PYSEC-2026-2862

Summary

PDM writes several project-local state or configuration files without symlink protection. If a malicious repository places those files as symlinks, local PDM operations can overwrite the symlink targets.

This creates an arbitrary file clobber primitive relative to the privileges of the invoking user.

Affected Behavior

• Project-local config writes can affect files outside the repository
• The most stable demonstrated sink is pdm.toml
• Related sinks include .pdm-python and .python-version

Affected Code

• src/pdm/project/config.py:303-350
• src/pdm/project/core.py:209-217
• src/pdm/cli/commands/use.py:187-189

Technical Details

Config.__init__() resolves the project-local pdm.toml path and _save_config() writes to the resolved target. If PROJECT_ROOT/pdm.toml is a symlink to another file, pdm config -l ... updates the target file instead of refusing the write.

The same general problem exists for other project-local persistence paths that are written directly with no lstat / O_NOFOLLOW protection.

For the pdm.toml PoC specifically, the target file must already contain parseable TOML. Otherwise the load step fails before the write path is reached. That parser constraint does not apply to the .pdm-python or .python-version sinks.

Impact

• Arbitrary file clobber as the invoking user
• Destructive modification of local files outside the repository root
• Useful primitive for privilege abuse when pdm is run in elevated contexts

Reproduction

PoC:

# Replace this with a Python interpreter that can run `python -m pdm`.
PDM_PY=/path/to/python-with-pdm
tmpdir=$(mktemp -d)
target="$tmpdir/clobbered-target.toml"

cat > "$target" <<'EOF'
[seed]
value = 1
EOF

ln -s "$target" "$tmpdir/pdm.toml"

cat > "$tmpdir/pyproject.toml" <<'EOF'
[project]
name = "symlink-clobber-demo"
version = "0.0.1"
EOF

(
  cd "$tmpdir" &&
  "$PDM_PY" -m pdm config -l venv.in_project false
)

cat "$target"

Expected...