fix(preset): refuse to source world-writable preset files

Preset files are executed as shell code. A world-writable preset file
could be modified by any local user to inject arbitrary commands.

Symlinks are resolved before the permission check because stat on a
symlink returns the permissions of the link itself (always 777 on
Linux) rather than those of the target. readlink -f is used with a
fallback to realpath for macOS/BSD compatibility.

1 files changed, 15 insertions, 0 deletions

diff --git a/transcode.sh b/transcode.sh
index 7a0a9cb..15dba25 100755
--- a/transcode.sh
+++ b/transcode.sh
@@ -381,6 +381,21 @@ load_preset() {
 
   debug "Sourcing preset file:" "$file"
 
+  # Reject preset files that are world-writable to prevent arbitrary users
+  # from injecting shell code.
+  #
+  # Symlinks must be resolved first: stat on a symlink returns the permissions
+  # of the symlink itself (always 777 on Linux), not the target. We use
+  # readlink -f (GNU) with a fallback to realpath for macOS/BSD.
+  local resolved_file
+  resolved_file=$(readlink -f -- "$file" 2>/dev/null || realpath -- "$file")
+  local preset_perms
+  preset_perms=$(stat -c '%a' -- "$resolved_file" 2>/dev/null || stat -f '%OLp' -- "$resolved_file")
+  if [[ "${preset_perms: -1}" =~ [2367] ]]; then
+    error "Preset file is world-writable, refusing to source:" "$resolved_file"
+    exit $EXIT_CONFIG_ERROR
+  fi
+
   unset -v ffargs
   ffargs=()