3

XXE 安全威胁目前没有。在 OWASP 十大 Web 应用程序安全威胁列表中排名第 4，因此我希望 Java 标准 XML 库能够阻止此类攻击。但是，当我以 Sonar 推荐的方式使用 Validator 类时，规则“XML 解析器不应受到 XXE 攻击 (java:S2755)”（规则链接）：

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));

在 Java 11 中，billionlaughs.xml 是

<?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>

我得到以下异常：

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)

所以我的问题是这是否被认为是减轻十亿次笑声攻击的正确方法（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 解析以简单地避免查看该<!DOCTYPE ..>部分。

4

1 回答 1

3

OWASP 十大条目和SonarSource规则都是关于XML External Entities的，而“Billion Laughs”攻击是使用XML Internal Entities构建的。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。

Java至少从 Java 1.5开始就有您遇到的实体扩展限制。

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 规则中提供的示例之一自行测试。例如，让您的验证器验证以下内容（假设您的操作系统是 Linux）：

<!DOCTYPE foo [
<!ELEMENT foo ANY >
<!ENTITY xxe SYSTEM "file:///etc/passwd" >]>
<foo>&xxe;</foo>

然后让你的代码输出你的值StringWriter writer。您会看到，在没有缓解的情况下，它包含/etc/passwd文件的内容。

如OWASP XML External Entity Prevention Cheat Sheet所述，在某些情况下，您还可以完全禁用 DTD（文档类型定义）以禁止外部和内部实体。

于 2020-09-03T15:08:57.487 回答