RASP漏洞防御之 XXE 漏洞
2022-9-4 20:30:51 Author: mp.weixin.qq.com(查看原文) 阅读量:0 收藏

简介

当应用是通过用户上传的XML文件或POST请求进行数据的传输，并且应用没有禁止XML引用外部实体，也没有过滤用户提交的XML数据，那么就会产生XML外部实体注入漏洞。

XXE 漏洞在owasp2021中位置：

A05:2021 – Security Misconfiguration CWE-611 Improper Restriction of XML External Entity Reference（XXE）

防护

使用语言中推荐的禁用外部实体的方法

这里以 Java 语言为例子说明。

使用XML库的Java应用程序特别容易受到XXE攻击，因为大多数Java XML解析器的默认设置是启用XXE。为了安全地使用这些解析器，必须在使用的解析器中显式禁用XXE。下面描述如何在最常用的Java XML解析器中禁用XXE。

01.DocumentBuilderFactory

javax.xml.parsers.DocumentBuilderFactory

DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();String FEATURE = null;FEATURE = "http://apache.org/xml/features/disallow-doctype-decl";dbf.setFeature(FEATURE, true);
FEATURE = "http://xml.org/sax/features/external-general-entities";dbf.setFeature(FEATURE, false);
FEATURE = "http://xml.org/sax/features/external-parameter-entities";dbf.setFeature(FEATURE, false);
FEATURE = "http://apache.org/xml/features/nonvalidating/load-external-dtd";dbf.setFeature(FEATURE, false);
dbf.setXIncludeAware(false);dbf.setExpandEntityReferences(false);

02.Dom4j

org.dom4j.io.SAXReader

saxReader.setFeature("http://apache.org/xml/features/disallow-doctype-decl", true);saxReader.setFeature("http://xml.org/sax/features/external-general-entities", false);saxReader.setFeature("http://xml.org/sax/features/external-parameter-entities", false);

03.Jdom

org.jdom2.input.SAXBuilder、 org.jdom.input.SAXBuilder

SAXBuilder builder = new SAXBuilder();builder.setFeature("http://apache.org/xml/features/disallow-doctype-decl",true);builder.setFeature("http://xml.org/sax/features/external-general-entities", false);builder.setFeature("http://xml.org/sax/features/external-parameter-entities", false);builder.setExpandEntities(false);Document doc = builder.build(new File(fileName));

04.XMLInputFactory

javax.xml.stream.XMLInputFactory

// This disables DTDs entirely for that factoryxmlInputFactory.setProperty(XMLInputFactory.SUPPORT_DTD, false);// This causes XMLStreamException to be thrown if external DTDs are accessed.xmlInputFactory.setProperty(XMLConstants.ACCESS_EXTERNAL_DTD, "");// disable external entitiesxmlInputFactory.setProperty("javax.xml.stream.isSupportingExternalEntities", false);

05.XMLReader

org.xml.sax.XMLReader

XMLReader reader = XMLReaderFactory.createXMLReader();reader.setFeature("http://apache.org/xml/features/disallow-doctype-decl", true);// This may not be strictly required as DTDs shouldn't be allowed at all, per previous line.reader.setFeature("http://apache.org/xml/features/nonvalidating/load-external-dtd", false);reader.setFeature("http://xml.org/sax/features/external-general-entities", false);reader.setFeature("http://xml.org/sax/features/external-parameter-entities", false);

