Any program can be a GitHub Actions shell

In GitHub Actions, you can use the shell keyword to specify the shell that runs a given run: block. This keyword is optional for workflows but mandatory for action definitions.

The shell normally defaults to something sensible for your runner, e.g. bash on Linux and macOS, and pwsh on Windows. But it can also be specified, and GitHub documents that specifying it explicitly also implies some flags of their choosing:

# setting bash explicitly implies bash --noprofile --norc -eo pipefail
- shell: bash
  run: |
    echo "Hello, world!"

Based on that, you might think that there's a fixed number of valid shell values¹, which GitHub tracks and adds their special flags to. But you'd be wrong!

As it turns out, you can set shell to any executable on the $PATH, and GitHub will happily use it to execute the run block. If the command doesn't already take a single file as input, you need to pass it the special {0} argument, which GitHub replaces with the temporary file that it generates the template-expanded run block into.

Thanks to this, we can do all kinds of cursed things.

Using C as our step runner works fine:

- run: sudo apt install -y tcc
- shell: tcc -run {0}
  run: |
    #include <stdio.h>
    int main() {
      printf("Hello, world!\n");
      return 0;
    }

...and so does dynamically modifying the $PATH via $GITHUB_PATH:

- run: |
    touch ./bash
    chmod +x ./bash
    echo '#!/bin/sh' > ./bash
    echo 'echo hello from fake bash' >> ./bash
    echo "${PWD}" >> "${GITHUB_PATH}"
- run: |
    echo "this doesn't do what you expect"
  shell: bash

Does this matter from a security perspective? It's hard to say — there are plenty of other ways in which file writes imply execution in GitHub Actions, including GITHUB_ENV itself.

On the other hand, it's surprising (IMO) that GitHub does $PATH lookups even for their "well-known" shell values; I would expect those to be fixed to specific values (e.g. bash to /bin/bash), especially since they inject flags into the run's command line based on those well-known values.