PYSEC-2026-4012

Vulnerability from pysec - Published: 2026-09-17 16:42 - Updated: 2026-09-30 16:13
VLAI
Details

pyvenv.cfg is a line-based format with no escape syntax. PyEnvCfg.write() wrote values verbatim, while PyEnvCfg._read_values() parses the file with str.splitlines(). A value containing a line boundary therefore became additional configuration lines, and because reading is last-wins, the injected keys replaced any key written earlier in the file.

The prompt value is the reachable input: it is set by --prompt, by the VIRTUALENV_PROMPT environment variable, or from the config file, and write() emits prompt before home. A crafted prompt can therefore set home in the generated pyvenv.cfg. home is what tooling reads to locate the base interpreter, so a consumer that trusts it can be pointed elsewhere. implementation, version_info, version, executable, command and virtualenv are also written before prompt and can be replaced the same way.

This requires the prompt to come from somewhere other than the person running the command, for example a CI job templating a branch name into it, tooling deriving an environment name from user-supplied data, or an inherited VIRTUALENV_PROMPT.

The boundary set is the one str.splitlines() recognizes, which is wider than \n: \r, \v, \f, the file, group and record separators, U+0085, U+2028 and U+2029 were all written through unchanged and all split the line when read back.

Fixed in 21.7.11: PyEnvCfg.write() now collapses those boundaries to spaces as it serializes each line, so it cannot emit a structurally invalid file regardless of what a caller places in content.

Impacted products
Name purl
virtualenv pkg:pypi/virtualenv
Aliases
To clipboard 

{
  "affected": [
    {
      "package": {
        "ecosystem": "PyPI",
        "name": "virtualenv",
        "purl": "pkg:pypi/virtualenv"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "21.7.11"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ],
      "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",
        "21.2.0",
        "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",
        "21.7.2",
        "21.7.3",
        "21.7.4",
        "21.7.5",
        "21.7.6",
        "21.7.7",
        "21.7.8",
        "21.7.9"
      ]
    }
  ],
  "aliases": [
    "CVE-2026-102938",
    "GHSA-9h9j-4vrj-gf7g"
  ],
  "credits": [
    {
      "name": "Bern\u00e1t G\u00e1bor",
      "type": "FINDER"
    }
  ],
  "details": "`pyvenv.cfg` is a line-based format with no escape syntax. `PyEnvCfg.write()`\nwrote values verbatim, while `PyEnvCfg._read_values()` parses the file with\n`str.splitlines()`. A value containing a line boundary therefore became\nadditional configuration lines, and because reading is last-wins, the injected\nkeys replaced any key written earlier in the file.\n\nThe `prompt` value is the reachable input: it is set by `--prompt`, by the\n`VIRTUALENV_PROMPT` environment variable, or from the config file, and\n`write()` emits `prompt` before `home`. A crafted prompt can therefore set\n`home` in the generated `pyvenv.cfg`. `home` is what tooling reads to locate\nthe base interpreter, so a consumer that trusts it can be pointed elsewhere.\n`implementation`, `version_info`, `version`, `executable`, `command` and\n`virtualenv` are also written before `prompt` and can be replaced the same\nway.\n\nThis requires the prompt to come from somewhere other than the person running\nthe command, for example a CI job templating a branch name into it, tooling\nderiving an environment name from user-supplied data, or an inherited\n`VIRTUALENV_PROMPT`.\n\nThe boundary set is the one `str.splitlines()` recognizes, which is wider than\n`\\n`: `\\r`, `\\v`, `\\f`, the file, group and record separators, `U+0085`,\n`U+2028` and `U+2029` were all written through unchanged and all split the\nline when read back.\n\nFixed in 21.7.11: `PyEnvCfg.write()` now collapses those boundaries to spaces\nas it serializes each line, so it cannot emit a structurally invalid file\nregardless of what a caller places in `content`.\n",
  "id": "PYSEC-2026-4012",
  "modified": "2026-09-30T16:13:20.447448Z",
  "published": "2026-09-17T16:42:47Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/pypa/virtualenv/security/advisories/GHSA-9h9j-4vrj-gf7g"
    },
    {
      "type": "WEB",
      "url": "https://github.com/pypa/virtualenv/pull/3247"
    },
    {
      "type": "FIX",
      "url": "https://github.com/pypa/virtualenv/commit/a30f995461043acb6cacbf3a890951563ccf7140"
    },
    {
      "type": "WEB",
      "url": "https://github.com/pypa/virtualenv/releases/tag/21.7.11"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/pypa/virtualenv"
    },
    {
      "type": "PACKAGE",
      "url": "https://pypi.org/project/virtualenv"
    }
  ],
  "summary": "virtualenv writes prompt values into pyvenv.cfg without sanitizing line boundaries, allowing configuration injection"
}



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