RASP防护

上面的修复方式在具体实施时存在缺陷：

  • 如果读取xml的代码在第三方包中（或者依赖框架），一般很难直接修改，必须pr框架/包提供者来修复；

这里以dom4j为例子来说RASP如何修复XX漏洞

package com.example.controller;
import org.dom4j.Document;import org.dom4j.Element;import org.dom4j.io.SAXReader;import org.springframework.http.ResponseEntity;import org.springframework.web.bind.annotation.PostMapping;import org.springframework.web.bind.annotation.RequestMapping;import org.springframework.web.bind.annotation.RestController;
import java.io.ByteArrayInputStream;
@RestController@RequestMapping("/xxe/dom4j")public class Dom4jController {        @PostMapping("/post/dom4j.do")    public ResponseEntity<String> documentBuilder1(String xml, int fix) throws Exception {        return ResponseEntity.ok(dom4j(xml, fix));    }
    public static String dom4j(String xml, int fix) throws Exception {        SAXReader saxReader = new SAXReader();        // 启用修复方式        if (fix == 1) {            saxReader.setFeature("http://apache.org/xml/features/disallow-doctype-decl", true);            saxReader.setFeature("http://xml.org/sax/features/external-general-entities", false);            saxReader.setFeature("http://xml.org/sax/features/external-parameter-entities", false);            saxReader.setFeature("http://apache.org/xml/features/nonvalidating/load-external-dtd", false);        }        ByteArrayInputStream byteArrayInputStream = new ByteArrayInputStream(xml.getBytes());        Document document = saxReader.read(byteArrayInputStream);        Element element = document.getRootElement();        return element.getText();    }}

dom4j版本

 <dependency>    <groupId>org.dom4j</groupId>    <artifactId>dom4j</artifactId>    <version>2.0.0</version></dependency>

POC

curl --location --request POST 'http://localhost:8080/xxe/dom4j/post/dom4j.do' \--header 'Content-Type: application/x-www-form-urlencoded' \--data-urlencode 'xml=<?xml version="1.0" encoding="UTF-8"?><!DOCTYPE root [<!ENTITY xxe SYSTEM "file:///etc/passwd">]><root>&xxe;</root>' \--data-urlencode 'fix=0'

读取 /etc/passwd文本内容

返回结果：

### User Database# # Note that this file is consulted directly only when the system is running# in single-user mode.  At other times this information is provided by# Open Directory.## See the opendirectoryd(8) man page for additional information about# Open Directory.##nobody:*:-2:-2:Unprivileged User:/var/empty:/usr/bin/falseroot:*:0:0:System Administrator:/var/root:/bin/shdaemon:*:1:1:System Services:/var/root:/usr/bin/false...

模块插件举例

    /**     * read重载方法最终调用 read(InputSource)     *     * @see org.dom4j.io.SAXReader#read(InputSource)     */    public void closeDom4jXXE() {        final String className = "org.dom4j.io.SAXReader";        final String methdName = "read";        new EventWatchBuilder(moduleEventWatcher)                .onClass(className)                .includeBootstrap()                .onBehavior(methdName)                .withParameterTypes("org.xml.sax.InputSource")                .onWatch(new AdviceListener() {                    @Override                    public void before(Advice advice) throws Throwable {                        if (!enableCheck) {                            return;                        }                        SAXReader saxReader = (SAXReader) advice.getTarget();                        saxReader.setFeature(FEATURE_DEFAULTS_1, true);                        saxReader.setFeature(FEATURE_DEFAULTS_2, false);                        saxReader.setFeature(FEATURE_DEFAULTS_3, false);                    }
                    @Override                    protected void afterThrowing(Advice advice) throws Throwable {                        requestInfoThreadLocal.remove();                    }                });    }

    需要注意的是，JRASP 对Dom4j的hook类做了进一步的优化，hook了更加底层的方法（如下所示）。

public Document read(InputSource in) throws DocumentException

（经过debug和实际dump字节码发现，open-rasp 存在重复插桩、防护逻辑执行两次的问题。相关issue：https://github.com/baidu/openrasp/issues/396）

选取hook类一般是十分慎重，必须了解hook类的功能，选取类的一般性原则如下：

  • 能防护漏洞；

  • 选取更底层方法，避免被被绕过；

  • 在保证功能的基础上hook类尽量少；

目前JRASP已经具备上面5类常用XML解析器的漏洞防护插件。

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XXE漏洞案例：

https://www.jrasp.com/case/CVE-2018-15531.html

https://www.jrasp.com/case/CVE-2018-1259.html

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