xxe安全威胁目前在owasp十大web应用程序安全威胁列表中排名第四,因此我希望java标准xml库能够防止此类攻击。然而,当我以sonar推荐的方式使用validator类时,规则“xml解析器不应该容易受到xxe攻击(java:s2755)“(链接到规则):
String xsd = "xxe.xsd";
String xml = "billionlaughs.xml";
StreamSource xsdStreamSource = new StreamSource(xsd);
StreamSource xmlStreamSource = new StreamSource(xml);
SchemaFactory schemaFactory = SchemaFactory.newInstance(XMLConstants.W3C_XML_SCHEMA_NS_URI);
Schema schema = schemaFactory.newSchema(xsdStreamSource);
schemaFactory.setProperty(XMLConstants.ACCESS_EXTERNAL_DTD, "");
schemaFactory.setProperty(XMLConstants.ACCESS_EXTERNAL_SCHEMA, "");
// validators will also inherit of these properties
Validator validator = schema.newValidator();
validator.setProperty(XMLConstants.ACCESS_EXTERNAL_DTD, ""); // Compliant
validator.setProperty(XMLConstants.ACCESS_EXTERNAL_SCHEMA, ""); // Compliant
StringWriter writer = new StringWriter();
validator.validate(xmlStreamSource, new StreamResult(writer));
使用Java11,使用billionlaughs.xml
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE lolz [
<!ENTITY lol "lol">
<!ENTITY lol2 "&lol;&lol;&lol;&lol;&lol;&lol;&lol;&lol;&lol;&lol;">
<!ENTITY lol3 "&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;">
<!ENTITY lol4 "&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;">
<!ENTITY lol5 "&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;">
<!ENTITY lol6 "&lol5;&lol5;&lol5;&lol5;&lol5;&lol5;&lol5;&lol5;&lol5;&lol5;">
<!ENTITY lol7 "&lol6;&lol6;&lol6;&lol6;&lol6;&lol6;&lol6;&lol6;&lol6;&lol6;">
<!ENTITY lol8 "&lol7;&lol7;&lol7;&lol7;&lol7;&lol7;&lol7;&lol7;&lol7;&lol7;">
<!ENTITY lol9 "&lol8;&lol8;&lol8;&lol8;&lol8;&lol8;&lol8;&lol8;&lol8;&lol8;">
]>
<lolz>&lol9;</lolz>
我得到以下例外:
Exception in thread "main" org.xml.sax.SAXParseException; lineNumber: 1; columnNumber: 1; JAXP00010001: The parser has encountered more than "64000" entity expansions in this document; this is the limit imposed by the JDK.
at java.xml/com.sun.org.apache.xerces.internal.util.ErrorHandlerWrapper.createSAXParseException(ErrorHandlerWrapper.java:204)
at java.xml/com.sun.org.apache.xerces.internal.util.ErrorHandlerWrapper.fatalError(ErrorHandlerWrapper.java:178)
at java.xml/com.sun.org.apache.xerces.internal.impl.XMLErrorReporter.reportError(XMLErrorReporter.java:400)
at java.xml/com.sun.org.apache.xerces.internal.impl.XMLErrorReporter.reportError(XMLErrorReporter.java:327)
at java.xml/com.sun.org.apache.xerces.internal.impl.XMLErrorReporter.reportError(XMLErrorReporter.java:284)
at java.xml/com.sun.org.apache.xerces.internal.impl.XMLEntityManager.startEntity(XMLEntityManager.java:1413)
at java.xml/com.sun.org.apache.xerces.internal.impl.XMLEntityManager.startEntity(XMLEntityManager.java:1337)
at java.xml/com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl.scanEntityReference(XMLDocumentFragmentScannerImpl.java:1842)
at java.xml/com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl$FragmentContentDriver.next(XMLDocumentFragmentScannerImpl.java:2982)
at java.xml/com.sun.org.apache.xerces.internal.impl.XMLDocumentScannerImpl.next(XMLDocumentScannerImpl.java:605)
at java.xml/com.sun.org.apache.xerces.internal.impl.XMLNSDocumentScannerImpl.next(XMLNSDocumentScannerImpl.java:112)
at java.xml/com.sun.org.apache.xerces.internal.impl.XMLDocumentFragmentScannerImpl.scanDocument(XMLDocumentFragmentScannerImpl.java:534)
at java.xml/com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse(XML11Configuration.java:888)
at java.xml/com.sun.org.apache.xerces.internal.parsers.XML11Configuration.parse(XML11Configuration.java:824)
at java.xml/com.sun.org.apache.xerces.internal.jaxp.validation.StreamValidatorHelper.validate(StreamValidatorHelper.java:176)
at java.xml/com.sun.org.apache.xerces.internal.jaxp.validation.ValidatorImpl.validate(ValidatorImpl.java:115)
at trial.Trial.main(Trial.java:35)
因此,我的问题是,这是否被认为是减轻十亿笑脸攻击的正确方法(毕竟,有64000个实体扩展的限制),或者是否有另一种方法来配置xml解析以避免查看 <!DOCTYPE ..>
部分。
1条答案
按热度按时间lg40wkob1#
owasp的前十条条目和sonarsource规则都是关于xml外部实体的,而“billion laughs”攻击是使用xml内部实体构造的。内部实体定义为:
[…]没有单独的物理存储对象,实体的内容在声明中给出。
自Java1.5以来,java已经达到了您正在经历的实体扩展限制。
但是,仍然需要推荐的缓解措施来防止xml外部实体攻击。您可以使用owasp站点或sonarsource规则中提供的一个示例来测试这一点。例如,让验证器验证以下内容(假设您的操作系统是linux):
然后让代码输出
StringWriter writer
之后。您将看到,在没有缓解措施的情况下,它包含/etc/passwd
文件。如owasp xml外部实体预防备忘单所述,在某些情况下,您还可以完全禁用DTD(文档类型定义)以禁用外部和内部实体。