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.
In the XSS Protection API module 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.
In the XSS Protection API module 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.