virtualenv writes prompt values into pyvenv.cfg without sanitizing line boundaries, allowing configuration injection
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.
Affected software
Click a technology to see all vulnerabilities affecting it.
| Platform | Package | Vulnerable versions | Fixed in |
|---|---|---|---|
| Python (PyPI) | virtualenv | < 21.7.11 | 21.7.11 |
References
- https://github.com/pypa/virtualenv/security/advisories/GHSA-9h9j-4vrj-gf7g
- https://nvd.nist.gov/vuln/detail/CVE-2026-102938
- https://github.com/pypa/virtualenv/pull/3247
- https://github.com/pypa/virtualenv/commit/a30f995461043acb6cacbf3a890951563ccf7140
- https://github.com/pypa/advisory-database/tree/main/vulns/virtualenv/PYSEC-2026-4012.yaml
- https://github.com/pypa/virtualenv
- https://github.com/pypa/virtualenv/releases/tag/21.7.11
- https://pypi.org/project/virtualenv
- CVSS vector
CVSS:4.0/AV:L/AC:L/AT:P/PR:N/UI:P/VC:L/VI:H/VA:L/SC:N/SI:N/SA:N- Weakness (CWE)
CWE-93- Platform
- Python (PyPI)
- Package
virtualenv- Disclosed
- 17.09.2026
- Updated
- 11.10.2026 15:18
Sources and licenses
OSV / GitHub Advisory Database - GHSA-9h9j-4vrj-gf7g - CC BY 4.0
OSV / GitHub Advisory Database - PYSEC-2026-4012 - PyPA Advisory Database; CC BY 4.0
Vulnerability history: virtualenv
All published vulnerabilities of this package (Python (PyPI)), newest first. Shows whether this is a one-off slip or a package that needs watching.
- Total entries
- 7
- First → latest
- 12.2011 09.2026
- Without a fix
- 0
- Exploited in attacks
- 0
Severity breakdown
- High 4
- Medium 3
Disclosures per year
- 2017: 0
- 2018: 0
- 2019: 0
- 2020: 0
- 2021: 0
- 2022: 0
- 2023: 0
- 2024: 1
- 2025: 0
- 2026: 5
earlier: 1
-
virtualenv writes prompt values into pyvenv.cfg without sanitizing line boundaries, allowing configuration injection this entry
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