiple security controls that rely on proper URL validation.
The operational impact of this vulnerability extends beyond simple XSS exploitation, as it undermines the fundamental security assumptions of the Sling XSS protection mechanism. Attackers can craft URLs that appear legitimate to the validation system but contain embedded malicious payloads, potentially leading to session hijacking, data theft, or unauthorized administrative actions. The vulnerability is particularly concerning in environments where Sling applications process user-generated content or external links, as it creates opportunities for persistent XSS attacks that can affect multiple users. This flaw directly relates to CWE-79 which addresses cross-site scripting vulnerabilities, and maps to ATT&CK technique T1203 which involves exploiting web application vulnerabilities to execute malicious code.
Organizations should immediately upgrade to patched versions of the Apache Sling XSS Protection API, specifically versions beyond the affected ranges mentioned in the vulnerability description. The mitigation strategy should include comprehensive code review of any custom implementations that rely on the vulnerable API methods, alongside thorough testing of URL validation logic in all web applications using Sling frameworks. Security teams should implement additional monitoring for suspicious URL patterns and consider deploying web application firewalls that can detect and block known XSS payload patterns. Regular security assessments should verify that all URL processing components properly handle encoding sequences and that input validation occurs at multiple layers to prevent similar issues from emerging in other parts of the application stack.