xxe十亿笑声攻击似乎没有像声纳推荐的防止xxe攻击的解决方案所预期的那样得到缓解

des4xlb0  于 2021-07-05  发布在  Java
关注(0)|答案(1)|浏览(482)

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。

lg40wkob

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):

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

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。

相关问题