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PDM: Project-Local State and Config Writes Follow Symlinks

Moderate severity GitHub Reviewed Published May 21, 2026 in pdm-project/pdm • Updated Jun 10, 2026

Package

pip pdm (pip)

Affected versions

< 2.27.0

Patched versions

2.27.0

Description

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.

References

@frostming frostming published to pdm-project/pdm May 21, 2026
Published to the GitHub Advisory Database Jun 10, 2026
Reviewed Jun 10, 2026
Last updated Jun 10, 2026

Severity

Moderate

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v4 base metrics

Exploitability Metrics
Attack Vector Local
Attack Complexity Low
Attack Requirements None
Privileges Required None
User interaction Active
Vulnerable System Impact Metrics
Confidentiality None
Integrity High
Availability Low
Subsequent System Impact Metrics
Confidentiality None
Integrity None
Availability None

CVSS v4 base metrics

Exploitability Metrics
Attack Vector: This metric reflects the context by which vulnerability exploitation is possible. This metric value (and consequently the resulting severity) will be larger the more remote (logically, and physically) an attacker can be in order to exploit the vulnerable system. The assumption is that the number of potential attackers for a vulnerability that could be exploited from across a network is larger than the number of potential attackers that could exploit a vulnerability requiring physical access to a device, and therefore warrants a greater severity.
Attack Complexity: This metric captures measurable actions that must be taken by the attacker to actively evade or circumvent existing built-in security-enhancing conditions in order to obtain a working exploit. These are conditions whose primary purpose is to increase security and/or increase exploit engineering complexity. A vulnerability exploitable without a target-specific variable has a lower complexity than a vulnerability that would require non-trivial customization. This metric is meant to capture security mechanisms utilized by the vulnerable system.
Attack Requirements: This metric captures the prerequisite deployment and execution conditions or variables of the vulnerable system that enable the attack. These differ from security-enhancing techniques/technologies (ref Attack Complexity) as the primary purpose of these conditions is not to explicitly mitigate attacks, but rather, emerge naturally as a consequence of the deployment and execution of the vulnerable system.
Privileges Required: This metric describes the level of privileges an attacker must possess prior to successfully exploiting the vulnerability. The method by which the attacker obtains privileged credentials prior to the attack (e.g., free trial accounts), is outside the scope of this metric. Generally, self-service provisioned accounts do not constitute a privilege requirement if the attacker can grant themselves privileges as part of the attack.
User interaction: This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable system. This metric determines whether the vulnerability can be exploited solely at the will of the attacker, or whether a separate user (or user-initiated process) must participate in some manner.
Vulnerable System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the VULNERABLE SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the VULNERABLE SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the VULNERABLE SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
Subsequent System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the SUBSEQUENT SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the SUBSEQUENT SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the SUBSEQUENT SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
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

EPSS score

Exploit Prediction Scoring System (EPSS)

This score estimates the probability of this vulnerability being exploited within the next 30 days. Data provided by FIRST.
(7th percentile)

Weaknesses

UNIX Symbolic Link (Symlink) Following

The product, when opening a file or directory, does not sufficiently account for when the file is a symbolic link that resolves to a target outside of the intended control sphere. This could allow an attacker to cause the product to operate on unauthorized files. Learn more on MITRE.

CVE ID

CVE-2026-47763

GHSA ID

GHSA-ghq2-5c67-fprm

Source code

Credits

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